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Evaluation of Survey
Positioning Methods
for Near shore Marine
and Estuarine Waters
CPA
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EPA Contract No. 68-01-6938
TC 3953-03
Final Report
EVALUATION OF SURVEY
POSITIONING METHODS FOR
NEARSHORE MARINE AND ESTUARINE WATERS
for
Marine Operations Division
Office of Marine and Estuarine Protection
U.S. Environmental Protection Agency
Washington, DC 20460
March, 1987
by
Tetra Tech, Inc.
11820 Northup Way, Suite 100
Bellevue, Washington 98005
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PREFACE
This manual has been prepared by EPA's Marine Operations Division,
Office of Marine and Estuarine Protection in direct response to the request
from EPA Regional Offices and coastal municipalities with sewage treatment
plants discharging into estuarine and marine coastal waters. The members of
the 301(h) Task Force of EPA, which includes representatives for the EPA
Regions I, II, III, IV, IX and X, the Office of Research and Development,
and the Office of Water, are to be commended for their vital role in the
development of this guidance by the technical support contractor, Tetra
Tech, Inc. Under regulations implementing Section 301(h) of the Clean Water
Act, municipalities are required to conduct monitoring programs to evaluate
the impact of their discharge on marine biota, to demonstrate compliance
with applicable water quality standards, and to measure toxic substances in
the discharge. This document provides essential information for the evaluation
of operating characteristics (e.g., accuracy, precision, reliability) and
system costs of station positioning equipment. Four main types of navigational
instruments are considered in this document: optical techniques, electro-optical
methods, electronic microwave systems, and satellite positioning equipment.
Positioning for 301(h) monitoring programs is discussed along with the
various advantages and disadvantages of each type of positioning system. A
system selection procedure is also outlined to enable the designation of an
appropriate positioning method to meet the needs of a given monitoring
program. Further, methods are suggested to enhance system operational
characteristics to minimize positional errors.
The information provided herein will be useful to U.S. EPA monitoring
program reviewers, permit writers, permittees, and organizations involved in
the design, implementation and review of nearshore monitoring studies.
Selection of appropriate station positioning methods from the wide range of
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available techniques is an important consideration of all monitoring programs,
therefore the guidance developed herein should have broad applicability in
the design of marine and estuarine monitoring programs.
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CONTENTS
Page
LIST OF FIGURES v
LIST OF TABLES vi
ACKNOWLEDGMENTS vii
1. I NTRODUCTI ON 1
REPORT PURPOSE 1
ORGANIZATION AND USE 1
AVAILABLE NAVIGATIONAL TECHNIQUES 3
2. MONITORING REQUIREMENTS 5
LOCATIONS OF OUTFALLS 5
MONITORING STATION LOCATIONS 5
ACCURACY LIMITATIONS 8
POSITIONING ERROR 9
3. OVERVIEW OF AVAILABLE SYSTEMS 14
OPTICAL POSITIONING TECHNIQUES 14
THEODOLITE INTERSECTION 15
SEXTANT ANGLE RESECTION 17
ELECTRONIC POSITIONING TECHNIQUES 22
SYSTEM CLASSIFICATIONS 23
COMPARATIVE ABSOLUTE ACCURACIES 24
OPERATING MODES 24
SHORT-RANGE SYSTEMS 27
MEDIUM-RANGE SYSTEMS 33
LONG-RANGE SYSTEMS 34
HYBRID POSITIONING TECHNIQUES 35
11
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,
4. SYSTEM SELECTION PROCEDURE
DEFINING POSITION REQUIREMENTS
ESTABLISHING SCREENING CRITERIA
37
37
RANGE CONSIDERATIONS
ACCURACY CAPABILITY
AVAILABILITY
CAPITAL AND OPERATING COSTS
REVIEW OF CANDIDATE SYSTEMS
38
38
40
40
40
41
PURCHASE/LEASE OPTION EVALUATION
5. SUMMARY OF RECOMMENDED PROCEDURES AND EQUIPMENT
41
45
CANDIDATE SYSTEM SELECTION
MULTIPLE HORIZONTAL ANGLES
MULTIPLE ELECTRONIC RANGES
RANGE AND ANGLE
45
46
46
48
SHALLOW-WATER POS IT ION I NG METHODS
REFERENCES
49
51
APPENDIX A - POSITION ERROR ANALYSIS
APPENDIX B - SYSTEM CHARACTERISTICS
A-l
THEODOLITES
SEXTANTS
B-1
B-2
ELECTRONIC DISTANCE MEASURING INSTRUMENTS (EDMIs)
TOTAL STATIONS
B-2
B-6
B-6
MICROWAVE SYSTEMS
TRISPONSER
FALCON 484 MINI-RANGER
MICRO-FIX
HYDROFLEX
AUTOTAPE DM-40AjDM-43
B-6
B-10
B-11
B-11
B-12
B-13
VARIABLE RANGE RADAR
MEDIUM-RANGE SYSTEMS
B-14
B-16
SYLEDIS
RAYDIST TRAK IV
B-16
B-18
f11
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HYPER-FIX B-19
ARGO OM-54 B-20
HYDROTRAC B-21
LONG-RANGE SYSTEMS B-22
LORAN-C B-22
VIEWNAV B-26
LAMBDA B-27
OMEGA B-27
SATELLITE SYSTEMS B-28
TRANSIT (NAVSAT) B-29
GEOSTAR B-31
NAVSTAR GPS B-32
SERIES B-35
AERO SERVICE GPS B-37
HYBRID POSITIONING TECHNIQUES B-38
AZTRAC B-38
POLARFIX B-41
ARTEMIS B-42
1v
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Number
10
11
A-I
A-2
A-3
A-4
A-5
B-1
FIGURES
1
Examples of some key 301(h) monitoring station locations
for a medium-large marine municipal discharge
2
3
Locations of ZID-boundary stations for selected ZID sizes
Station positioning by theodolite intersection
4
Station fix using position circles
5 .
Three-arm protractor for sextant resections
6
Shore target locations to avoid the danger circle
7
Range-range radio-navigation operating mode
8
9
Hyperbolic radio-navigation operating mode
Hyperbolic grid formed by two position line patterns
Range-azimuth positioning system area of coverage
Navigation system preliminary screening criteria
Line of position measurements from two shore stations, de-
picting LOP uncertainty, and associated error indicator
Angle-of-cut effects on fix accuracy
Page
7
12
16
18
19
21
26
28
29
36
39
A-2
A-4
Density function of two jointly distributed random variables A-5
Illustration of radial error
Ellipticity versus drms/CEP
NAVSTAR/GPS test network accuracy
v
A-11
A-13
B-34
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Number
A-1
A-2
6-1
6-2
6-3
6-4
6-5
6-6
6-7
6-8
6-9
6-10
TABLES
1
Example lID-boundary station locations
2
3
Electronic positioning system categories
Typical 1985 equipment rental costs
4
Summary of recommended systems
Probability versus Rio for elliptical bivariate
distributions with two equal standard deviations
Circular error probabilities as a function of measure-
ment standard deviation ratios and error circle ratios
Summary of vernier transit and scale-reading theodolite
characteristics
Summary of micrometer and digitized theodolite character-
istics
Marine sextant characteristics
Electronic distance measuring instruments
Total station characteristics
.Short-range positioning system characteristics
Variable range radar system
Medium-range positioning system characteristics
Long-range positioning system characteristics
Range-azimuth positioning system characteristics
vi
Page
11
25
43
47
A-7
A-9
B-3
6-4
B-5
B-7
6-8
B-9
6-15
6-17
6-23
6-39
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ACKNOWLEDGMENTS
This document has been reviewed by the 301(h) Task Force of the Environ-
mental Protection Agency, which includes representatives from the Water
Management Divisions of U.S. EPA Regions I, II, III, IV, IX, and X; the
Office of Research and Development - Environmental Research Laboratory -
Narragansett (located in Narragansett, RI and Newport, OR); and the Marine
Operations Division in the Office of Marine and Estuarine Protection, Office
of Water.
This technical guidance document was prepared by Tetra Tech, Inc.
for the U.S. Environmental Protection Agency (Marine Operations Division,
Office of Marine and Estuarine Protection, Office of Water) under the 301(h)
post-decision technical support contract No. 68-01-6938, Allison J. Duryee,
Project Officer. This report was prepared under the direction of Dr. Thomas
C. Ginn, Project Manager. The primary author was Dr. William P. Muellenhoff.
Ms. Theresa M. Wood performed techni ca 1 editing and supervi sed report pro-
duction.
Mention of trade names or commercial products herein does not constitute
endorsement for use by U.S. EPA or Tetra Tech, Inc.
vi i
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1.
INTRODUCTION
REPORT PURPOSE
The purpose of this report is to provide information to assist U.S. Environ-
mental Protection Agency (EPA) reviewers of 301(h) monitoring programs,
permit writers, permit applicants, and permittees in the process of selection
and review of survey positioning techniques which will be used to locate
sampling stations in nearshore marine and estuarine 301(h) monitoring programs.
It is designed to be a reference document for quickly identifying candidate
navigational systems to use in particular situations, based on their operating
characteristics and cost. Descriptions and assessments of available locational
techniques and equipment, and assistance for minimizing locational error
are provided. As such, it complements two other 301(h) program guidance
documents that address monitoring program variables, station siting, sampling
procedures, sampling equipment, sampling frequency, sample handling, analytical
methods, data analysis, and reporting procedures (Tetra Tech 1982a,b).
ORGANIZATION AND USE
The report is arranged to enable users with different levels of familiarity
with navigational equipment to access information in the detail needed,
and to then proceed through the in-depth selection of one or more systems
that will satisfy a particular application. Following a brief summary
of available navigational techniques in the next part of the Introduction,
Section 2 addresses navigational requirements for monitoring programs conducted
near coastal outfalls. Typical discharger locations are discussed along
with zone of initial dilution boundaries, types of monitoring stations
usually specified, and recommended accuracy limits for positioning at such
stations. Section 3 is an overview of available systems, including categories
of equipment, typical ranges, advantages and disadvantages, and expected
accuracies. A system selection procedure is outlined in Section 4, which
includes defining positional requirements, selecting screening criteria,
1
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reviewing candidate systems, and finally selecting a best system for a
particular need. Also, the options of renting or leasing systems and daily
costs are discussed. Recommended procedures and equipment are presented
in Section 5, with a quick reference chart indicating those systems most
applicable in coastal areas. The most effective shallow water navigational
techniques that are available are also discussed.
Details regarding positioning errors have been appended, along with
the design and performance characteristics of representative systems.
This enables interested readers to access this material without unnecessarily
burdening others not requiring such information. Refer to the Table of
Contents Appendix B listing for the particular details of each system con-
sidered.
Readers should be alert for advancements in navigational techniques
that may have occurred since publication of this document. In particular,
users should monitor the development of sufficiently accurate systems that
are expected to become available in the near future at reasonable costs
(e.g., satellite systems). Manufacturers cited herein, other manufacturers,
and government agencies (e.g., the U.S. Coast Guard and the National Ocean
Survey) can provide perm.it wri ters and monitoring program designers with
the most current information on available systems. The 1985 prices quoted
herein are for minimum systems that allow range-azimuth, range-range, or
hyperbolic positioning. They do not typically include the costs of optional
equipment for data processing, position plotting, or guidance control.
Where ancilliary equipment is needed, or deemed highly desirable in the
future, its availability should be considered in final selection of a system.
A lthough a number of manufacturers, trade names, and commerci al products
are cited in this report, their inclusion does not signify endorsement
or recommendation for use by the U.S. EPA or by Tetra Tech, Inc. Similarly,
the exclusion of a product does not signify disapproval. Recommendations
are confined to techniques or categories of equipment for specific appli-
cations. Commercial products are mentioned merely as examples, and no
2
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attempt has
information,
been made to include
document users should
dealers in their area.
all products now available. For such
contact manufacturer representatives
or equipment
AVAILABLE NAVIGATIONAL TECHNIQUES
Positioning techniques can be classified as mechanical, optical, electro-
optical, electronic, or a combination of these. Mechanical distance measuring
techniques (e.g., tapes, wire, measuring wheels, and rods) which are not
amenable to offshore positioning are not further discussed herein.
The most popular optical techniques include use of theodolites and
sextants. A theodolite is an instrument used to measure horizontal and
vertical angles. It is used to visually sight on the vessel (~sually a
target mounted on the mast) to determine the angle between it and a target
of known position onshore. Performed simultaneously with another theodolite
at a different position onshore, or at a later time with the same instrument
at the" second position, this sighting enables a geometrical determination
of the vessel's position. Alternately a sextant can be used on the vessel
to observe the angles between three objects of known position onshore,
again enabling a plot of the vessel's position. Range is limited to line-
of-sight.
Electro-optical systems include the laser, which enables very accurate
distance measurements. In coastal navigation, the most useful electro-
optical system measures both the angle to the vessel (siting optically
as with a theodolite) and the range using electronic (typically microwave)
signals. Referred to as total stations or range-azimuth systems, these
systems eliminate the need for more than one shore station. Although many
are limited to the visual range, some are now all electronic, permitting
operation even with low visibilities.
Electronic systems use the transmission of electromagnetic (EM) waves
between two or more shore stations and the vessel to define vessel location.
This is accomplished by measuring differences in either time delays or
the phases of arriving signals. The systems are based on a knowledge of
3
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the EM wave travel velocity and an ability to predict variations in this
velocity as a function of travel path and period of transmission. Most short-
range systems operate at microwave frequencies, whereas longer range systems
typically operate at much lower frequencies. As indicated in Appendix B,
there are a number of systems operating at a variety of ranges, with the
longer range systems typically having lower accuracies. The popular Loran-C
is one of the systems discussed in detail in Appendix B.
,
Satellite navigation systems are also long-range, although they do
not operate at low frequencies. Positional measurements are based on the
Doppler frequency shift which occurs when the distance between the satellite
transmitter and vessel receiver changes, and knowledge of the satellite's
orbit. The U.S. Navy Transit (NAVSAT) system, the Navstar Global Positioning
System, and private satellite systems will soon afford the user, at a reasonable
cost, a selection of highly accurate systems usable in most areas.
4
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2.
MONITORING REQUIREMENTS
LOCATIONS OF OUT FALLS
Over.200 municipalities from Alaska, Hawaii, Puerto Rico, and eastern
and western U.S. coastal areas have applied for variances from the secondary
treatment requirements of the Clean Water Act. Due to the large number
and diverse geographic locations of the applicants, the offshore distances
and the water depths at the proposed monitoring stations vary widely.
Some of the smaller applicants in New England and Alaska have proposed
outfall locations very close to shore in shallow waters. By contrast,
some of the larger applicants have requested discharge locations as far
as 8,085 m (26,525 ft) offshore, in typical water depths of 60 m (197 ft).
Near Hawaii, discharges have been proposed for waters up to 150 m (492 ft)
deep, although offshore di~tances are small due to the presence of significant
bottom slopes. Puerto Rican discharges are 1,000-3,000 m (3,281-9,843ft)
offshore in waters 10-40 m (33-131 ft) deep. Because of the diversity
in locations of proposed discharges, navigational techniques and error
allowances recommended herein apply to a wide range of water depths and
distances offshore.
MONITORING STATION LOCATIONS
The U.S. Environmental Protection Agency requires that receiving water
quality standards will be met and that water quality conditions will be
adequate to ensure the protection and propagation of a balanced indigenous
population of shellfish, fish, and wildlife and allow recreational activities
in and on the water beyond the boundary of the zone of initial dilution
(U.S. EPA 1982). The ZID, as this zone is referred to, is the region of
initial mixing surrounding or adjacent to the end of the outfall pipe or
diffuser ports. The zone may not, however, be larger than allowed by applicable
water quality standards (U.S. EPA 1982). The ZID represents the area within
which the critical initial dilutions occur (i.e., the lowest initial dilution
5
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and those which occur when the environment is most sensitive). U.S. EPA
uses a simplified method to calculate the size and shape of the lID. The
width is twice the depth of the water plus the width of the diffuser, and
the length is twice the water depth plus the length of the diffuser (U.S. EPA
1979). The lID boundary is centered on the diffuser, and extends from
the water surface to the bottom (Tetra Tech 1982a).
To meet the monitoring requirements of Section 125.62 of the Final
Rule governing coastal municipal discharges (U.S. EPA 1982), receiving
water and sediment sampling stations need to be located at or very near
the lID boundary, at reference sites, and in potential impact areas. lID
boundary stations should be located on both the upcurrent and downcurrent
sides of the lID, with the downcurrent site being occupied at any given
time to ensure that the waste field drift flow is intercepted as it is
carried across the lID boundary (Tetra Tech 1982a).
Compliance with conditions of a secondary treatment variance requires
monitoring at a site-specific array of sampling locations. The importance
of both absolute accuracy (i.e., correct geographical location) and repeatable
accuracy (i.e., return capability) is a function of the particular sampling
station in question. The types of stations usually specified in 301(h)
monitoring programs are depicted in Figure 1. Absolute and repeatable
accuracies are most critical for the within-ZID and lID-boundary stations
(lO, l1, l2)' Applicants must be able to sample at a specific boundary
location on any given occasion, and to return to nearly the same location
on subsequent trips. At gradient (G1, G2, G3' G4) and control or reference
(C1) stations, initial accurate location is not as critical. However,
it is important to relocate these stations accurately during subsequent
surveys to be able to quantify any temporal changes in the parameters sampled
(e.g., benthic community characteristics). This requirement for high repeatable
accuracy also applies to stations in or near special habitats (HI, H2).
The ability to conduct sampling at the appropri~te depth contour is also
very important.
6
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'. .' ,.'
,"""'" 'X""""'"
S, .
':'::"':>:: ",:::.:' ..., " ,.10 m""" . HABITATS
;'\:.;\%}(~):: .
\,
......,..,..,.,.....X~
'\
'\ ,.""
/~ /-
--
ZID BOUNDARy---.rn- - - - - - -'~'-60-~'~X~- - '.x...""
................X m T
. X Z G 1 G2 5 GJ
..,Z2 Zo 1
...' I ----------
..........,.. , 1, I ----------
PREDOMINANT
..
"." ."
....,
."X ""''''''
........." R, ..........,..~
~......,.... ..."
KEY:
G '" GRADIENT
H '" HABITAT
R '" REFERENCE
S '" NEARSHORE
T '" TRAWL
Z .. ZONE OF INITIAL DILUTION
"'" '" """"" "" ..,..."" .",
.",.. ,.,..' .,30m"'" ."
CURRENT
... ......~~................. ..... ...oom ....." ........ ....
Figure
1.
Examples of some key 301(h) monitoring
large marine municipal discharge.
for a medium-
station
locations
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ACCURACY LIMITATIONS
Both the procedures and equipment used to establish a position contribute
errors that determine the overall accuracy of a fix. Absolute or predictable
accuracy is a measure of nearness to which a system can define a position
by latitude and longitude (Bowditch 1984)." Repeatable or relative accuracy
is a measure of a system's ability to return the user to a position whose
coordinates were previously measured with the same system. The difference
between these two accuracies can be quite significant. For example, depending
on one's location in the coverage area, Loran-C has a repeatable accuracy
in offshore areas from 15 to 90 m (49 to 295 ft), but an absolute accuracy
of somewhere between 185 and 463 m (607 and 1,519 ft) (Dungan 1979).
In many instances, repeatable accuracy is more important than absolute
accuracy (e.g., retrieval of crab pots, return to desirable fishing grounds,
avoidance of underwater obstructions, and locating an important sea buoy).
For coastal outfall monitoring, both repeatable and absolute accuracy can
be important, depending on the type of sampling site. For within-lID and
lID-boundary stations, both accuracies are important because sampling stations
must be located within or very near the boundary and be repeatedly occupied
during the program. For gradient, special habitat, and reference stations,
repeatable accuracy is more important than absolute geographic location.
Once such a station is established within a special habitat, it is often
necessary to return to the same site to identify temporal variations in
the previously sampled biological community. Thus, it is important to
select navigational procedures and equipment with both the absolute and
repeatable accuracies needed to meet the monitoring program objectives.
Because repeatable accuracy of navigational equipment is usually at least
one order of magnitude better than absolute accuracy, the latter frequently
limits the location of a sampling vessel during coastal monitoring programs.
Therefore, the fOllowing discussion focuses on absolute accuracies that
can be achieved by various procedures and associated equipment.
Practical considerations also limit how well an offshore positional fix
can be made. Resolution of a position line to better than 1-2 m (3.3-6.6 ft)
becomes meaningless when measuring the location of a moving vessel (trawling),
8
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or one which is on station but pitching and rolling. Antenna movement
alone usually precludes any higher resolution in position coordinates.
Exceptions to this rule can occur when conditions are unusually calm or
when the platform is highly stable (e.g., a semi-submersible drilling rig).
POSITIONING ERROR
Many factors contribute to the total error in position of the water
column or benthic sampling point. These include movement or drift of the
"on-stationll vessel, knowledge of the offsets between the navigational
system antenna and equipment deployment point and between this point and
the subsurface sampling or profiling equipment, and error in the ship's
initial location itself. Most of these factors are very site or operationally
specific, and can be estimated with varying degrees of confidence. Because
the accuracy to which the actual sampling point is known is highly dependent
on all such factors, they should be carefully considered in both the design
and conduct of monitoring programs. The discussion herein is limited to
only one of the overall error components, namely the location of the vessel
itself at a desired point on the surface.
One important factor to consider when selecting a navigational location
technique is the locational accuracy required of the vessel at each of
the candidate stations. Of importance are both the absolute or geographic
accuracy, and the repeatable accuracy enabling return to the same stations
within desired limits of positional error. The following example method
may prove useful as an aid in establishing the error limitations suitable
for a particular set of monitoring stations.
In evaluating navigational accuracy requirements during this review,
a method was sought to specify the positional error as a function of one
or more pertinent parameters. A lID boundary error proportional to some
percentage of the lID dimension was selected as the controlling parameter.
Because lID size is proportional to the water depth, the allowable error
in position would also be proportional to depth. For example, the lID-
boundary stations could be located at a distance from the diffuser axis
equal to one-half the lID width plus 20 percent of the water depth at mean
9
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tide level. The allowable maximum error in the location of these stations
would then be ~20 percent of the water depth. As a result, the closest
that sampling would occur is at the lID boundary, and the farthest that
sampling would occur is 40 percent of the water depth beyond this boundary.
Nominally, however, sampling would be performed within a distance from
the lID boundary equal to 20 percent of the water depth. Example lID-boundary
station locations using this approach for a variety of lID sizes are listed
in Table 1. The lID-boundary and sampling station locations for discharges
at the 100-, 60-, and 20-m (328-, 197-, and 66-ft) depths are shown in
Figure 2.
When discharge depths are less than approximately 15 m (49 ft), the
example 20 percent error allowance results in a highly restrictive positional
error [i.e., less than ~3 m (~9.8 ft)]. Therefore, a positioning error
of ~3 m (~9.8 ft) would be more appropriate when sampling station depths
are less than 15 m (49 ft). Although the percent error as a function of
water depth increases at shallower depths, this minimum error is reasonable
given available navigational techniques for small sampling vessels in other
than extremely calm waters. Stations beyond the lID may be similarly located
using the 20 percent depth rule to the 15-m (49-ft) contour and using the
~3-m (~9.8-ft) error limitation for shallower locations. As indicated
earlier, it is recognized that the ability to reoccupy a given site can
be as important as knowing its exact geographical location. However, relocation.
probably will not be a problem because the same navigational equipment
used to absolutely locate lID-boundary stations will likely be used elsewhere.
By using this equipment a repeatability of less than ~2 m (~6.6 ft) can
be expected.
This approach may be useful for determining accuracy requirements
for equipment selection. However, if this approach is used when specifying
allowable positional error, higher accuracy is required at the shallower
stations. This would result in more stringent requirements for shorter
outfalls, normally associated with municipalities having smaller volumetric
flow rates. Such municipalities often have much more limited resources
available for monitoring programs than do larger dischargers. Thus, it
is important to recognize such a constraint. Fortunately, as discussed
10
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TABLE 1. EXAMPLE lID-BOUNDARY STATION LOCATIONS
Average Av erag e Recommended Rec ()11mend ed
Diffuser Di ffuser ZID Station A 11 owab 1 e
Depth Di ameter Wid th Loca t i ana Errorb
(m) (m) (m) (m) (m)
100 4.0 204.0 122.0 t20
90 3.6 183.6 109.8 t18
80 3.4 163.4 87.7 t16
70 3.2 143 . 2 85.6 :H4
60 3.0 123.0 73.5 :H2
50 2.5 102.5 61. 3 t10
40 2.2 82.2 49.1 t 8
30 2.0 62.0 37.0 t 6
20 1.8 41.8 24.9 t 4
15 1.5 31.5 18.8 t 3
10 1.5 21. 5 13.8 t 3
5 1.0 11.0 8.5 t 3
3 0.5 6.5 6.3 t 3
a Distance from the zone of initial dilution centerline to the stations
based on 0.5 times the ZID width plus 20 percent of the average water depth
of the diffuser when over 15 m (49 ft)'.
b Error magnitude is equal to t20 percen t of the average diffuser depth,
when over 15 m (49 ft).
11
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/ OUTFALL PIPE
ZID BOUNDARY
STATION
LOCATION
I
I
I
I
I
I
ERROR-: 40 m
LIMIT I
,
ZID/
BOUNDARY
DIFFUSER
204 m
100 m DEPTH
4.0 m DIFFUSER
122 m
24 m-+1
.
I 123 m '
I I
I I
I w 1
73.5 m I
I ,,- "I
I I
f-- I
I I
I I
~
60 m DEPTH
3.0 m DIFFUSER
20 m DEPTH
1.8 m DIFFUSER- -
,rh,
I 4'~ m I
I 24 m I
8m-t
I
I
Figure 2.
Locations of ZID-boundary stations for selected
ZID sizes.
12
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in Section 5 of this report, inexpensive navigational techniques applicable
to short-range/shallow-water situations that allow positioning well within
a few meters are available. Also, the equipment can be rented for the
infrequent use a small discharger will have, as discussed in Section 4.
13
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3.
OVERVIEW OF AVAILABLE SYSTEMS
There are many methods for positioning sampling vessels in coastal
waters and for returning to previously occupied sampling sites. These
methods range from simple extensions of well-established onshore survey
techniques using theodolites to highly sophisticated electronic navigation
systems. In this section, a number of positioning methods and their associated
theoretical accuracies are presented, with emphasis on procedures for consis-
tently meeting the recommended levels of accuracy. Also addressed are
site-location errors and ways in which these can be minimized.
OPTICAL POSITIONING TECHNIQUES
Optical positioning requires visual sighting to determine alignment
.
on one or more ranges, or the distances and angles between the vessel and
shore targets. Methods most frequently used include:
.
Graduated line (tape) or wire
.
Intersecting ranges
.
Range line and uniform speed or angle
.
Vertical angle ranging (subtense bar or stadia)
.
Angle and distance to vessel
.
Two angles from shore or vessel.
Use of a tape or wire is impractical except in the case of very short
distances from shore. Intersecting ranges are used when a number of established
landmarks permit easy selection of multiple ranges that intersect at the
desired sampling point, and accuracy is not critical. Range line and uniform
14
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speed or angle between the line and an onshore target are more applicable
to hydrographic surveys than discrete sampling station locations. Vertical
angle ranging requires a graduated shore line target known as a subtense
bar. A sextant is set so the individual marks on the bar subtend a given
angle at desired distances from shore. This method is limited to extremely
confined areas, relatively short distances, and calm waters.
The remaining optical methods are most applicable for offshore posi-
tioning. The angle and stadia method was used historically in very calm
waters. This method requires vertical positioning of the stadia (or rod)
on the vessel and careful measurement of angles to the stadia hairs, enabling
a calculation of distance from the observer to the rod. A position fix
is made by measuring the azimuth from a baseline of known orientation.
Although practical only in calm seas, this same method is now effectively
employed by electronically determining the distance using either a total
station or a range-azimuth system, such as those discussed later in this
report. The traditional optical positioning method involves the observation
of two horizontal angles, either from shore using two theodolites, or from
the vessel using a sextant. Each of these techniques is discussed in the
remainder of this section.
Theodolite Intersection
Position of the sampling vessel can be established using theodolites
by two onshore observers who simultaneously measure the angle between a
reference object or shore traverse and the vessel (Figure 3). A rod or
other aiming point is normally erected on the vessel. Radios, flag signals,
or lights are used to set the moment at which angle measurements are made.
Using a theodolite with an accuracy of ~15 sec for single angle measurement
at intercept angles near 450 and a range of 5 km (3.1 mi), should yield
a position error less than ~1 m (3.3 ft) (Ingham 1975). A method for calcu-
lating expected errors for specific positioning situations is presented
in Appendix A.
Although the accuracy of this method appears high, its use in open
waters has several distinct disadvantages. Complex arrangements are usually
15
-------
:::) BE"NCHM"AFii(:
Itll~tjf.iiIJ'Y{i[
,
."
:
Figure
3.
Station
positioning
by
theodolite
intersection.
16
-------
needed to ensure that angles are simultaneously measured by the two onshore
observers at the time of the desired fix. Although not a problem when
the vessel remains on station for a long period of time, it becomes a problem
when the vessel is trawling. Lines from the two theodolites should ideally
intersect at nearly right angles at the vessel's location. Weak position
fixes or corresponding large positional areas of uncertainty result when
the angles measured are less than 300 or more than 1500. To avoid such
extreme angles, onshore observers may have to move frequent1y~ resulting
in delays of a predetermined sampling schedule. Finally, as with all optical
methods, target movement and path interferences (e.g., fog, heavy rain, or
heat waves) can confound the measurements. In spite of these disadvantages,
the procedure offers relatively high accuracies at low cost and has been
successfully applied in small-scale coastal surveys during favorable weather.
It is also advantageous in very restrictive areas or in harbors.
Sextant Angle Resection
An offshore position can also be fixed from the vessel by measuring
the two horizontal angles between 1ines-of-sight to three identifiable
targets whose positions are known (Budlong 1977). When a vessel is underway,
the sextant angles must be measured simultaneously by two observers. The
measurement of the first angle between the center ind one outside target
allows determination of a circle of position (COP) on which the sampling
vessel must lie. For example, when the measured angle is 520, the first
circle of position is plotted by subtracting this angle from 900 and drawing
lines seaward from the siting targets at the resultant angle of 380 from
the baseline (Figure 4). These lines cross at the center of the first
COP, which can then be drawn with a compass. This procedure is then repeated
using the center and remaining target, resulting in the plot of the second
COP. In the example, the second angle is 670, requiring a plot of radius
lines at 230 from the baseline. The intersection of the two position circles
marks the vessel's location.
Position fixes are normally plotted using a station pointer or a three-
arm protractor (Figure 5). The two measured angles are set by moving and
then locking the two outer arms of the protractor. The protractor is then
17
-------
.....
Q:)
","
"," 230
"'''):30 --
" ---
" ---
--
--
-
Figure 4.
Station
fix
using
position circles.
-------
"".' ,
,.'."." .
. '"..,.'
',. ','
... ," '..
'," "'.,.
" ".. ,
.': :.' '. ~., . "
. ..
".:.':...::".','."
'.."." '."
:"..'..' ,",
" -' ' , '
:: ' .~.; '::-: .:
..';' ..".
,". ""',',
, " .
-. .'::".'
::,.':: :":::-
VERNIER
MOVABLE
SIDE
ARMS
.INDEX ERROR
ADJUSTMENT
SCREWS
FIXED
CENTER
ARM
...
",. "
~'>::":''':''
:'.'.:,; ":,': ,::..;,
~ ~ '; ::.:. :. " :
1'M,~,;,::
]~!~','l
Figure 5.
Three-arm protractor for sextant resections.
19
-------
moved over a nautical chart until all three arms are aligned with the preplotted
locations of the shore targets. The vessel's position is t1en recorded
at the center of the protractor. Because this procedure can be implemented
in 10-15 sec with some experience, it has been used historically in hydrographic
surveys where moving vessel positions are needed. To minimize the parallax
error, the two sextant operators should stand as close as possible when
making the measurements. Sextant angles can be routinely measured to approx-
imately 1 min of arc or better, depending on the instrument quality and
operator ability. Within 5 km (3.1 mi) of shore stations and at acceptable
COP crossing angles, the resulting accuracy in position is 1 part in 2,500
or about ~2 m (6.6 ft) (Ingham 1975).
Sextant angle resection has been the most widely used positioning
technique in coastal surveying. This is due to the relatively high accuracies
that can be achieved, ease of implementation, and nominal cost of the sextants
and the three-arm protractor. Also, no shore party is required. The procedure
does have some limiting factors, however. Range is ultimately limited
by visibility and by the sizes, elevation, and placement of the shore targets.
Also, it is imperative to follow certain rules in locating targets to avoid
indeterminate or weak fixes. For example, a fix cannot be obtained when
the vessel and all three shore targets lie on a common circle (called the
danger circle) as shown in Figure 6. This can be avoided by alignment
of the three shore targets along a straight line, which, in effect, causes
the radius of the danger circle to become infinite. Other recommended
practices to assure strong fixes are placement of targets so that intercepted
angles fall between 300 and 1500 (ideally between 450 and 600), thereby
maintaining large position circle cut angles. Shore targets may also be
placed on a curve convex to the observer, with the middle target nearest
the sampling vessel. Alternately, targets may lie on a curve concave to
the vessel, provided the anticipated positions are within the triangle
formed by them, they are practically equidistant from the vessel, the observed
angles are not less than 600, or the vessel is well outside the circumscribing
circle (Clark 1951; Davis et al. 1966). Selected acceptable target configur-
ations and an unacceptable alignment are also depicted in Figure 6.
20
-------
N
-
..-
,;-
/
/
I
+-
\
\
\
"
'"
Adapted from Ingham 1975
.w;,*
I '- 1\
( , - - -->:,1 )
\ ""'"."'""t"
,n I
"-...- --01.
Figure 6.
Shore target locations to avoid the danger circle.
-------
When at the limits of visibility, sextant angle fixes are likely to
be weak because the angles are small. In such cases, large positional
errors can be caused by very small errors in the angles themselves (Umbach
1976). This problem can be partially offset by using a telescope mounted
on the sextant. Sextants must be in perfect adjustment and angles must
be read with extreme care. Also, the sex"tant must be of superior quality
so that the angles may be read to the necessary precision. When working
at locations very near shore, the sum of the two angles can approach 1800,
with one angle often very large and the other very small. Under such condi-
tions, care must be taken to mark the two angles simultaneously if the
vessel is moving, or to make several measurements if the vessel is on station,
because the angles change rapidly with little vessel movement.
Split-fixes (no common center object) may be taken when a three-point
fix is not possible. The vessel position is at the intersection of two
angle loci. A fix is considered strong when the intersection angle is
greater than 450. Split fixes should be used only when absolutely necessary.
They are inefficient due to the required recording procedures and plotting
time, and they cannot generally be entered into automatic data processing
and plotting systems.
Considering the accuracy achievable when the double horizontal sextant
angle method is properly implemented, this procedure offers an inexpensive
candidate positioning method for limited coastal surveys. However, the
method may be ineffective during poor visibility or other periods when
monitoring may nonetheless be required. Also important is the need to
construct, survey, and maintain a sufficient number of properly located
shore targets to provide unambiguous positioning. Overall, the method
has merit when the cost of an electronic positioning system cannot be justified.
ELECTRONIC POSITIONING TECHNIQUES
Electronic positioning systems utilize the transmission of electromagnetic
(EM) waves from two or more shore stations and a vessel transmitter to
define a vessel's location. The systems are based on the EM wave travel
velocity of 299,793 km/sec (186,282 mi/sec) and an ability to predict variations
22
-------
in this velocity as a function
measuring differences in signal
comparing the phases of received
(hyperbolic ranging).
of travel path. Position is determined by
arrival times (range-range mode) or by
signals to those of the transmitted signal
At their respective max1mum ranges, electronic positioning methods
have higher accuracies than visual methods. Measurements can be obtained
regardless of weather and visibility conditions and operating ranges are
typically much greater than for optical methods. Range can be extended
to 50-100 km (31-62 mi) by simply elevating transmitting and receiving
antennae until signal attenuation becomes a limiting factor. Shore stations
need not be attended, minimizing personnel requirements. Positional readouts
are available as distances or coordinates rather than wavelengths or time
delays, and deck units are usually compatible with data processing and
automatic plotting equipment. Position information is continuous, enabl ing
maintenance of a desired location by dynamic positioning or by traversing
along a predetermined path.
The short-range systems of primary importance in coastal monitoring
programs are compact, lightweight, durable, easily calibrated, and relatively
stable. Disadvantages of electronic systems can include cost, particularly
for smaller monitoring program requirements, the inconvenience of orienting
shore and onboard units, vandalism of shore stations, and unknown signal
propagation effects (although this should not be a problem over relatively
short distances). As discussed later, however, the cost factor can be
minimized by sharing among multiple users, by leasing equipment during
survey periods, or by contracting for both survey personnel and equipment.
System Classifications
Electronic positioning systems are often classified by range capability,
which depends largely on propagation characteristics of the operating signal.
Band width and signal power also influence reception distance. For purposes
of discussion, electronic positioning systems are herein categorized as
short-range, medium-range, long-range, and global or very long-range.
Although short-range systems are emphasized, other categories are also
23
-------
examined because systems such as Loran-C are frequently proposed by coastal
dischargers. Satellite navigation systems are also presented, because
their system prices are declining and capabilities (i.e., coverage and
access time) are expected to increase. Representative commercially available
equipment in the various range categories is presented in Table 2.
Short-range [0-40 km (0-25 mi)] microwave systems are portable and
best suited for use in the range-range mode (see "Operating Modes," below).
Medium-range systems [to 150 km (93 mi)] are also transportable, although
components are usually bigger and heavier. They are effective in either
the range-range or hyperbolic mode (see below). Long-range [to 2,000 km
(1,243 mi)] and global systems transmit from permanently installed, widely
dispersed shore stations or satellites for multi-user operation.
Comparative Absolute Accuracies
Although it is difficult to specify the positional accuracies achievable
by instruments in each category, some generalizations can be made. Whereas
optical methods provide accuracies of ~2 m (6.6 ft) for ranges up to 5 km
(3.1 mi) offshore, short-range electronic positioning systems provide accuracies
of ~1 to 3 m (~3.3 to 9.8 ft) for ranges up to 40 km (25 mi) from shore
stations. Comparable me~ium-range system accuracies are ~5.0 m (16 ft)
up to 150 km (93 mi). Long-range systems typically have accuracies of
~50 to 100 m (164 to 328 ft) within 350 km (217 mi) of shore stations,
and over ~100 m (328 ft) at greater ranges.
Operating Modes
There are two principal radio navigation system operating modes:
the two-range (or range-range) mode and the hyperbolic mode. Some systems
utilize the advantages of each in a combination mode. In the range-range
mode, position fixing is accomplished by measuring the extremely small
time intervals required for EM signals to travel from a master transmitter
onboard a vessel to one or more slave stations onshore, and back (Figure 7).
For a known propagation velocity, the time interval is converted to a distance
(range) from the slave, defining a single circle of position on which the
24
-------
TABLE 2.
ELECTRONIC POSITIONING SYSTEM CATEGORIES
Representative
Category Range Systems
Very long range >2,000 km OMEGA
TRANSIT (NAVSAT)
Very low frequency GEOSTAR
Satellite NAVSTAR GPS
SERIES
AERO SERVICE GPS
Long range 0-2,000 km LORAN-C
VIEWNAV
Low frequency LAMBDA
Med i urn ran ge 0-150 km SYLEDIS
RAYDIST TRAK IV
Medium-high frequency HYPER-FIX
ARGO OM-54
HYDROTRAC
Short range
Radar
Microwave
0-40 km
DECCA
FURNO U.S.A.
KODEN/5I-TEX
RA YTHEON MARl NE'
TRISPONDER
MItHRANGER
MICRO-FIX
HYDRO FLEX
AUTOTAPE
AZTRAC
POLARFIX
ARTEMIS
0-100 km
25
-------
SLAVE 1
.. SLAVE 2
LANE WIDTH.. SIGNAL WAVELENGTH
:::'::.~::.~;'
Figure
7.
Range-range radio-navigation operating mode.
25
-------
vessel may lie. The intersection of two or more such circles (based on
signal returns from two or more slave stations) results in a posi tion fi x.
This operating mode is usually restricted to a single user, although single
side-band techniques have been employed to allow multi-user operation (Ingham
1975). Lane width remains constant regardless of distance from the system
baseline. In effect, the lane-width aspect of system resolution does not
increase at increasing ranges from the baseline, as is the case in the
hyperbolic positioning mode.
In the hyperbolic mode, the receiver onboard a vessel detects the
phase difference of signals arriving from multiple shore-based transmitters.
Lines of constant phase between a master and slave transmitters form a
hyperbolic pattern of position lines (Figure 8). By measuring the phase
difference between arriving signals, the vessel can be located along one
of the position hyperbolas~ Adding a second master-slave transmitter pair
superimposes another hyperbolic pattern, resulting in a grid network with
pattern crossings (Figure 9). Measurement of the signal phase difference
from the second transmitter pair allows vessel positioning on the second
pattern, and therefore "unambiguous" location at the applicable grid crossing
point. Phase differences usually are resolved to 1/100 of the lane width.
Because of lane widening for increasing range from the baseline, the system
resolution decreases with vessel distance from the master-slave transmitter
baseline. As indicated earlier, the angle-of-cut of arriving signals also
affects the magnitude of position error. In hyperbolic systems, extending
the baseline length improves cut-angles of the arriving signals and decreases
lane spreading (Ingham 1975).
Short-Range Systems
Electronic Distance-Measuring Instruments--
Electro-optical and microwave distance-measuring instruments (EDMI)
are used extensively in land-based surveying. The EDMI master generates
a carrier signal which is directed towards a reflector (in the case of
light beams) or a repeater (for microwaves). The light or microwave beam
is modulated at two or three different frequencies, usually under the control
27
-------
. :~ i~ :~::X;~.
..:: '.~:: ::'::':"
:.'::':'.:. ':".:.
, .,.-:....
\)\{)
',~~;: :~.::"~ : ~ \:.'
...,'".:.
,,",".
'." .
,.:.."
;:::. :,',:'
"':':.' :-:'
:,,:.'.','
"(
LINE OF POSITION
\mt~~}~;
\
\
\
\
'\
\
\
w.ww
z "...'
~ "{/tA.
~
"..' . .
'..';".'
',.'.'" ,
';:.' : ',:: .
:' .'.':' :.
'." ""
. , , ',' ','
LANE WIDTH - (BASELINE LINE WIDTH)(COSEC "(/2)
BASELINE LANE WIDTH = 1/2 SIGNAL WAVELENGTH
\
\
\ it~?~..
VARIABLE
LANE
WIDTH
. ',.',
. .::..~.'
. "..'.
. ,'.'
::, :.". ::'
": '..'~.:"
,'. """.
.: )~:~} ~:;t~ASTER
Figure 8.
Hyperbolic radio-navigation operating mode.
28
-------
VARIABLE
LANE
WIDTH
position
1 i ne
Figure 9.
Hyperbo 1 i c
patterns.
grid
by two
fonned
29
-------
of a precision quartz crystal oscillator. A phase comparison of the incoming
and outgoing beams at the master enables very accurate distance determinations.
The EOMI is a relatively new development in the field of surveying.
The Geodimeter of the early 1950s, which used a modulated light beam, was
replaced in the late 1950s by the Tellurometer which used a modulated microwave
signal. This improvement increased the range and allowed operation in
moderate rain, fog, and darkness. Newer EOMIs have shorter ranges aad
are much more compact, less power-intensive, and easier to read due to
the use of solid-state electronics. The latest EOMIs use highly coherent
laser light, have longer ranges, require even less power, are portable,
and are easy to operate. The so-called "total station" consists of a theodolite
for measuring angles and an EOMI for measuring distances, with outputs
recorded on magnetic or paper tape for subsequent analyses. Under favorable
conditions, typical EOMI ranges are 1.6 km (1 mi) for light-based systems,
80 km (50 mi) for laser systems, and 150 km (93 mi) for microwave systems.
Properly adjusted and calibrated, an EOMI has very few sources of
error. Ground wave reflection can cause error when measurements are made
over water, because reflected signals result in faulty distances due to
the longer path lengths. The swing, or cyclic manner in which reflections
are recorded, must be correctly interpreted. At very close range, EOMI
accuracy is limited by a constant of uncertainty. Beyond 0.5-1.0 km (0.3-
0.6 mi), accuracies of 1 part in 25,000 are easily achievable. If meteoro-
logical conditions over the signal path are sufficiently well known, accuracies
of 1 part in 100,000 can be achieved (Moffitt and Bouchard 1982). Most
manufacturers report EOMI mean square error (MSE) capabilities at ~ (5 mm
+ 2-5 ppm). At a distance of 5 km (3.1 mi), this represents distances
of from 1.5 to 3.0 em (0.6 to 1.2 in). It is apparent, therefore, that
accuracies achievable with electronic distance measurement devices are
more than adequate to meet the positioning requirements for coastal surveys.
In fact, the angle-measuring devices used with an EOMI, and not the EOMI
itself, limit positional accuracy. Probably the major disadvantage of
the EOMI is the irregular motion and resultant misalignment of the reflector
(in electro-optical systems). Use of microwave patterns eases directivity
requirements.
30'
-------
Total Stations--
An electronic tachymeter, commonly referred to as a total station,
is an instrument for determining the range, bearing, and elevation of a
distant object. As used in coastal surveys, it is a shore station instrument
used to sight the survey vessel reflectors, enabling positional information
to be recorded onshore for subsequent communication to the ves~el operat~r.
In a manual station, the same telescope optics (co-axial) are used to measure
.both distance and angles. They are basically theodolites with built-in
EDMI units. With such manually operated units, slope reduction of distances
is done by optically reading the vertical angle and keying it into a built-in
or hand-held calculator (McDonnell, Jr. 1983). A semiautomatic total station
contains a vertical angle sensor for automatic slope reduction of distances,
while horizontal angles are read optically. With an automatic station,
both horizontal and vertical angles are read electronically for use with
slope distances in a data collector or internal computer. A theodolite
with a mount-on EDMI is not usually classified as a total station. An
exception is the modular total station, designed to allow addition of other
equipment. Such units are usually built around an electronic (digitized)
theodolite such as the Kern E1. Many total stations use hand-held calculators
for data storage, computations, access to control registers, testing, cali-
bration, and orthogonal offset determinations. Most manufacturers offer
optional data collectors that serve as electronic supplements to field
books. This permits a convenient interface with a computer and remote
transmission of data using an acoustic modem.
. Although some manufacturers report displaying angles or accuracies
to 1 sec or less, experienced surveyors know that this level of accuracy
requires careful or repeated pointings at good targets. Fortunately, locating
coastal monitoring stations does not require such an accuracy out to the
5- or 6-km (3.1- to 3.7-mi) range that is needed in some cases. The range
capability of a total station is dependent on the number of prisms available
for signal reflection. Such prisms are directional (i.e., must be pointed
towards the shore station), as opposed to omnidirectional prism arrangements
used for range-azimuth navigation systems described later in this report.
31
-------
For coastal monitoring programs, single station capability is attractive.
Setup and calibration efforts are minimized, and the logistics of station
movement are much simpler than with a multi-station net. A total station
can be used on many other projects when not in use for periodic monitoring
programs. Instrument capabilities and costs are reported in the free,
bimonthly journal Point of Beginning [P.O.B. Publishing Company, Wayne, MI
(313-729-8400)J.
Microwave Navigation Systems--
Short-range electronic positioning systems generally operate at microwave
frequencies. These frequencies limit the system ranges to "radio line-
of-sight" because any obstructing objects cause a complete loss of signal
and can result in undetected erroneous distance readings (Umbach 1976).
Typically, such systems are effective between 25 and 100 km (16 and 62 mi)
offshore, depending on antenna heights and power outputs. Position measurements
are indirect (i .e., by timing the travel of multiple pulsed signals from
a master to two or more remote stations and back; alternately, phase differences
between arriving signals can be measured). Available systems operate in
the range-range mode, the hyperbolic mode, or both. The position fix is
defined by the intersection point of two position circles or hyperbolic
constant-phase lines. Most systems consist of a receiver-transmitter with
antenna on the vessel, and two shore stations also having a receiver trans-
mitter, antenna, and power source. Typically, when the vessel's signal
is received at each shore unit, a response signal is triggered. Ranges
are then automatically determined based on the time delays of returning
signals.
Because microwave systems have nominal accuracies of ~1-3 m (~3.3-9.8 ft)
from very short ranges to 25-40 km (16-25 mi), they provide adequate positioning
capability for coastal monitoring programs. This is advantageous in large
sampling efforts involving more than one vessel, or separate programs in
which sponsors share system costs but wish to'access the unit at any time.
32
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Medium-Range Systems
Systems in this category typically operate in the medium- to high-
frequency bands (i.e., 1.5-400 MHz), achieving greater ranges using EM
waves that propagate around the earth's surface. This enables greater
spheroidal distances to be measured, limited normally by transmitter power
outputs (Ingham 1975). Typical ranges are up to 150 km (93 mi) from base
stations, although system modifications in some cases have extended this
up to 400 km (249 mi). The need for much longer baselines requires careful
selection of station sites and usually results in the establishment of
semipermanent master-remote chains. Although the typical operating mode
is hyperbolic, some systems allow range-range operation. All systems are
capable of lane integration or counting. However, a break in reception
may require a vessel to return to a known point to allow the lane count
to be reset.
Positional accuracies are very dependent on knowledge of. the EM wave
propagation velocity over the entire transmission path, which is influenced
by the nature of the path. Systems calibrated for over-water transmissions
can err when land masses (e.g., islands) are located between the survey
vessel and shore stations. Also important is the extent to which transmission
path atmospheric characteristics (e.g., temperature, pressure, and humidity)
differ from that of the "standard atmosphere." Variations in velocity
values can also be caused by differences in the conductivity of the surfaces
over which the EM waves travel. Poor conductance results in low propagation
velocities, high attenuation, and reduced effective range (Umbach 1976).
Manufacturers usually state a system's resolution in terms of a lane-width
fraction (e.g., 0.01 lane) or a distance on the baseline. Resolution in
this case is a measure of the equipment's precision or ability to detect
and display signal changes corresponding to small changes in distances.
Positional accuracy, on the other hand, is a function not only of the error
in each of the hyperbolic position lines, but the combined error associated
with two or more position lines and their associated crossing angles (Appendix
A). Positional accuracies of medium-range systems vary from a few meters
near the base line to tens of meters at the system's range limits (Ingham
1975) .
33
-------
Medium-range systems must be used with caution in nearshore coastal
areas due to the severe landmass attenuation and water-land interface effects.
Such effects are usually manifested as large calibration variations within
a limited survey area. These systems generally present a less desirable
option for coastal monitoring than do the short-range systems because a
much higher price is required for a range capability beyond that needed.
Also absolute positioning accuracy is generally less than the short-range
systems discussed earlier.
Long-Range Systems
Long-range and global navigation systems generally operate at low
(30-300 kHz) or very low (less than 30 kHz) frequencies. As in the case
of medium-range systems, EM waves at such frequencies travel for very long
distances, limited typically by transmitter power. Onshore station chains
are usually permanent, for use with an appropriate vessel receiver and
published hyperbolic lattice charts (Ingham 1975). Because long-range
systems are designed primarily for general navigation rather than accurate
positioning, achievable accuracies are typically much lower than those
of shorter-range systems. Long-range systems included in this category
are OMEGA and Loran-C units, the Decca Lambda, and satellite systems.
All these systems are described further in Appendix B. Particular emphasis
is on Loran-C which is commonly used for coastal navigation and provides
15-90 m (50-300 ft) repeatable accuracy at a cost of $1,000-$2,000. Improved
performance can be achieved using the differential mode with the expense
of another Loran-C receiver and communication system for transmitting offset
corrections. In addition, satellite systems are detailed in Appendix B
because the developing positional accuracies and costs appear promising
for future use in coastal navigation.
34
-------
HYBRID POSITIONING TECHNIQUES
A number of hybrid positioning systems combine positional data from
various sources to obtain fixes. Suth systems usually involve the intersection
of a visual line-of-position with an electronic line-of-position (Umbach
1976). Visual data may be in the form of sextant angles or theodolite
azimuths. Electronic positional data are normally obtained from a microwave
system.
Of particular interest to coastal monitoring programs are dynamic
positioning systems that require only a single shore station and that use
the simultaneous measurement of angle from a known direction and range
to the survey vessel. This range-azimuth method has the advantage of circular
coverage around the shore station (Figure 10). A single station minimizes
logistical requirements and geometric limitations. Line-of-position inter-
sections are the ideal 900 everywhere within the coverage area. Because
it is independent of the absolute azimuth angle, growth in the error ellipse
is due only to distance from the shore station. Accuracy improves as range
decreases, even fairly close to the shore station, and only one unobstructed
line-of-sight is needed. However, such systems allow only a single user.
Characteristics of three representative range-azimuth systems are
summarized in Appendix B. Two have fully automatic shore stations, requiring
attendance only during setup and alignment. The third requires an onshore
operator at all times. For multiple-day surveys, an automatic station
eliminates the tedium of continuously tracking the survey vessel. The
systems desc~ibed offer much greater flexibility than range-range or hyperbolic
systems with multiple onshore stations. Where positioning requirements
extend into ports, estuaries, or up rivers, the single-station system offers
distinct advantages in covering restricted or congested survey areas and
in establishing an unobstructed signal path. Each system described is
unique in either its operating medium (optical, microwave, laser) and/or
procedure (i.e., manual or automatic tracking). The higher costs of these
systems compared to standard microwave systems may be justified where the
system can also be used for onshore work during non-survey periods.
35
-------
COVERAGE
AREA
Figure
10.
Range-azimuth
positioning
system area
of coverage.
3i
-------
4.
SYSTEM SELECTION PROCEDURE
The major steps in selecting a navigation system include design of
the overall monitoring program, definition of specific positioning require-
ments, and evaluation of available systems. Actual implementation of the
system involves approval and procurement of the selected system, assembly
and installation of shore stations, and equipment calibration and field-
testing.
DEFINING POSITION REQUIREMENTS
The maximum allowable positional error of the selected navigational
system is based on the required absolute positional accuracy at each proposed
monitoring station (e.g., within-lID, lID-boundary, near-lID, gradient,
reference, and nearshore stations). To determine the needed accuracy for
position locations, a variety of information can be used. For example,
defining the characteristics of each station by type, location, water depth,
estimated time on-station, and frequency with which it will be occupied
would be useful for determining. accuracy requirements. Using the depth
criterion selected, such as that described in Section.2 of this report,
the maximum recommended positional error can be calculated for each station.
Using this method, either the lID-boundary or a shallower station away
from the lID could result in the most stringent accuracy requirement.
Additional error allowances for non-lID boundary stations and state agency
requirements (e.g., mixing zone limitations) may also be determined. Ranges
to each station from the midpoint and extreme upcoast and downcoast limits
of the sampling area can be determined from a nautical chart. Biological
trawl paths and navigational hazards can also be indicated on a monitoring
station diagram which depicts the outfall, lID, special habitats, potentially
impacted areas, and the fixed monitoring stations.
Based on a general knowledge of available systems (i.e., range-azimuth,
azimuth-azimuth, range-range, or hyperbolic), candidate locations for onshore
stations should be examined. Estimates of positional errors should be
37
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based on anticipated LOP or angle errors expected at each station. Alternate
shore station locations should be examined to define one or more locations
that will provide coverage of the entire sampling area using different
types of systems. A summary of limiting factors within the survey area
and at individual sites should be developed, based on an inspection of
each candidate shore station site. Line-of-site obstructions, traffic
frequency, competing transmitters, air-water boundary irregularities, accessi-
bility, and security should be evaluated. Having established accuracy
requirements and survey area characteristics, the planner can then proceed
with selection of appropriate screening criteria.
ESTABLISHING SCREENING CRITERIA
It is important to establish a set of screening criteria appropriate
for the intended uses of the navigational equipment. Possible screening
criteria include achievable range compared with the distance to the farthest
monitoring station, equipment accuracy compared to the levels specified
for each station, availability of equipment,. and initial cost relative
to available f~nds. Systems surviving the initial screening may be examined
further. Categories of positioning systems, and the effect of applying
such criteria are presented in Figure 11.
Range Considerations
Eight equipment categories are shown as input to the Level I range
screening. Visibility and infrared limitations may eliminate theodolites,
EDMls, total stations, and two of the range-azimuth systems, depending
on distance capability needed. Use of multiple prisms and possible directional
clusters must be considered at this step. Screening effects are shown
here only graphically as dashed lines going into Level II. However, in
an actual screening, a number of specific instrument models in each of
these categories may be dropped from further consideration due to inability
to operate at the distances needed. Fortunately, sea conditions conducive
to sampling are coincident with relatively high visibilities. The desire
to continue sampling under other than ideal conditions may be balanced
against the inconvenience of resampling on a better day. The presence
38
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'"
g
~
~ i..
~ :a
~ .. ~
~ h ~~
~~ i~
'" ..
~'"
-:a
~ ;;: ~
e i a ~
:a~
p
I I
~I : ~ ~
:r-i: ! 4-~1:~ U
t!' ~ ~ ~ ~ -
~' :tt~ ~ ~
: e ~ ~..
~~ a Ct
~~\;
\;~
I!:..
:::a
~ ~ ~
~ ~ ~ ~
:a~
~ ~
~..
'"
+
CANDIDATE ~ SYSTEM
Figure 11.
Navigation system preliminary screening criteria.
39
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of navigational hazards or the difficulty in returning to port may preclude
sampling under low-visibility conditions.
Accuracy Capability
At the accuracy levels used in the earlier example, long-range positioning
systems may virtually be dropped from further consideration. Even when
operated in a differential mode, such systems do not provide the selected
positioning accuracy. They have been designed for navigating or distant
positioning in open seas, where absolute accuracy is usually unimportant.
An exception might be to retrieve oceanographic equipment. Even then,
to be within 50 m (165 ft) is usually sufficient to sight the resurfaced
package, which is normally equipped with a radar antenna. Some of the
medium-range systems and all of the short-range systems pass the screening
at the selected accuracy level. In this example, the number of candidate
satellite systems is reduced by this screening because Transit-based systems
cannot achieve accuracy requirements in the required relatively short time
frame. A~l other systems provide the absolute accuracies selected, and
thus are not further eliminated at Level II.
Availability
Assuming that a monitoring program must be implemented by mid-1986,
virtually all candidate satellite systems that can provide the required
accuracy would have to be dropped from further consideration. Several
are early in the planning stage (e.g., GEOSTAR). For others, the manufac-
turers do not anticipate full operation before the end of 1986 or mid-198?
(e.g., ISTRAC SERIES and Aero Service Marine GPS). All other systems are
available, limited only by the procurement period.
Capital and Operating Costs
Remaining medium-range systems, except Syledis, exceed $100,000.
Microwave systems typically cost $40,000-$65,000, with one (Autotape) at
$90,000. The three range-azimuth systems (which may offer greater flexibility
for other work and reduced logistical problems) range from $64,500 for
40
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an AZTRAC plus Falcon 484 to $100,000 for POLARFIX. ARTEMIS is intermediate
in cost, at $70,000. Remaining systems (total stations for $9,000 manual,
$15,000-$25,000 automatic; EDMI and theodolites for $10,000-$12,000; a
pair of sextants for $8,000 maximum; two theodolites for $4,000-$8,000)
are all significantly lower in initial cost. However, these are typically
much more labor-intensive and complicated logistically. It may be necessary
to factor these additional "costs" in over the life of the system. Thus,
all but one of the medium-range systems and one of the short-range systems
might be dropped from further consideration. However, representatives
from each category passing Level III may remain after the Level IV screening
based on initial cost alone. Further cost-benefit analyses are appropriate
at this step. The option of renting or leasing the selected system might
also be considered.
REVIEW OF CANDIDATE SYSTEMS
The initial screening process eliminates systems without the required
.range or accuracy capabilities (as described in the previous section).
Systems surviving this initial cut may then be carefully examined for capability
and cost. Important additional factors include compatibility with any
existing equipment, ability to accommodate future system expansion, and
availability of ancillary items (e.g., data logger, plotter, tracking monitor,
or computer). Flexibility to use the system for other work may also be
relevant. For example, street improvement projects or dredging of a navigation
channels may also require use of the navigation equipment. Such projects
may be in high-traffic areas, near signal interference sources, near tall
structures that block line-of-sight, or perhaps up a sinuous river. In
such cases, attributes of a particular system type (e.g., range-azimuth)
may warrant more careful examination of available systems in that category.
PURCHASE/LEASE OPTION EVALUATION
Short-term rental or long-term leasing of equipment may be considered
as an alternative to purchase of a positioning system. A short-term rental
limits costs to the actual surveying periods only and avoids maintenance
41
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responsibilities. Long-term leasing affords continued access for applications
other than coastal surveying, at reduced daily or monthly rates. These
two options may be particularly attractive to smaller municipalities unable
to justify or fund the initial cost of surveying equipment. These arrangements
are also attractive during periods of rapid navigation system development
if such developments indicate that adequate systems will be available in
the near future at lower capital costs. Current efforts in the satellite
navigation field are an example of such development.
Three principal sources of rental equipment are manufacturers (usually
only lease/purchase option plans), companies with a large pool of surveying
and navigation equipment devoted exclusively to equipment rentals, and
service firms that normally provide staff and equipment to support a field
survey. Some environmental consulting firms also maintain equipment lease
pools, since equipment is used routinely in the conduct of field work.
The monthly rental cost of equipment is often set at 10 percent of the
suggested retail cost. However, short-term rates for many items are categorized
by the number of rental days (e.g., 1-10, 11-90, and 91 plus). Rent/purchase
options are available, enabling ownership transfer to the renter for a
nominal amount at the end of a fixed rental period. In some cases, eventual
purchase of a rented item is optional, with a set amount of the rental
fee (e.g., 50-80 percent) applied to the purchase price when the option
is exercised.
Typical 1985 rental costs for candidate positioning systems are shown
in Table 3. As indicated, a theodolite/EDMI pair can be rented for approxi-
mately $50-$85 per day for short periods, or $20-$35 for periods over 3
months. Total stations rent for $40-$100 per day on a short-term basis,
depending on how elaborate (e.g., automatic) the station is. For longer
periods, the daily rate drops to $15-$60. Hydrographic systems range from
approximately $175 to $200 per day (e.g., Trisponder 202, Mini Ranger III,
or Cubic Autotape). Monthly rates are typically $3,000 for basic systems,
but can exceed $15,000 for more elaborate systems.
Although the cost information presented herein will change, the range
of costs indicated should enable the potential user to consider rent/lease
42
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TABLE 3. TYPICAL 1985 EQUIPMENT RENTAL COSTS
Category Daily Cost (Dollars)
1-10 Days 11-90 Days 91 + Days Monthly
Theodo1 ites I" 25-35 18-21 12-15
6-10" 22-25 13-15 9-12
20" 15-18 10-13 7-10
E OM Is 1. 0-2.0 km 25-30 15-19 10-13
2.0-6.0 km 25-65 18-39 15-20
>6.0 40-50 25-30 16-22
Tota 1 Stations Automa tic 75-110 45-70 30-60
Manual 40-50 25 -4 5 16-20
Hydrographic Systems
Trisponder 202 175 2,750
Cubic Auto Tape
(3 range) 200 105 90
Mo to r 0 1 aMi n i
Ranger II
(inc1 ud ;ng 2 trans-
pond ers). 200 105 90 3,750
(add it ;ona 1 trans-
pond ers) 36 18 12
43
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as well as procurement when evaluating the cost-effectiveness of applicable
systems. It may also be appropriate to include the cost of one or more
equipment technicians or operators (typically at $300 per day plus expenses)
to provide reliable navigation data for each survey.
44
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5.
SUMMARY OF RECOMMENDED PROCEDURES AND EQUIPMENT
Based on this evaluation, the systems recommended for coastal positioning
include theodolite, sextants, EDMIs, total stations, and microwave and
range-azimuth systems. Although satellite systems offer adequate.accuracy,
they require further development. However, satellite systems should be
considered in future applications. This final discussion is a summary
of the recommended positioning techniques for coastal navigation.
CANDIDATE SYSTEM SELECTION
Positioning techniques and associated equipment were described in
previous sections of this report. No single system is best for all coastal
monitoring purposes, as needs will vary according to size and complexity
of the planned monitoring program, nature of the immediate and surrounding
areas, and other navigational or surveying requirements of a municipality.
Positioning techniques fall into three principal measurement categories:
.
Multiple Horizontal Angles
Theodolite Intersection
Sextant Angle Resection
.
Multiple Electronic Ranges
Distance-Measuring
Range-Range Mode
Hyperbolic Mode
Satellite Ranging
Instruments
45
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.
Range and Angle
Theodolite and EDMI
Total Station
Range-Azimuth Navigation Systems.
Systems within these categories which will meet or exceed the positional
accuracy levels recommended herein are summarized in Table 4.
Multiple Horizontal Angles
In the first category, theodolites were found to have the angular
accuracies required, out to the maximum ranges anticipated. Their costs
range from $1,000 to $4,000 (30-sec vs. 10-sec accuracy) and are readily
available due to general use as a surveying instrument. At least two theodo-
lites, two operators, a vessel siting target, and a three-way communications
link to coordinate fixes are required. Visibility can be a limiting factor.
By comparison, sextant angle resection can be performed using one
instrument if the vessel is stationary, or using two instruments simultaneously
if the vessel is moving. Achievable angular accuracy of =10 sec is adequate
and relatively inexpensive sextants ($1,000-$2,000) are readily available.
Again, visible range can be limiting. Shooting an accurate fix from a
nonstationary platform in any significant sea or swell could be more difficult
than shooting with theodolites from shore. A distinct advantage of sextant
angle resection is location of the navigators on the survey vessel. The
method generally requires highly visible shore targets and a three-arm
protractor for plotting positions.
Multiple Electronic Ranges
Positioning using multiple ranges can be accomplished with two staffed
EDMI stations. Accuracies were found to be more than adequate but ranges
were found to be marginal [if needed beyond 3 km (1.9 mi)] unless multiple
prisms were used. Because such prisms are directional, procurement of
multiple clusters for more than one direction could result in a significant
46
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TABLE 4.
SUMMARY OF RECOMMENDED SYSTEMS
Category
Theodolite
Sextant
EDMI
Total Stations
Microwave
Navigation
Systems
Range-Az imu th
Systems
Satellite Systems
Representative
Equipment
Table B-1
Table B-2
Table B-3
Table 8-4
Table 8-5
Table 8-6
Table 8-10
Tab 1 e 8-9
Accuracy
10-30 sec
.: 1 m (3.3 ft)
+ 10 sec
.: 2-m (6.6 ft)
1.5-3.0 cm
5-7 cm
.: 1-3 m
0.010 and 0.5 m
1-10 m
Cost
$1,000-$4.000
$1.000-$2,000
$3.500-$15,000
$8.000-$30.000
$40.000-$90.000
$65.000-$100.000
$150.000-$300.000
(i!,itial units)
Advantages
Traditional
method.
Inexpensive.
High accuracy.
Successfully
applied.
Restricted areas.
Rapid. Easy to
implement. Most
wide ly used.
Low cost. No
shore party.
High accuracy.
Extremely accur-
ate. Usable for
other surveying
projects. Cost.
Compact. port-
able. rugged.
Single onshore
station. Other
uses. Minimum
logistics.
No visiblity
restrictions.
Multiple users.
Highly accurate.
Radio line-of-
sight.
Hi gh accuracy.
Single station.
Circular coverage.
High accuracy.
Minimum logis-
tics. Use in
restricted/con-
gested areas.
Future cost. No
shore stations.
Disadvantages
Line-of-sight.
Two manned shore
stations.
Simultaneous
measurements.
Limits on inter-
section angles.
Area coverage;
station movement.
Simultaneous
measurement of two
angles. Target
visibilities.
location. mainte-
nance. Line-of-
sight. Best in
calm conditions.
Limits on accep-
table angles.
Motion and
directionality of
refl ectors .
Visibility, unless
microwave. Two
shore stations.
Ground wave reflec-
t ion.
Reflector movement
and directionality.
Prism costs.
Cos t . Mu 1 tip 1 e
onshore stations.
Logistics. Security.
Single user. Cost.
Current coverage.
Initial develop-
ment cost.
47
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cost beyond the initial investment of $3,500-$5,000 each for the two shorter-
range units, or $8,000-$15,000 each for the two longer-range units. Several
microwave navigation systems with more than adequate range and sufficient
accuracy are available in the $40,000-$90,000 range. Limitations include
geometry of shore stations, position of the vessel in the coverage area
(i.e., crossing angle limitations), and possible interferences due to line-
of-sight obstructions, sea-surface reflective nulls, and land-sea boundaries.
The hyperbolic mode provides multiple user capability, but at the cost
of an additional shore station.
Satellite ranging holds promise because required accuracies should
be achievable in the near future. Transit satellite-based systems do not
offer sufficient accuracy, except with multiple passes, and multiple passes
are impractical when a given sampling station is occupied only briefly.
Accuracies ne~ded will undoubtedly be achievable in the future using differen-
tial GPS techniques ($10,000-$40,000 for first units; as low as $1,000
for subsequent production models). Commercial geosynchronous satellite
networks, such as GEOSTAR, may become available at a proposed system inter-
rogator cost of $450 plus a monthly fee. However, this system is in the
very early stages of planning, having only recently received FCC approval
of requested frequencies. Finally, the codeless GPS systems (SERIES or
Aero Services Marine GPS System) currently under development could be used,
but at a current cost of over $250,000.
Range and Angle
Systems in the range-azimuth category show great promise. Required
angular and range accuracies are available, only one shore station is needed,
and costs depend on system refinements. At the low end of the scale, an
EOMI and theodolite could be paired with a communication link for approximately
$10,000-$12,000. Total stations developed specifically for this requirement
range in cost from $8,000 for a manual station to $15,000-$30,000 for a
v
fully automatic station. Optical and infrared range limitations apply
to these systems. Finally, the three range-azimuth navigational systems
examined provide sufficient positional accuracy with a single station at
48
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costs ranging from $65,000 for manual tracking to $70,000-$100,000 for
fully automatic tracking.
SHALLOW-WATER POSITIONING METHODS
When sampling stations are located in relatively shallow water, they
can be identified by relatively inexpensive methods (in addition to those
discussed earlier in this report). Provided the center of the ZID over
the outfall can be located (e.g., by diver-positioned surface float) an
optical range finder may be used to establish the required distances to
nearby water quality or biological sampling stations. An optical range
finder is used by simply focusing a split-image on the target float, enabling
the slant distances to the target to be read off the instrument scale.
When combined with a careful compass reading, this distance reading allows
po~itioning of the sampling vessel. Positioning in this way enables profiling
of the water column or collection of grab samples of bottom sediments.
A survey of accuracies claimed for commercially available instruments
suggests that the ~3 m (9.8 ft) recommended minimum accuracy can be achieved
within approximately 100 m (328 ft) of the surface target. The Lietz Model
1200, for example, provides an accuracy of ~1 m (3.3 ft) at 100 m (328 ft).
Beyond this distance, instrumental errors increase rapidly. For the instrument
cited, a ~9 m (29.5 ft) accuracy is quoted at 300 m (984 ft). The suggested
U.S. list prices of optical range finders vary from $35 to $120 (Folk,
L., 21 March 1985, personal communication).
An acceptable alternate method of collecting bottom samples from desired
locations in shallow water is to use divers. Provided visibility is adequate,
divers may measure radial distances to desired locations by holding a tape
at the outfall and traversing the appropriate distance over the bottom
in the proper direction.
Visual ranges have sometimes been used to establish a station position.
This method requires that a minimum of two objects are in alignment, enabling
the vessel to be placed on a common axis extending to the vessel's position.
Simultaneous siting on a second set of at least two objects places the
49
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vessel at the intersection of the two common axes. The accuracy of each
visual range is highly dependent on the quality of the visual range (e.g.,
alignment), the distance from the alignment objects to the vessel, and
the angle between each range. Also, the number of visual ranges used affects
the magnitude of the positional error. Although this technique is frequently
used for positioning single sampling stations in bays and harbors or otherwise
where a number of convenient alignable targets can be selected, the method
is not considered acceptable for coastal monitoring at lID boundary stations.
Even when a desired sampling station has been accurately located using
one of the previously discussed methods, and at that location at least
three aligned target-pairs are visible, it is not likely that sufficient
alignment target-pairs will be present for all other desired locations.
Also, the unpredictability of positional error each time a station is reoccupied
detracts from the value of this method.
Permanent installation of a marker buoy at the outfall terminus or
midpoint of the diffuser allows easy return to this point on subsequent
.
sampling trips. Using the previously discussed range-finder technique
or a line of desired length enables positioning at desired distances from
the marker buoy. However, it is not uncommon to lose such a buoy due to
vandalism, impact, or severe wind and wave forces whic~ cause its release
and movement from the site. Therefore, it is necessary that the sampling
party be prepared to relocate the outfall station (e.g., by diver, sonar,
or pinger mounted on the outfall itself), if location of other stations
are dependent on locating this station first.
Because the techniques described here are inexpensive to implement
(as are use of the sextant resection or theodolite intersection methods),
they are attractive to small coastal municipalities. However, as discussed
in Section 4, use of more sophisticated and less labor-dependent techniques
may be achievable at moderate costs by renting or leasing, rather than
buying such equipment. Such options may be considered, provided the recommended
ab~olute positional accuracies can be achieved by the technique selected.
50
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pp. 53666-53685.
Waltz, D.A. 1984. An error analysis of range-azimuth positioning. pp. 93-
101. In: Proc. of National Ocean Service Hydrographic Conference. April
25-27, Rockville, MD.
Waltz, D.A. 18 January 1985. Personal Communication (phone by Dr. William
P. Muellenhoff). National Ocean Service, Norfolk, VA.
Weems, L. 2 January 1985. Personal Communication (phone by Dr. William
P. Muellenhoff). Sercel, Inc., Houston, TX.
Whalen, W:L. 1984. Geostar positioning system using satellite technology.
Sea Technol. 25(3):31-34.
Whitcomb, J.H. 14 January 1985. Personal Communication (phone by Dr. William
P. Muellenhoff). ISTAC, Inc., Pasadena, CA.
~
-------
APPENDIX A
POSITION ERROR ANALYSIS
-------
APPENDIX A
ANALYSIS OF POSITION ERROR
Traditionally, position errors have been classified into three categories:
gross, systematic, and random (Mikhail and G"racie 1981). Gross errors
are mistakes due to observer carelessness and must be detected and eliminated
from positional measurements. Systematic errors occur due to a determ-
inistic system, in this case the sextant, range finder, or electronic position-
ing equipment. Such errors can be expressed by functional relationships
and can therefore be corrected for in the final measurement. Random errors
reflect variations in me~surements that remain after eliminating gross
errors and allowing for systematic errors. As their name implies, such
obse~vational errors are random and have no known functional relationship.
Random errors, the subject of the following discussion, are dealt with
on a statistical basis using probability models.
All positional fixes are in error to some extent. This is true even
if the transmitting stations have been very accurately located and bias
or systematic errors in position coordinates have been eliminated. Random
errors will still cause a variable displacement of position lines about
their computed positions.
Positional accuracy is usually stated in terms of the probability
of being within a certain distance of a desired geographical location.
The uncertainty in each line of position can be described by its standard
deviatjon (0), which is the square root of the sum of the squares of deviations
from the mean line-of-pos"ition (LOP) value, divided by one less than the
number of repetitive measurements. The LOP can therefore be considered
a strip whose width is a function of the LOP uncertainty, or standard deviation
(Figure A-la). Because a fix involves at least two LOPs, each having an
uncertainty, the LOP crossing boundary forms a parallelogram (Figure A-1b).
A-I
-------
, /- MEASURED LINE OF
,/ . POSITION NO.2
MEASUREMENT
VALUE SPREADS
SHORE STA. 1
BASELINE
SHORE STA. 2
a.
ERROR
PARALLELOGRAM
EQUAL
PROBABILITY
ELIPSE
b.
SOURCE: MODIFIED FROM BOWDITCH 1984
Figure A-I. Line of position measurements from two shore stations,
depicting LOP uncertainty (a), and associated error
indicator (b).
A-2
-------
The parallelogram area is dependent on the size of the standard deviations
of the two LOPs and the angle at which they cross. The parallelogram area
is given by:
A = (20'1)(20'2) csc = 40'10'2 csc~
(A-i)
where 0'1 and 0'2 are the LOP measurement standard deviations. As the cosecant
function becomes large for angles less than 300 and more than 1500, so
too does the size of the error parallelogram (Figure A-2). Thus, to reduce
positional uncertainties, shore station locations should be located such
that very large or very small cut angles do not occur within the survey
area.
Probability theory indicates a 68.3 percent chance of being within
~1.00' of the mean of a set of normally distributed measurements. However,
the probability that the measured position is within the parallelogram
is less. For two sets of measurements, the joint or product probability
is (0.683)2 x 100 or 46.6 percent. A probable parallelogram is defined
as one for which the probability of a measurements being within its boundary
is 50 perc,ent. This occurs at distances slightly larger than ,!.1.00' (i.e.,
+1.050'). Using +2.00' results in a parallelogram within which it is 91 percent
probable that the 'positional location occurs. A parallelogram constructed
to reflect a specific probability that the actual position will lie within
its boundaries, is easy to overlay ,on a nautical chart. The principal
drawback is that the error parallelogram is not a curve of equal probability.
The chance of being located in a small area adjacent to the midpoint of
one of the sides is greater than the chance of being in an equal size area
located in one of the corners.
An equal probability contour from a two-LOP fix is an ellipse centered
on the LOP intersections (Figure A-lb). This can be seen by examining
the density function of two jointly distributed random variables, called
a bivariate normal distribution. The joint distribution has the form of
a bell-shaped surface as shown in Figure A-3 (Mikhail and Gracie 1981).
Any plane parallel to the x,y coordinate plane cuts the bivariate density
surface in an ellipse. As with the parallelogram, the area and configuration
A-3
-------
>-
I
..-
\
\
\
\
\
\
\
\
\
I
I
I
I
30°
ANGLE OF
CUT
\
\
\
\
\
\
\
\
AREA OF
UNCERTAINTY
"
"
"
"
,
,
,
/
/
/
/
/
/
/
/
150°
ANGLE OF
CUT
.......------
----
---
---
--
-----~--
--
/
/
/
/
/
-
---
--
--
---
-
--
---..----...
--
--
,
"
"
,
"
"
,
"
,
90°
ANGLE OF
CUT.
Figure A-2.
/
/
/
/
/
/
/
/
"
"
"
,
"
,
"
,
/ "
/ "
X/ ",
/ / POSITION
/ LINES
Angle-af-cut effects an fix accuracy
-------
f(x. Y)
f(x}
.
y
SOURCE: MIKHAIL AND GRACIE 1981
Figure A-3.
Density function of two jointly distributed
random variables.
A-5
-------
of the error ellipse are dependent on the crossing angle and standard deviations
in each set of measurements. The ellipse in which 50 percent of the expected
measurements lie is referred to as the probable ellipse. The major axis
of the ellipse will pass through the true position and lie within the LOP
acute crossing angle. Techniques for determining the orientation and the
magnitudes of an error ellipse for various probability levels are presented
by Mikhail and Gracie (1981).
When the error density distributions of two sets of measurements are
equal and the cut angle is 900, the error figure becomes a circular normal
distribution. At a radial distance from the true position equal to 10,
there is a 39.9 percent chance of being located within the circle. A 50 percent
probability occurs for a circle with a 1.180 radius and a 90 percent probability
occurs for a circle with a 2.20 radius. These probabilities change as
the cut angle varies from 900, and as the two LOP standard deviations differ
from each other. For example, if 01 = 02 and the cut angle is 400, the
circle having 'a radius of 10 corresponds to a 26 percent probability.
A 50 percent probability occurs at 1.620. A radius greater than 3.40 is
needed to achieve a 90 percent probability that the position lies within
the circle boundary.
It is more common to establish the probability that a position is
located within a circle of a particular radius than to define an error
ellipse. The resultant circular probable error (CPE or CEP) is the radius
of a circle within which 50 percent of the fixes should be located (Figure
A-1b). Bowditch (1984) provides two methods for determining the sizes
of error circles at a desired probability level, given the standard deviation
in each LOP measurement and the associated pattern crossing angle. Alternately,
the probability that the measured position is within a circle of a selected
radius can also be determined.
As the cut angle approaches 900, the probability of measurements bei~g
within a circle of given radius increases. Or, alternately, the radius
within which a certain percentage of all measurements will fall decreases.
Table A-1 can be used in the special case where the 0 values of the two
fix measurements are equal. For example, for a circle error radius (R)
A-6
-------
TABLE A-l. PROBAB ILITY VERSUS RIo FOR ELLI PT ICAL B IVARIA TE
DISTRIBUTIONS WITH TWO EQUAL STANDARD DEVIATIONS
~ .. I'. ... u. u. ". II. u. .,. u. u. ". u. ". u. ... u. ...
0.0 .000
0.1 .OOS
o 2 .0111 .004 .001> .001 .009 .011 .011 .012 .014 .O:S .011, .017 .011 .011 .019 .019 .019 .020
O.J .OM .009 .012 .01S .019 .022 .02S .028 .031 .034 .036 .038 .040 .041 .042 .043 .044 .044
0.4 .008 .014 .021 .021 .033 .019 .045 .050 .05S .059 .Db3 .01>7 .070 .072 .074 . aT6 .1111 .m
'.5 .009 .019 .0%9 .039 .IUI .057 .01>6 .014 .083 .090 .0% .102 .101> oliO .114 .116 .117 .111
0.' .016 .0%9 .043 .OS7 'aTO .082 .095 . I UfI .117 .127 .135 .In . ISO .a56 .159 .1.2 .1" .U5
'.1 .020 .019 .057 .aTS .093 .110 .126 .141 .155 .168 .179 .189 .198 .205 .211 .215 .211 .211
0.1 .02S .CMI .012 .095 .111 .134 .I~ .1~7 .195 .211 .226 .238 .249 .251 .2.5 .210 .213 .ra
I.' .131 .0.0 .018 .11. ."3 .1119 .191> .217 .238 .257 .215 . 21JO .303 .3" .322 .321 .332 .U3
I.. .US .010 .104 .131 .169 .199 .228 .2!.11 .281 .305 .3:. .341 .3~ .371 .381 .)11 .3.2 .Jt3
1.1 .040 .010 .120 .1S8 . 1910 .231 .265 .296 .325 .3S2 .306 .39b .414 .421 ....0 ..... .452 .454
1.2 .044 .092 .136 .111 .22t .2114 .302 .331 .37U .100 .&21> .4&9 .&69 .4A5 .&91 .SU6 .SIl .sn
1.3 .on .104 .m .2~ .251 .2911 .339 .318 ...4 .H" . 475 .SUI .522 . S&O . SS3 .563 .569 .STO
1.& .051 .IIS .111 .22'11 .271 .327 .314 .&11 .4!.11 .491 .S:l .550 .S73 .512 ..Db ..11 .6%3 .US
..5 .0.3 .126 .11' .247 .304 .358 .408 .H5 .497 .53\ .Sc.8 ..cr. .621 .6&1 ..511 .1>67 ..73 .615
"6 .0.9 .131 .201 .2'119 .330 .384 .4&2 .491 .S36 .S7/o ."11 ."40 .... .".. .70: . ~13 .720 .7%2
1.7 .014 .1&8 .220 .lI9 .3SS ...7 .471 .5:11 .573 ..15 ...541 .1182 .707 .~~I .7'" . 7511 .762 .7"
"I .111' .IS9 .2" .310 .380 ....S .Su~ . S5'1 .111111 ...51 ...SA .~7'" .14S .1'" .112 .193 .100 .102
.." .ON .111'11 .251 .329 .4113 .4;1 .53f1 .."'1 .1111 ...AS .~.:~ .1!.& .780 ..00 .1111 .127 .U3 .136
l.a .089 .119 .2116 .349 .426 .498 .5113 .621 .1172 .11" .753 "~II~ .110 .83:1 .1&6 .856 .162 .16S
2.1 .093 .119 .210 .347 ..... .5'13 .5119 .1149 . i- 0 .1« .780 .81: .1.11 . 8S~ .111 .UI .U7 .190
2.2 . "'" .199 .29S .316 .&1C! .S&6 .IIIS ...a .~'l7 .iil .8Ui .138 .1.1 .UII .1" .911) .9u9 .911
2.' '.104 .209 .309 .404 .490 .570 .".1'1 .':'00 .n: .~95 .831 .8..0 .883 ,.900 .913 .922 .9n .92'
2.' .109 .219 .324 .422 . SI2 .592 .l1li3 . i:& . ~75 .818 .8S~ .88U .'Iul .'118 .93u ..38 .9&2 .9'"
:.5 .IIS .229 .338 ....0 .532 .1114 ..8b .7"... . ~9j .118 .111 . 8'1~ .9\8 .,,)1 .U.I ."!.I .955 .956
2.' .119 .23' .352 .H7 .551 .63& .701> . ihb .811> .1'>6 .U~ . ~12 .931 ."~j ...51 .'l1li1 ...4 .-
2.7 .12' .2&8 .306 .&14 .510 .654 .~2" . j8ft .8H .113 . '}('1 . '!Zo .94) . ~5S .'l1li1 .969 ..73 .r.4
:.1 .129 .254 .319 .490 . sa. .111& .1&6 .805 .8>1 .8RR .'1." . '117 . '11,1 ....., . '117 I .97" .'17' .980
2.' .IU .2118 .393 . Sf11 .1107 .692 .1bJ .8~1 ....7 .'(I~ ..,:8 . '117 .~I .971 .978 .~! .914 ..IS
3.0 .139 .2'17 .406 .S22 .624 .110 .181 .837 .880 .IIU . '138 ....... . 'H>I . "~II .98: ..... .981 .98!>
3.1 .1'" .281 .419 .S'I .641 .121 .7'1i .8>2 .8YJ .Y!4 . "I~ .....1 . ~7 ~ .~; . 96.. .~.. .'11'11 .99:
3.2 ."9 .n. .43l .Ss) .657 .741 .81% .8":, . 'J\I5 .9H ."S. .0;0." . y:~ .1111;; ;'l'J0 .9'JZ .''1) .994
3.J .154 .3010 ."5 . !.III .673 .1S9 .8:11 .878 .'1111 ."U . '16:! . '17, .911) ...... ...9: ."'1 .995 ...91>
3.4 .1S9 .31S .451 .513 .1144 .113 .140 .88'1 . 9:~ . 'ISI .Y(,R ...19 .~.. . 'I'JI .9'14 .~ .991 .997
'.S .164 .32& .410 .5'; . r!~ .184 . IS: .'IUII .IIH . 9S~ . 'J73 .981 .'111111 . 'l'JJ ..,..., . ')91 I ..,... .9111
3.' .1119 .333 .&82 .611 . I. .101 .86& .IIIU .942 ....1 .'171 ...... .""1 ."'1, .~7 .- ..,... ."""
3.7 .178 .J41 .4" .62S .731 .114 .87S .'11' ."&'1 .'H>II .'.1 . "}II .....1 .""" . 1I'J8 ...... . '"" .-
3.1 .179 .352 . SOlo .63' .1&4 .8:11 .886 .9:i . .,5S .q~3 ."14 ."'.' . "'1) . 'l'J7 . "'HI .- . 'I" .-
3.' .113 .360 .511 ..SI .1S7 .131 .I9S .935 .""1 . .J~~ ."87 . 'I'll ."'''' ...... .-' .,,,.. ...ow 1.11\'"
4.0 ..19 ~m .5%9 .664 .110 .1&8 .90~ . '142 ..... .9811 .'JII.. . 'I'll . ,,"'7 ...'111 ."'''' .- 1."1'" 1.1'"''
... .In .!.&I .671> .781 .R" .913 .9U .'7u .'IIIJ . '~)I . 4IY~. . 'l'J8 ."'19 .- I.I~'" I. \,,~. I.''''''
4.2 .198 .117 .552 .1>11 .793 .8119 ..211 . .5. . ",7 I . 91th ..."'3 ,,'" .'11"" .9'1" .41'1" 1.I\(kl I.'''''' I. lillo'
&.3 .20J .3911 .$6& .100 .104 .8,. . .27 ..,58 .978 .988 . 'I'I~ .w~ . 'I"" .~CN 1.11"" L'''''' I. ,.111' I.''''''
..4 I. ,~~,
Reference:
Bowdi tch 1984.
A-7
-------
equal to 10 (i.e., the ratio of R/o=I), the probability of measurements
being within a circle this size varies from 3.5 percent at a 50 cut angle
to 39.3 percent at a 900 cut angle, with a 26 percent probability occuring
at 400 as mentioned earlier. At a given cut angle, as the Rio ratio or
error circle increases, so too does the probability that measurements will
fall within the circle. At an angle of 400, for example, as Rio varies
from 0.2 to 4.3, the probabilities vary from 1.2 to 95.8 percent. Thus,
Table A-I can be used to estimate the size of the error circle, for a given
cut angle, when the two measurement standard deviations are equal. For
example, when 01 and 02 = 20 m (66 ft) and the cut angle is 500, the radius
at which 50 percent of all measurements will be contained is determined
by interpolating in Table A-I between Rio = 1.4 at a P = 0.491 and Rio = 1.5
at P = 0.535 resulting in an Rio = 1.42 at P = 0.50. Thus, the desired
radius R = 1.420 or 28.4 m (93.2 ft).
Error circles about a measured position can be constructed for different
LOP standard deviations and cut angles. This can be done by first calculating
Ox and Oy, the standard deviations along the major and minor axes of the
error ellipse (discussed later in this section), knowing the crossing angle
and the standard deviations in each measurement (01 and 02). Table A-2
is then used to provide probabilities as a function of c, the ratio of
the smaller to larger of Ox and Oy, and K as the ratio of the error circle
radius to the larger of Ox or Oy. For example, for a 500 angle of cut,
a 01 = 15 m (49 ft) and 02 = 20 m (66 ft), the probability of location
within a circle of 30 m (98 ft) is:
(01)2 = 225 m2
(02)2 = 400 m2
sin2, = 0.5868
Substituting into equations A-4 and A-5 (given in the following discussion
on error ellipses), Ox is 29.9 m (98.1 ft) and Oy is 13.1 m (43.0 ft).
A-8
-------
TABLE A-2. CIRCULAR ERROR PROBABILITIES AS A FUNCTION OF MEASUREMENT
STANDARD DEVIATION RATIOS AND ERROR CIRCLE RADIUS
~ 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.11 1.0
0. 1 . 07116557 . 0443987 .02421111 .0164176 .0123875 . 0099377 . 0082940 .0071157 . 00622911 . oo5S4oo . 0049875
0.2 . 1585194 . 13311783 . 0884533 . 06283116 .0482U3 .0390193 .0327123 .0281U5 . 0246824 .0219757 .0198013
0.3 . 2358228 .2213804 . 17311300 .1318281 . 10311193 .085153!o .0719102 .01121386 . 0546598 . 04876311 . ~00'l5
0.4 .3108435 .3010228 .2635181 .2139084 . 1742045 . 1451808 . 1237982 . 1076237 . 0950495 . 0850326 . 076&837
0.5 . 382112411 . 3755684 .34SI7go . 3003001 . 25321153 .2152880 . 1857448 . 102G8211 . 1443941 . 1296286 . 1175031
0.6 . 4514938 . 4457708 .' 255605 . 3646374 . 3357364 .2914682 .2548177 .2251114 . 2009797 .1811783 . 1647298
0.7 .5160127 .5115048 . 4960683 . 4633258 .4170862 . 3699305 . 3280302 . 29256r.4 . 2629373 .2381583 .2172955
0.8 . 5762892 . 5725957 . 5604457 . 5349387 . 4941882 .4474207 . 4025628 .3627122 . 3283453 . 2989700 . 2738510
0.11 .6318797 . 6288721 .6191354 .5993140 .5651564 .5213998 .4759375 . 4333628 . 31153279 . 362013~ . 3330232
1.0 . 68268115 . 6802325 : 6723586 . 6568242 . 621112411 . 5gQOg53 . 54613111 . 50257go . 4621421 . U57553 . 39346113
1.1 . 72866711 . 7266597 . 7202682 .70711681 . 6859307 . 65244811 .6116316 . 5687467 . 5272462 . 4887873 . 4539256
1.2 . 7698607 . 7682215 . 7630305 .7532175 . 7359558 . 70791173 .67142611 .6306168 . 58113494 . 5498736 .5132477
1.3 . 8063990 . 8050648 . 8oo85~ . 711211968 . 77113550 .7567265 . 7249673 .6873122 . 6474394 . 6079822 . 5704428
1.4 . 8364867 . 1\374049 . 8340018 . 8277048 . 8169851 . 7989288 . 77208811 . 73830811 . 7007900 . 6623035 . 82168811
\.5 . 8663858 . 8655127 . 8627728 . 8577362 . 6493071 .8350816 .81211287 . 78331162 . 74811500 .7122S48 .8753475
\.6 . 8g()40\4 . 68117008 .8875060 . 883411\4 . 8768644 . 88575511 .6478393 . 8226246 .7111711M . 7574708 .7219877
1.7 . 111086111 .11103102 .90856111 . go53766 .IlOO1740 . 8915536 .8773116 . 8562471 .8291137 . 7977882 . 7162539
1.8 .1I28131M . 92761164 . 11263125 . 92379811 .11197275 .11130680 .11019110 . 8846624 .8613233 .8332175 .8021013
I. II . 9425869 .11422182 . 1M 112911 .9391586 . 11359855 .11308815 . 9222277 . 9083609 . 8866731 .86391411 . 8355255
2.0 . 1I~4997 . 1I~2272 . 9533775 .11516415 .9493815 . 94~~8 . 11388418 . 9278799 . 11115762 . 8go\495 . 8646647
11 .11642712 . 11640598 . 1MI34011 . 11622127 .9803170 . 9573205 . 115229S1S1 . 11437668 . 11305013 . 11122714 . 88117111S
2.2 .97211131 . 9720304 . 9715237 .9706109 .11691597 . 9668845 .9631017 . 9565522 . 9459386 . 11306821 .9110784
2.3 .11785518 . 9784275 . 9780408 . 11773450 .117624111 . 97452311 . 117161134 . 11667308 . 115837311 . 11458085 . 9289944
2.4 . 98380411 .9835108 .9832180 . 9826918 .9818594 .9805703 . 9784681 .9747495 . Sl682698 . 11580804 . 11438652
2.5 . 11875807 .9875100 . 9872900 . 9868953 . 9862720 .9853112 . 9837569 .9810035 . 11760522 .9679136 .95.'0031
2.6 . 9908776 . 9908249 . 9g()4612 .9901674 . 9897045 . 9889934 . 9878527 . 9858331 .9821023 .9756969 .96.1952.1
2.7 . 9930681 . 9930271 . 9929062 . 9926894 . 9923483 .9910260 . 9909944 . 9895268 .9867530 .9817837 .9738786
2.8 . 9948897 .9948612 . 9947727 .9946\41 .9943649 . 9939842 .9933821 . 9923219 . 9902886 . 9864876 . 9801'~9
2.11 . 9962684 . 9962477 . 9961834 . 9960884 . 99.,8878 .9958126 . 99.~1798 . 9944216 . 9929482 . 9900803 . 98.10792
3.0 . 9973002 . 9972853 . 99723111 .9971564 . 9970266 . 996821M . 9965205 . 9959854 . 9949274 . 9927925 .9888910
3. 1 . 9980848 . 9980542 .9980212 . 91179622 . 9978699 . 9977296 .9975109 .9971348 . 99638.~ I .9948168 .991&113
3.2 . 11986257 .9986182 . 9985949 . 9985533 . 9984880 . 9983892 . 9982356 . 9979733 . 9974478 . 9963105 .9910210
3.3 . 9990332 . 99gQ2711 . 99go116 . 11989824 . 9989368 . 99881177 . 9IIR7607 . 9985792 .9982147 . 9971004 . 995682'1
3.4 . 9993261 . 9993225 .9993112 . 9992909 . m25~3 .99112115 .9991376 .99goI29 . 9987626 .9981868 . Sl989113
3.5 . l19li5347 . 9995323 . 9995245 . 9995105 . 9994888 . 9994559 . 9994053 . 9993204 .9991502 . 11987480 .997812.\
3.6 .11996818 . 999680 I . 11996748 . 9996653 . 99116505 . 11996281 . 911115938 . 9995364 .9994218 . 9991442 . 9984662
3.7 . 9997844 . 9997832 . 11997797 . 9997733 . 9997633 . 9997482 .l19li7251 . 9996867 .9998102 . 9994208 . 9989352
9.8 . 99985.U . 9998545 . 9998522 .9!!98478 . 9998412 . 9998311 . 1191181.17 . 9997go2 . 99117396 .991181111 . 9992882
3.11 . 1199go38 . 99ggQ33 .9999018 .911989811 . 9998945 . 9998878 -. 9998778 . 9998608 . 99118276 . 99974 28 . 999.1020
'-0 . 9999367 . 9999363 . 91199353 . 99911334 . gggg305 . gggg261 .99991115 . 91199085 . Sl998870 . 11998309 . 9998845
'-I . gggg587 . 9999585 . 9999578 . 9991156& . 9999.\47 .9999519 . 99119475 . 9999404 . 991111266 . 9998900 . 9997783
'-2 . 11999733 . 9999732 . 11999727 . 9999720 . 9999707 . 9999689 . 99911681 .9999616 . 9999.,27 . 9999292 . 999~.\23
'-3 . 99119829 . 9999828 . 9999826 . 9999821 .99119813 . 999980 I . 9999783 .9999754 . 999118118 . 91199548 . 9999034
'-4 . 9999892 . 9999891 . 119998811 . 9999886 . 9999881 . 9999874 . 9999883 . 9999845 . 9999809 .9999715 .9999J1.1
'-.I . 9999932 . 11999932 . 9999931 . 9999929 . 9999925 . 9999921 .9999911 . 9999902 . 999988\ . 9999822 . 9999599
'-6 . 9111111958 . 9999957 . 9999957 . 99991155 . \l99?1154 . 9999951 . 9999917 . 119999311 . 9999926 . 9999889 . 9999146
'-7 . 99119974 .9l19li974 . 9999973 . 99119973 . 9999971 . 9999970 . 9999967 . 9999963 . 99999M . 9999932 . 9999840
'-8 . 9999984 . 911911984 . 99911984 . 91199983 . 9999'383 . 9909982 . 9999960 .9999977 . 9999972 . 999119~9 .9999901
'-II . 11999990 . 99999go . 11999990 . GGGGSl90 . 9999990 . ggggg81 . 9991191\8 . 9111191188 .99119983 . 991111975 . 99999311
$.0 . 9999994 . 9119S1994 .9999994 . 91199111M . 9999994 . 9999993 . 9999993 . Sl9ll9992 . IImSl90 .99msa . 9999963
$. I . GGGG9117 . mSl997 . 9999997 . 9999996 . 9999996 . 9999996 . 11999996 . 9999995 . 911119994 . 9~9991 . 9999978
$.2 . Sl9ll9998 . 911110118 . 9999998 . 99119998 . 9999S198 . 99999118 . 911991198 . 11999997 . 91199997 . 11999995 . 9999~7
5.3 . 9999919 . 99119\1911 . Sl9911m ."911999 . 9999V99 . 9999SI99 .9119991111 . 9999998 . 999S19911 . Sl999997 . 9999992
5.4 . 8199909 . 8199909 . SI99SI999 . SI999SI99 . SI99SI999 . 9999999 . 9II9S1S199 . 9999999 . 99999SII . 11999998 . gggggg~
5.5 1. 00000oo 1. 00000oo 1. 00000oo I. 00000oo 1. 00000oo I. 00000oo I. 00000oo . 9999999 . 99999911 . 9IISI999II . gggggg7
5.6 I. 00000oo I. 00000oo . 0999999 . gggggg8
$.7 I. 00000oo . 9999999
5.8 \. 00000oo
$.9
6.0
Reference:
Bawd itch 1984.
A-9
-------
Therefore:
c = 13.1/29.9 = 0.44
and
K = 30/29.9 = 1.003
Entering Table A-2 for values K = 1.0 and c = 0.4 gives. a probability of
62 percent.
The error circle approach has the advantage of being able to designate
positional accuracy with a single number (namely radius). As with the
error parallelogram, however, the error circle is not an equal probability
contour. Also noteworthy is that the area of the CEP circle becomes greater
than the corresponding basic ellipse as the ratio of ellipse minor to major
axes decreases from 1.0 (Bowditch 1984).
Positional accuracy is often quoted using radial error, root mean
square (rms) error, or drms (also written DRMS), all of which have the
same meaning. As with CEP, the error figure is a circle having a radius
equal to:
drms = (ox2 + oy2)1/2
(A-2)
where Ox and 0y are the 10 error components along the major and minor axes
of the probability ellipse (Figure A-4). This is often referred to as
the 1drms or 10 fix accuracy. Similarly, a 20 fix is given by:
2drms = [(20x)2 + (2ay)2]1/2
(A-3)
Given the LOP standard deviations 01 and 02, the standard deviations along
the basic ellipse axes can be determined by:
ox2 = [1/(2 sin2;)] { 012 + 022 + [(012 + 022)2 - 4(sin2;) 012Q22]1/2} (A-4)
and
A-IO
-------
"
"
"
"
"
"
"
"
~I
-", _l
"
/"
~2
" CT2
'-vi
,
SOURCE: BOWDITCH 1984
Figure A-4.
Illustration of radial error.
A-ll
-------
oy2 = [1/(2 sin2~)] { 012 + 022 - [(012 + 022)2 - 4(sin2~) 012022]1/2} (A-5)
The probability of being within a 1drms circle varies from 63
depending on the ellipticity of the error ellipse. For a
the probability varies from 95 to 98 percent.
to 68 percent
2drms circle,
Equipment manufacturers quote accuracy probabilities using various
terminology. Most commonly, CEP and d values are Provided. The relationship
rms
between these two particular accuracy measures is shown in Figure A-5.
For equal standard deviations (ox/Oy = 1), the drms is 1.2 times the CEP.
Thus, quoting the CEP would be more advantageous to a manufacturer than
quoting a drms value.
It is common to describe two-dimensional error distributions by two
separate one-dimensional standard deviations along each error axis (01
and 02). However, when system accuracy is given as 100 m (328 ft) at the
20 level, two clarifications are needed: 1) whether absolute or repeatable
accuracy is being quoted; and 2) whether 0 measures linear or circul~r
error. If the error is circular, the number describes an 86 percent probability
level, whereas if a 20 drms is referred to, then the probability of the
positioning being within the circle is between 95 and 98 percent, depending
on error ellipse axis lengths. Therefore, unless the basis of an accuracy
figure is very explicitly stated, it may be necessary to clarify its meaning.
Only then can the accuracy performance of competitive systems be compared.
A-12
-------
drm,
CiP .. 30
..~
-
- '-
~ 1 d rml
" ~
" "
'" "-
'- ..............
",, " ",, "
I.4~
1.40
.."
1.25
1.20
I. I~
o
o
0.2
0.3
0.4 0.' 0.8
c . ra/r,
0.7
0.'
0.'
1.0
0.1
SOURCE: BOWDITCH 1984
Figure A-5.
Ellipticity versus drms/CEP.
A-13
-------
APPENDIX B
SYSTEM CHARACTERISTICS
-------
APPENDIX B
SYSTEM CHARACTERISTICS
Characteristics of the kinds of systems described in the report are
provided in this appendix in the following order:
.
.
Theodo 1 ites
Sextants
Electronic Distance
Total Stations
Microwave Systems
Trisponder
Falcon 484 Mini-Ranger
Page
B-2
B-2
B-6
B-6
B-6
,
.
Measuring Instruments
.
.
.
Micro-Fix
Hyd rofl ex
Autotape DM-40A/DM-43
Variable Range Radar
Medium-Range Systems
Syledis
Raydist Trak IV
Hyper-Fix
Argo OM-54
Hydrotrac
Long-Range Systems
Loran-C
B-22
B-14
B-16
.
.
Viewnav
Lambd a
Om eg a
Satellite Systems
Transit (Navsat)
B-28
Geostar
8-1
-------
Navstar GPS
Series
Aero Service GPS
.
Hybrid Positioning
Aztrac
Polarfix
Artemis
Techniques
8-38
b
This more detailed information is provided to enable quick-reference
review by document users interested in the performance characteristics
and costs of systems representative of each class.
THEODOLITES
A theodolite is an instrument designed to measure horizontal and vertical
angles. Two rather distinct theodolite classes exist. Those having verniers
or scale divisions are less accurate than those having micrometer or digitized
readouts. As indicated in Table 8-1, the former can typically be read
to the 10- to 30-sec angular increment, whereas the more sophisticated
units can be read between 0.1 and 1.0 sec (Table B-2). Corresponding price
ranges are $650 to $4,000 for vernier models; $1,900 to $16,000 or more
for digital models. As discussed in the text, reading accuracies in the
15 sec range should be adequate for most purposes.
SEXTANTS
Sextants are classified as vernier or micrometer drum models, the
latter being preferred by most users. A well-constructed metal marine
sextant is capable of measuring angles with an instrument error not exceeding
10 sec or 0.1 min of arc (Bowditch 1984). However, as indicated earlier,
positional error will be highly dependent on operator ability. For accurate
work, a sextant having an arc radius of 162 mm (6.4 in) or more should
be selected. Characteristics of representative sextants are presented
in Table B-3. As indicated instruments with sufficient accuracies are
available in the $600 to $1,600 price range.
B-2
-------
TABLE B-1. SUMMARY OF VERNIER TRANSIT AND SCALE-READING THEODOLITE CHARACTERISTICS
COMPANY MOOEL VERNIER MICROMETER READABLE U.S. SUGGESTED
OR SCALE DIVISION TO: LIST PRICE"
Benchmark JENA 020 20" 10" $2495
Berger Bronze 65/4~ 20" /1' 20"/30" $212513850
Aston 67 20" 20" $1835
Project 100 I' 30" $1100
ST-l/6 1'/20" 30"/20" $699/1499
ST-8/9 20" 10" $1699/1899
Kern K1-S/ST 30" 6" $3895/3995
Leitz BT20/10C 20" 10" $169511995
115 I' 30" $795
TS20AlS6 I' 20"/6" $249513695
Hikon NT-2S MK III l' 0.2' $3195
Schneider 700/400 20"/1' 6"/20" $1235/650
Pentax GT-4B/6B 20" $169511895
TH-60S/60E I' 6" $2500/1895
Te1edyne OP 107/100-A-20 20" 20" $1295/1185
G-15 20" 20" $1350
400W I' $650
Topcon AG-30B 30" 15" $1595
TL-60SE I' 20" $2100
Warren-Knight 10-2220/3200 20"/1' 10"/30" $269511295
White TR-300 I' $749.
TR-303/303PH 20" $1879/1995
T-307AT/309T 1'/20" 6"/10" $2695/2350
Wi1d T-16 12" $1895
T-05 20" 10" $3950
Zeiss Th-42/43 20" 10" $3950
*January 1985
B-3
-------
TABLE B-2. SUMMARY OF MICROMETER AND DIGITIZED THEODOLITE CHARACTERISTICS
COMPANY MOOEL VERNIER MICROMETER READABLE U.S. SUGGESTED
OR SCALE DIVISION TO: LIST PRiCe
Benchmark JENA 010A 1" 0.1" $4295
JENA 015B 6" 1" $3495
Kern DKM2-A/T 1" 0.1" $5890
lJKM3/DKM3-A 0.5" 0.1" $12295
Lietz DT-20E 20" Accuracy 10" Display $2995
TM6/10E/20H 6"/10"/20" 2"/5"/10" $3995/3695/3295
TM-1A 1" 0.1" $5995
Nikon NT-1/5A 20"11" 6"10.25" $1895/Cj950
NT-4DI3D/2D 6"/10"120" 1"/2"14" $3695/3595/3095
Pentax TH-20D/10D/06D 20"/10"/6" 5"12" /1" $2850/3500/3690
TH-01W 1" 0.5" $4900
White TH 10-20110 20" 110" 2"/1" $285013500
T-308AT/208AT 20"110" 10"/5" $2995/3495
TH 10-1 I" 0.1" $5500
Topcon DT-20 LCD Readout 20" $2695
TL-20DE/10DE/6DE 20"/10"/6" 10" /5" / 3" $2859/3500/3690
Wild T-0/T-1 20"/6" 20"13" $3995
T -2/T -3 1"/0.2" 0.1"/0.1" $5995/11995
T-2000 0.5" 0.1" $13995
Zei 55 Th-2 1" $5950
ITh-2 0.6" $16500
.
January 1985
8-4
-------
TABLE B-3.. MARINE SEXTANT CHARACTERISTICS
SPECIFICATIONS C. PLATH NAVSTAR TAMAYA TAHAYA TAHAYA WEEHS&
NAVSTAR PROFESSIONAL SPICA JUPITER VENUS PLATH
CLASSIC UNIVISION
Arc Range -5 to +1250 -5 to +1250 -5 to +125° -5 to +125° -5 to +125° -5 to +125°
Instrument < :tl0" < tl0" < tl0" < t12" < 18" < tl0"
Accuracy
Vernier 0.2' 0.2' 0.2' 0.2' 0.2' 0.2'
Sca 1 e
Arc Radius (mm) 162 162 162 162 138 162
Telescope 4X40 4X40 4X40(70) 4X40(7°) 3X26 4X40
OJ 6X30 7X35(6.50) 7X35(6.50) 6X30
I
U1
Bri ghtness 100/25 100/25 75
Index Mirror (mm) 57X41 57X30 57X42 57X42 45X30 57X41
Horizon Mirror Dia. (mm) 57 57X30 57 57 45 57
III umination Arc & Drum Yes Arc & Drum Arc & Drum No Arc & Drum
Fnme Material Brass/Alloy Makrolon Aluminum Aluminum Aluminum Brass
Arc Material Brass/Alloy AI. Alloy Bronze Bronze Bronze Brass
Case Mahogany Plastic Plastic Plastic Wood Mahogany
U.S. Suggested $1270 $965 $919 $715 $585 $1674
List Price (1/85)
-------
ELECTRONIC DISTANCE-MEASURING INSTRUMENTS (EDMIs)
Elettro-optical and microwave EDMIs are commonly used in land surveying
where very small cllTlulative errors are acceptable. As indicated in Table B-4,
the range of an instrument is highly dependent on .its signal character-
istics and number of reflectors on the target. Optical range is on the
order of 1.6 to 6 km (1 to 3.7 mi), whereas microwave units can reach 14 km
(8.7 mi). Accuracies at 5 km (3.1 mi) are in the low centimeter range.
Costs are from $3,500 to as much as $15,000, and a laser unit, which can
operate to 25 km (15.5 mi), is available for $20,500. For most coastal
outfall survey work, two units would be required at approximately $5,000
to $6,000 each.
TOTAL STA nONS
Total stations are instruments capable of measuring not only horizontal
and vertical angles, but also distance to the target object. Some units
operate manually, requiring slope reduction of distances to be done by
optically reading a vertical angle and keying it into a calculator. Semi-
automatic units contain a vertical angle sensor for automatic slope reduction,
but horizontal angles are read optically. Automatic stations read horizontal
and vertical angles, and distance, electronically. At a 5 km (3.1 mi)
distance, single measurement range error is typically 3 cm (1.2 in); angular
error is typically +2-3 sec or 5-7 em (2-2.8 in). As shown in Table B-5,
costs range from $8,000 for a manual unit to as high as $30,000 for a fully
automatic station.
MICROWAVE SYSTEMS
Due to relatively high demand and fairly common usage, there are many
commercially available microwave systems. Representative systems described
in Table B-6 have nominal absolute accuracies of !. 1 m (3.3 ft), and cost
from $39,000 to $90,000. Ranges extend between 40 and 150 km (25-93 mi),
well beyond required accuracy for coastal monitoring programs. Some systems
. offer a multiple user option. Because of their common usage, the character-
istics of representative systems are described in the following paragraphs.
8-6
-------
TABLE B-4. ELECTRONIC DISTA~CE MEASURING INSTRUMENTS
COMPANY MODEL RANGE(m) ACCURACY U.S. SUCliESTEO
SINGLE PRISM MEAN LIST PRICE
TRIPLE PRISM SQUARE
MAXIMUM(pd sms) ERROR
BenChmark Surveyor III-X 1600 ~(5nvn" 5 ppm) 53.495
Orl ando. FL 3000
(305)281-5000 3500(6)
Geodimeter. Inc. Geodirneter 14-A 6000 t(5 m/ft .. 3 ppm) 511, 300
Novato. CA 8000
(415)677-1256 15000
Geodimeter 112/122 2500 t(5 m/ft .. 3 ppm) 56.250(112)
3600 510.950(122)
6000(8/16)
Geodimeter 220 1600 t(5 nvn .. 3 ppm) 58.850
2400
3200(8)
Kern Instruments OM 503 2000 t(J nm .. 2 ppm) 58.995
Brewsur. NY 3500
(914)279-5095 4500(7)
Keuffel & Esser. Co. Ranger V-A 8000 t(5 nm .. 2 ppm) 520.561
Marri s town. NJ (HeNe Laser) 16000
(201)285-5000 25000
PulseRanger 1000 t(30 cm .. 30Ppm) $7.500
3000
The Lietz Company RED 2A/2L 2000/3800 ~ (5 nm'.. 5 ppm) $4.695(2A)
Overland Park. KS 2800/5000 $5.795(2L)
(913)492-4900 ----17000(9)
MK Electronics. Inc. MK-III. MK-III VS 1600 ~(5 11111 .. 2 ppm) $5.950( 111)
Littleton. CO 3000 $7 .950( I I I-VS)
(303)795-2060 .4000
Nikon. Inc, NO 31 1900 t(5 nvn .. 5 ppm) $5.885
Ga rden City. NY 3200
(516)222-0200
Pentax. Corp. PM-81 1400 t(511111 .. 5 ppm) $4.790
Englewood. Co 2000
(303)733-1101
Teludist. Inc, Tellemat CMW20 25000 t(5 rnn + 3 ppm) S16. 500
Mastic Beach. NY (mi crowaye)
(516)399-5843
Topcon Instrument Corp, OM-S3 2000 t(5 nun .. 5 ppm) 55.390
Paramus. NJ 2500
(201)261-9450 2900(9)
Wi 1 d Heerbrugg CHat i on-450 1600 t(5 mm + 5 ppm) $3.995
Farmingdale. NY 2300
(516)293-7400 4000( 11)
01-4L Oistomat 2500 t(5 nm.. 5 ppm) $8.995
3500
7000( 11)
01-20 Distomat 6000 !,3 rnn .. 1 ppm) $14.995
7000
14000( 11)
B-7
-------
TABLE B-5. TOTAL STA nON CHARACTER ISTICS
ACCURACY
COMPANY MODEL TYPE RANGE" PRISMS RANGE ANGLE U.S. SUG:::S-:-:
(km) LIST PRICE
L M H
Carl Zeiss, Inc. RMS3 Semi -Auto a . 1 !,5-10 mm + 2ppm +2" 58,260
1. 5 b
Thornwood, NY 2.0 3
(914)848-1800 3.0c 9
El ta 3 Automatic 1.6 3 !,10 mm + 2ppm +2" 518,725
2.5 6
3.0 18
Elta 46R Automatic 2.0 3 !,10 mm '+ 2ppm +3" 512,820
Geodimeter, Inc. 140 Automatic 1.2 2.2 3.0 1 :(Smm + 5 ppm) :2" S19,950
Novato, CA 1.8 3.0 4.0 3
(415)883-2367 2.4 3.8 5.5 6
3.6 4.8 6.0 8
Kern Instruments El/DM503 Automatic 1.5 2.5 3.0 1 :(3 mm + 2 ppm) :2" S19,175
Brewster, NY E2/DM503 2.0 3.5 4.5 3 :(3 mm + 2 ppm) :0.5" S22,375
(914)279-5095 2.4 4.5 5.5 7
Lietz SMD- 3 Manual 0.8 1.2 1 : (5 mm + 5 ppm) 10" di9ital S8,800
Overland Park, KS SDM-3ER Semi -Auto 1.4 1.8 3 5" direct S10,800
(913)492-4900 SET-I0 Automatic 2.5+ 9 S14,000
MK electronics MK- I II Automatic 3.0 3 :(5mm + 2 ppm) 6" 512,950
Littleton, CO MK-IV 4.0 3 Stationary S14,950
(303)795-2060 MK-HYDRO 3 or 4 3 :(20 mm + 5 ppm) S16,950
Moving
Nikon Instruments NTD-4 Manua 1 1.2 1.6 1 : (5 mm + 5 ppm) 6" digital $8,595
Garden City, NJ 1.8 2.3 3 3" subdivision
(516)222-0200 NTD-l Automatic 1.2 1.6 1 : (5 mm + 5 ppm) 1" 515,985
1.8 2.3 3
Pentax PX-I0D Manual 1.4 1 :(5mm + 5 ppm) 10" digital 58,450
En91 wood, CO 1.7 3 5" direct
(303)773-1101 2" estimation
PX-06D Manual 1.4 1 !,(5 mm+ 5 ppm) 6" digital SI:I,765
1.7 3 I" estimation
Topcon GTS-2B Manual 1.4 1.7 1 :(5 mm + 5 ppm) 6" $7,990
Paramus, NJ 2.0 2.4 3
(201)261-9450 2.6 3.0 9
ET-l Automatic 1.4 1.7 1 :(5 mm + 5 ppm) 2" $14,250
2.0 2.4 3
2.6 3.0 9
Wil d Hee rb rug T2000+ Automatic 1.2 2.5 3.5 1 : (5 mm + 5 ppm) 0.5" S24,OOO
Farmingdale, NY DI4L or 1.5 3.5 5.0 3 :!:(3 mm + 2 ppm) 525,000
(516(293-7400 DI5 1.7 4.5 6.0 7
1.8 5.5 7.0 11
T2000+ Automatic 2.0 6.0 9.0 1 :!:(3mm + 1 ppm) 0.5" $30,000
DI20 2.3 7.0 11.0 3
2.6 8.0 13.0 7
2.7 9.0 14.0 11
* Atmospheric visibility: a. low. hazy, 5km b. medium. clear, 15km c. high. very clear, 30km
8-8
-------
TABLE B-6.
MICROWAVE POSITIONING SYSTEM CHARACTERISTICS
SYSTEH RANGE fREQUENCY NOMINAL USER ANTENNA COST
(km) (HHz) ABSOLUTE CAPABILITY Vessel Shore
ACCURACY Stat ion
(m)
TRISPONOER (520/540 OOHU) 5 9320-9500(261) +1 up to 8 3600H B7061800H $40.00C
DEL NORTE Technology, Inc 80 9329-9500(217E) (more 200V 6 /500V
EUle~s, IX optional)
(811)261-3541
80 8800-9000(218E)
aJ 36g0H
I fALCON 484 Hinl-Ranger 37 5410-5600 +2 up to 20 700H $39,300
~ Hotoro1a, Inc. 25 V 150V
Tempe, AZ
(602)897-4376
MICRO-FIX 80 5480 ~t up to 16 36g0H 900H
Raca 1 DECCA Survey, I nc. 5520 20 ,300V 60/300V $43,000
Houston, TX 5560
(713) 783-8220
HYDROFLEX 100 3000 +1 single 36g0H L~q-ra~ $63,500
Tel ud i s t, I nc. pTus 3xl0-6 multiuser 18 V 24 H, 22
Mastic 8each, NY distance(m) option
(516)399-5843 fan Beaw
180V.60 H
AUTOTAPE OH-40A/DH-43 150 2900-3100 -0.5 single 36g0H 30°. 1200H $90,000
Cubic Western Data +1:100,000 range 10 V 100V
San Diego, CA
(619)268-3100
*January 1985
-------
Trisponder
The Del Norte Trisponder is an X-band (8,800-9,500 MHz) positioning
systen canposed of a digital di,stance-measuring unit (DDMU), a master station
(usually on the vessel), and two remote stations located at known geographic
positions. Each station is a combined transmitter and receiver. The master
station antenna is omnidirectional and each remote station has a directional
antenna. Distances to remote stations are observed on the DDMU using the
range-range mode. A time-sharing feature allows up to eight users.
The manufacturer quotes a typical range accuracy of +1 m (+3.3 ft),
with an instrument resolution of 0.1 m (0.3 ft). In The Hydrographic Manual,
Umbach (1976) cites a range error for this system of +3 m (+10 ft) under
gooo field coooitions based on the Trisponder Basic Operation Manual published
in 1974. Also cited are tests conducted by the National Ocean Survey (dat~
unknown) which indicated that temporal electronic drift may cause variations
in measurements. Recalibration is suggested over a measured base line
after every 200 h of operation. However, the tests were conducted on a
Model 202 Trisponder Surveyor System for which the manufacturer claimed
a resolution of 3 m (10 ft) and a positional accuracy of +3 m (.:,10 ft),
after field calibration.
The currently available microwave system operates with Model 217/218E
transponders and Model 520 or 542 DDMUs. The Model 261 transponder, usable
as a master or remote, has a 5-km (3.1-mi) line-of-sight range. The Model
217/218E transponders operate up to 80 km (50 mi) from shore due to higher
output power. The transponders are designed for use with either model
DDMU. The Model 520 will collect four ranges and display two. The Model
542 interrogates four remotes, outputs four sets of data, and also provides
a positioning guidance capability. The cost of a compete system including
a Model 520 DDMU, a master and two remote 217/218E transponders, and antenna
is $40,000 (Buchanan, C., 31 December 1984, personal communication). For
the additional guidance capability of the Model 542 DDMU, the system cost
is $44,500. A less expensive "black box" version of the DDMU (Model 562)
is available for use with an existing shipboard computer system, allowing
'8-10
-------
more than eight users.
is $39,500.
Cost of the Model 562 and other required components
Falcon 484 Mini-Ranger
The Motorola Falcon 484 is a C-band (5,410-5,600 MHz) microwave ranging
and positioning system that operates in the two-range mode from 100 m (328 ft)
to 40 km (25 mi). The system consists of a vessel receiver-transmitter
assembly with an ommidirectional antenna, a range console, and shore-based
radar transponders with directional antennae. Pulsed radar from the survey
vessel transmitter interrogates radar transponder reference stations located
at geographically known points. Elapsed time between transmitted interrogations
and the reply from each transponder is used as the basis for range determi-
nations. Two ranges are used for trilateral positioning. If three or four
ranges are available, range residuals, sum of squared residuals, and error
circle radius data are output for the least squares position solution.
The manufacturer claims a range accuracy of +2 m (+6.6 ft). Up to 20 users
can operate in the same area.
A basic Falcon 484 system, including a range processor control display,
receiver/transmitter with ommidirectional antenna, two reference stations
with directional antenna, miscellaneous cables, and manuals currently costs
$39,300 (Jolly, J., 31 December 1984, personal communication). In an effort
to eliminate one of two major problems with most microwave positioning
systems (i.e., magnatron failures and the need to service beacons), Motorola
is currently developing a solid-state beacon using a gun diode. The associated
increase in long-term reliability may also result in decreased range.
However, range capability should remain adequate for positioning needs
of coastal monitoring programs. Costs of the modified transmitters were
not available at time of publication.
Micro-Fix
The Racal Survey Micro-Fix is a range-range microwave positioning
system. With line-of-sight to shore-based transmitters, it is capable
of operating up to 80 km (50 mi) ~ffshore. The system normally operates
8-11
-------
at 5,480 MHz, with options at 5,520 and 5,560 MHz. The master station
can interrogate up to eight remote stations (from a possible 32), with
each remote transmitter/receiver unit (T/R) preset to recognize its own
unique station code. Up to four separate station groups can be deployed
in the same area without interstation interference. Multi-user capability
allows a maximum of 16 users for each deployed chain. Master and remote
units are interchangeable.
The basic system consists of a master station with a Control Measurement
Unit (CMU), a T/R unit, and two tripod-mounted remote T/R stations. The
vessel's master station interrogates the remote stations sequentially,
triggering reply pulses received by the master T/R and processed by the
CMU to display the corrected ranges. The CMU capabilities include automatic
and continuous self-calibration, track guidance, plotter drive, x-y conversion
(full spheroid and multi-range solution), and slant range correction.
Nominal accuracy is stated by the manufacturer to be ~1 m (~3.3 ft). The
cost of a basic Micro-Fix system, including training, is $43,000 (Harris, E.,
3 January 1985, personal communication).
The manufacturer is developing the capability to use the system in
a hyperbolic mode, a combination range-range/hyperbolic mode, or a range-
azimuth mode, but this capability is not yet available. The system also
uses circular polarization techniques to avoid reflective (water surface)
signal cancellation nulls, thereby eliminating the need for a second antenna
on the vessel. This has been accomplished by an antenna design that prevents
signal entrance from reflective angles.
Hydroflex
Hydroflex is a short- to medium-range microwave navigation system
designed for survey applications where a high degree of accuracy is required
in fixing or tracking the position of a moving vessel. The system operates
at frequencies of 2,920-3,300 MHz and has a range of 100 m to 100 km (328 ft
to 62 mi). It consists of a master unit controlled by an HP85 computer
with a customized software package, an ammidirectional antenna, and connecting
cables. Each remote unit consists of a transceiver and either a large
8-12
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range or fan-beam antenna for mounting on a customer-provided tripod.
Accuracy is claimed to be 1 m (3.3 ft) +3x10-60, where 0 is the distance
measured in meters. The system can be operated in either a two- or three-
range mode. Although single-user is the normal operating mode, a multi-user
option is available. The cost of a master and two remote stations is $63,500
(Baker, W., 11 January 1985, personal communication).
Autotape OM-40A/DM-43 0
The Cubic Western Autotape is an S-band range-range microwave positioning
system that operates at ranges of up to 150 km (93 mi). System components
include a shipboard interrogator and range responders at each fixed onshore
station. The OM-43 is capable of working with three geographic sites.
Range information is computed by comparing the phase shift of the modulated
signal transmitted between the interrogator and responder phase unit to
an interrogator reference signal. An Automatic Position Computing System
(APCS) is available for steering information, real time analog plot of
the vessel's track, and magnetic data recording. The system does not accom-
modate multiple users. The manufacturer claims a range accuracy of 0.5 m
plus 1:100,000 times the range distance. This error is due to internal
random errors, systematic errors, temperature variation bias errors, signal
strength, component aging, and initial calibration errors. External errors
are said to far exceed internal noise, systematic, and bias errors. Index
of refraction error, which can approach 5 m (16.~ ft) at 100 km (62 mi),
is usually small enough at short ranges to be ignored. Multipath errors
(dependent on orientation of reflective objects near and behind the interrogator
omnidirectional antenna) have been observed from 0 to 3 m (0 to 9.8 ft).
The manufacturer states that internal averaging plus external data smoothing
will reduce the effect on Autotape to a small fraction of a meter, provided
the antenna is moving. For this reason, the sys~em is best suited to appli-
cations where the interrogator is on a moving vehicle.
The cost of a basic Autotape system including an interrogator, two
range responders, and associated antennae is $90,000. The same system
with the OM-43 and three shore stations is $124,000 (Hempel, C., 10 January
1985, personal communication).
8-13
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VARIABLE RANGE RADAR
Using variable range radar (VRR) a position can be fixed by measuring
the distances to three identifiable targets on the radar screen. A variable
range marker is used to measure the distance to the object (as identified
by its radar reflection). This distance is then drawn with a compass as
a 1 ine of position on the nautical chart. The intersection of the multiple
LOPs establishes the vessel's position.
Most commerc ia 1 vessels are equ i pped with radar for safety and navigation
purposes. . Coastal vessel radar usually have ranges from 26 to 116 km (16 to
72 mi). A variable range marker (VRM), whether built-in or an add-on,
removes a large portion of operator error in estimating distances. For
positioning ataccuracie~ less than 300 m (984 ft), a VRM is almost always
needed. Range accuracies with the VRM are usually 1 to 2 percent of the
range scale, or +25 m (82 ft) at 0.5 km (0.3 mi), the smallest scale.
Accuracy decreases as the range scale is increased. Bearing accuracy is
usually less than 10. Characteristics of representative VRRs are presented
in Table B-7. Generally, analog systems have better resolution than digital
systems, but are not as versatile.
. Each position fix relies on the resolution and identification of the
radar reflection. Resolution will depend on target position and al ignment.
The location of the reflection is not always easy to identify. Placement
of the cursor away from the actual refl ection surface will introduce error
into the fix. Estimation of the location of the radar target in relation
to the mapped structures on the chart is important. Misidentification
of a reflection source will result in the plotting of a position at the
wrong coordinate, but will not affect repeatable accuracy. Certain regions,
such as sloped headlands and tidal flats, give inaccurate reflection because
it is impossible to relate the reflection to a map location. The most
accurate radar range fixes are obtained from solid reflections between
0.16 and 6.4 km (0.1 and 4 mi). This keeps the range scale low resulting
in higher accuracy and avoids erroneous readings that can be caused from
very close reflections.
8-14
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TABLE B-7. VARIABLE RANGE RADAR (VRR) SYSTEM CHARACTERISTICS
U. S.
Variable Rang e Nom ina 1 Accuracy Suggested
Systems Model Type Range Markers (kID) Range (m) Angle List Pricea
DECCA R017D VRM/VP3 Digital 1 (add-on) 77 +0.50 $3,990
Racal DECCA Marine ROl10 8T Digital 1 77 +30 at 0.4 km +0.50 $6,000
Redmond, WA
(206) 885-4713
Furuno U.S.A. FR-360 MKII Analog add-on 58 +25 at 0.5 km <+10 $4,400
San Francisco, CA without VRM
(415) 873-9393 FR-810 Digital 2 116 +36 at 0.5 km <+10 $7 ,400
a:J FR-1011 Analog 1 77 +25 at 0.5 km <+10 $8,300
I
..... KODEN/S1-TEX T-I00 Digital 1 26 +22 at 0.5 km +0.50 $2,200
U1
Norwell, HA
(617) 871-6223
Raytheon Marine 3604 Analog 1 58 +10 $4,900
Seattle, WA 3610 Analog 1 116 +10 $5,400
(206) 285-6843
a Dee ember, 1985.
-------
Many vessels are already equipped with VRR and, if necessary, variable
range markers can be added for $1,000 to $2,000. The newer digital systems
offer multilevel processing for better target pickup (a problem with earlier
digital models) and map plotting ability on the screen for $5,000 to $10,000.
MEDIUM-RANGE SYSTEMS
Characteristics of representative medium-range systems are presented
in Table 8-8. As indicated, absolute accuracies of less than 10 m (33 ft)
are reportedly achievable for ranges well beyond those needed for coastal
positioning. The additional costs, typically between $100,000 and $200,000,
are not justified unless a much longer range system is needed for other
navigating or surveying work.
Syled;s
The Sercel Syledis is a short- to medium-range radio positioning system
that operates in the 420-450 MHz frequency range at distances of up to
300 km (186 mi) from shore. Through the use of long-coded pulses, the
system reportedly retains the advantages of short-duration pulses (i.e.,
accuracy and multi path discrimination) without corresponding limitations.
The technique is use:! to increase the amount of transmission energy available,
enabl ing range extension well beyond line-of-sight at moderate transmitting
powers. At operating frequencies, the system is said to be free of velocity
variations over ground paths or due to sky waves, and the effects of surface
reflected waves are reduced. The system can be operated in an active range-
range mode with up to three users in the same net'ftUrk, in a passive (hyperbol ic)
mode with an unlimited number of users, or in a combined mode (hyperbol ic
network and single range) with up to 11 users. The last mooe has the advantage
of retaining range-range accuracy while minimizing transmit times for each
user.
The basic system includes a mobile onboard interrogator and an array
of accurately positioned beacons along the coastline. Absolute accuracy
is claimed to be +1 m (~3.3 ft) at distances less than line-of-site, +3 m
8-16
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TABLE B-8. MEDIUM-RANGE POSITIONING SYSTEM CHARACTERISTICS
SYSTEMS RANGE FREQUENCY ABSOLUTE USER ANTENNA COST*
(km) (MHz) ACCURACY (m) CAPAB I LI TV VESSEL SHORE
SYLEDIS 300 420-450 i1 LOS Up to 4 Not Provided $ 51,350
Serce 1, Inc. 406-434 i3 2X LOS (Range-range)
Houston, TX Unlimited
(713)492-6688 (Hyperbolic)
RAYDIST 278 night 1.5-2.5 !3 4/net Omnidirectional $102,200
Hastings-Raydist 740 day (Range-range)
Hampton, VA Unlimited
(804 )723-6531 (Hyperbolic)
m
I HYPER-FIX 700 day 1.6-3.4 5-10 Up to 6 Omnidirectional $113,000
-
...., Racal DECCA Survey 250 night 0.01 lane (Range -range)
Houston, TX display Unl imited
(713 )783-8220 resolution Hyperbolic
ARGO OM-54 740 day 1.6-2.0 4-5 Up to 12 Omnidirectional $188,00
Cubic Western Data 370 night 0.05 lane (Range-range)
San Di ego, CA display Unl imited
(619)268-3100 resolution (Hyperbolic)
HYDROTRAC 300 day 1.6-4.0 1. 5-4 Single Omnidirectional $100,000
Odom Offshore Surveys 200 night 0.01 lane (Range -range)
Baton Rouge, LA display Unl imited
(504 )769-3051 resolution (Hyperbolic)
*January 1985
-------
(.:!:.9.8 ft) up to two times line-of-site, and +3 m x 10-50 (in meters) beyond
t his po i n t.
For purposes of coastal monitoring programs, the most likely system
configurations are range-range or hyperbol ic. In the range-range configurat ion,
components include an SR3 receiver, S03 power duplexer, two B2 (two-user
limit) or B4 (four-user limit) beacons, and shipboard/shore-base antennae.
Excluding the antennae (estimated at $1,000 each by the system ~anufacturer),
the basic system cost is $51,350 (including two B2 beacons). To operate
in the passive pseudoranging mode, hyperbolic geometry dictates the need
for a third shore station. Assuming three B2 beacons, the system would
cost $70,185. Use of B4 beacons ($1,210 each) would result in an additional
cost for the range-range system of $2,420 and an additional cost for the
hyperbolic system of $3,630 (Weems, L., 2 January 1985, personal communi-
cation).
Raydist Trak IV
With a Teledyne Hastings-Raydist radio navigation system, users can
identify geographic positions within +3 m (~9.8 ft) at distances up to
278 km (173 mi) from land-based transmitters during the night and 741 km
(460 mi) during the day. A major attribute of this 1.5-2.5 MHz system
at medium ranges is improved accuracy over Loran-C. In the range-range
mode, Raydist can accommodate four users with a vessel receiver/transmitter
(R/T) and two shore R/Ts. Alternately, the hyperbolic mode can be used
by relocating the vessel transmitter to a third suitable location on shore
and adding a vessel receiver. This mode allows an unlimited number of
users. A Radist-T configuration involving a fourth onshore transmitter
is available to increase the area of coverage having good line-of-position
intersections. In this configuration, an unl imited number of users can
be accommodated in the range-range mode (Comstock and Rounion 1982). The
system offers elliptical and halop grid patterns in addition to hyperbol ic
nets. Halop grids assure right-angle intersections throughout the coverage
area. The Radist system has a stated resolution of approximately ~0.75 m
(+2.5 ft) and repeatability of +2 m (+6.6 ft).
8-18
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The minimum system for range-range operation consists of an onboard
TAI00 transmitter, 10.7-m (35-ft) whip antenna, RA89 navigator receiver,
and GA62 two-dimensional position indicator. Latitude-longitude readouts
require an additional KA18-23 Radist director. Position can be plotted
with a PB22B 12-in flatbed plotter. Analog signals can also be retained
with an RB15-1 strip-chart recorder. Onshore components consist of two
AA77 base stations and two medium-range 14-m (46-ft) antennae. Excluding
. .
the KA18-23 Raydist Director, PB22B plotter, and the RB15-1 recorder, the
system cost is $102,181. Additional components for hyperbolic mode operation
(i.e., QB66B active probe mobile unit antenna and two antenna extensions)
cost $1,200. The KA18-23 Raydist Director costs $44,533; the PB22B plotter
costs $5,752; and the RB15-1 position signal recorder costs $4,085 (Gibbs, F.,
2 January 1985, personal communication).
Hyper-Fix
The Racal DECCA Survey Hyper-Fix is a 1.6-3.4 MHz positioning system
that operates at distances from shore up to 700 km (435 mi) in dayl ight
hours and 250 km (155 mi) at night (temperate latitudes). Actual operating
range depends on location. In the Gulf of Mexico, for example, 24-h operation
is quoted up to 250 km (155 mi)J. The system is operable in the range-
range, hyperbolic, or a combined mode using additional shore stations.
As with any 2-MHz system, accuracy is a function of geometry (i.e.,
distance off baseline and pattern crossing-angle). The receiver/controller
is capable of a 0.001 lane resolution, rounded to 0.01 lanes on display.
This is equivalent to less than 1 m (3.3 ft) on the baseline. Positional
acc uracy is sa id to be on the order of 1 m (3.3 ft) in the hyperbol ic mode
with optimal geometry and 1.5 m (5 ft) in the ranging mode.
Vessel and onshore station components for range-range operation consist
of a power amplifier, receiver/controller, and an antenna tuning unit.
In the hyperbo1 ic mode, the same onshore components are needed for each
shore station. Only a receiver/controller and an active antenna are required
on the vesse 1. The manufacturer reports re1 ati ve1y campl ete area coverage
in the Gulf of Mexico. Where multiple user demand exists, Racal Decca
.8-19
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Survey, Inc. will establ ish a cha in of shore stations and lease accessibil ity
to them. Users then need only a receiver/controller and an antenna. Based
on a comprehensive evaluation of several mid-range systems available, the
manufacturer reports wide acceptance of the system by the U.S. Navy (Harris, E.,
3 January 1985, personal communication).
A range-range mode system costs approximately $113,000, including
$32,276 for a 90515 mobile Receiver/Controller, 90512 Power Amplifier/Battery
Charger, and 90516 Antenna Tuning Unit; $715 for a 90538 shipboard antenna;
and $40,000 for each of two short-range shore transmitter stations. The
hyperbolic mode requires an additional shore station at $40,000. In the
Gulf of Mex ico, users may purchase the mobile recei veri controll er and shi pboard
antenna for approximately $33,000, and lease access to shore stations for
an initial monthly cost of near $12,000. Training in the optimal use of
shipboard equipment in conjunction with the established onshore chain is
included. Monthly costs decline over time to approximately $9,500 (Harris, E..,
3 January 1985, personal communication).
ARGO OM-54
The Cubic Western ARGO OM-54 is a medium-frequency positioning system
that operates between 1.6 and 2.0 MHz, and at an addi tional frequency 10 percent
higher when lane identification is used. The system determines mobile
station range from each of the fixed stations by accumulating whole-lane
counts and canputing fractional lane distances. The ARGO uses a multiplexing
sequence and a shortened radio frequency pulse to service up to 12 vessels
in the two-range mode, or an unlimited number of vessels in the hyperbolic
mode. Up to 16 frequency pairs can be factory-programmed for alternate
use. Maximum system range is 740 km (460 mi) in dayl ight and 370 km (230 mi)
at night. Range accuracy is given as 0.02 lanes instrumental and 0.05 lanes
achievable field accuracy. With lane widths of 74-94 m (243-308 ft), this
represents accuracies of 3.7-4.7 m (12-15 ft). Overall position accuracy
is stated to be on the order of ten s of meters by the manufacturer (Hempel, C.,
10 January 1985, personal communication).
8-20
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The standard ARGO OM-54 configuration consists of four fixed land-
based stations and multiple mobile or vessel stations. Each fixed station
includes a Range Processing Unit (RPU) and an Antenna Loading Unit (ALU).
In addition, mobile stations have a Control and Display Unit (CDU). Options
include a two-range strip-chart recorder, a digital printer, tilting yoke
mount, power supply, cables, and various antennae. The cost of a basic
onshore station (RPU and ALU) is $54,075, not including the antenna [$4,160
for a 30-m (100-ft) model]. In addition, the CDU for the mobile unit costs
$14,960, excluding the antenna [$2,500 for a ll-m (35-ft) whip]. A range-
range system consisting of two shore stations and a mobile station with
antennae costs approximately $188,000.
Hydrotrac
The ODOM Offshore Survey's Hydrotrac is an over-the-horizon medium-
range positioning system that operates in the 1.6 to 4 MHz range. The
system is capable of operating up to 300 km (186 mi) during the day and
200km (124"mi) at night in the Gulf of Mexico. Range in other operating
areas (e.g., Alaska) may be greater. The system can be operated in either
the range-range mode with two shore stations or the hyperbolic mode with a
master and two slave stations. By incorporating a third slave station, network
coverage can be tripled. Existing U.S. station coverage is primarily in the
Gulf of Mexico, where 12 Hydrotrac systems have been operating since 1973.
. With ideal crossing angles and predictable signal path conditions,
the claimed system accuracy is 2 m (6.6 ft) out to 300 km (186 mi). However,
in the hyperbol ic mode, land expansion and nonideal crossing angles may
result in a lower accuracy. The random errors in the receiver are sa id
to be less than 0.01 lane. A special filter limits the amount of additional
error from fluctuation in propagation conditions and incidence of sky waves.
In the range-range mode, vessel system components include a Model
700 Receiver ($25,625), a Model 1000 Amplifier ($6,875), a Model 610 Antenna
Coupler ($2,045), a cable set ($812), DC power supply ($468), and a 9-m
(30-ft) tilt-base center-loading antenna ($2,034). Each of the two shore
stations consists of a Model 701 Slave Drive Unit ($16,250), a Model 1000
B-21
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Slave Power Amplifier, ($6,875), a Model 610 Antenna Coupler, a cable set,
and an antenna of appropriate height for the station location and desired
maximum operating range. [A 30-m (100-ft) Texas tower type antenna would
cost approximately $5,000; a 9-m (30-ft) whip antenna would cost less than
$2,000.] The overall system, including the larger antennae onshore, would
cost $99,823 (Apsey, B. 10 January 1985, personal communication).
LONG-RANGE SYSTEMS
Characteristics of selected long-range and global systems are shown
in Table B-9. Most of the systems listed do not provide the absolute accuracies
recommended herein for coastal monitoring when operated in a normal mode.
Of significance, however, are the characteristics of differential Loran-C
and proposed satellite systems, as discussed below.
Loran-C
Loran, an acronym for long range navigation, is a pul sed low-frequency
electronic navigation system that operates at 90 to 110 kHz in the hyperbolic
mode. Loran-C receivers match cycles to measure time differences between
arriving master (M) and secondary (W, X, Y, Z) signals, which are pulse-
and phase-coded to enable source identification (Panshin 1979). The microsecond
arrival time differences are displayed and can be plotted on a special
Loran-C latticed chart as 1 ines-of-position. Fully autOO1atic Loran-C receivers
simultaneously process signals from two master-secondary station pairs,
displaying LOP information to permit course tracking.
Because Loran-C stations radiate peak powers of 250 kW to 2 MW, range
capability varies. Due to the use of low frequencies and large baseline
distances [i.e., 1,850 km (1,150 mi) or more], Loran-C can provide positional
information of reasonable accuracy out to 2,225 km (1,380 mi) with sky
waves (Maloney 1978). Range achievable at a particular station is dependent
on transmitter power, receiver sensitivity, noise or interference levels,
and signal path losses (Canadian Coast Guard 1981).
8-22
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TABLE B-9. LONG-RANGE POSITIONING SYSTEM CHARACTERISTICS
SYSTEM RANGE FREQUENCY NOMINAL USER COST
(km) (KHz) ABSOLUTE ' CAPABILITY
ACCURACY
(m)
OMEGA Global 10-14 1BOO day Un1tmited $4,000-10,000
Multiple Receiver 3100 night
Manufacturers
LORAN-C 2500 day 90-110 185-460 Un1tmited $1,000-2,000
Multiple Receiver 1850 Night 15~90 repeatable
Manufacturers
LAMBOA 400-800 100-200 137 day Single 2-range Ca 11 Agent
DECCA Survey, Ltd. 730 night Unlimited-hyperbolic
England
~ VI EWNAV Some 90-110 '!to Multiple $40,000 and
. Navigation Sciences Coastal $2,000 annual fee
N Bethesda, MD Areas
&.oJ
(301)952-5225
TRANS IT Global 150 & 400 MHz 90, 1 pass/1 freq. Unlimited $2,500-10,000 basic
Multiple Receiver 37-46,1 pass/2 freq. $30,000-45,000 elaborate
Manufacturers 3-5,Mult1ple pass
and 2 freq.
GEOSTAR U. S. Land & 5117- 6533 1-7 Un lim1 ted Transceiver $450 and
Geostar Corporation Coastal Areas 1618-2492 link Rental $10-30/mo.
Princeton, NJ (Estimated)
(609)452-1130
NAVSTAR/GPS Global 1575 MHz 40,C/A,CEP P-M11i tary $10,000-140,000 initial
Mulitple Receiver 1228 MHz 8-9, P, CEP CIA Conrnerc1al $1,000 eventually
Manufacturers Foreign
SERIES Global 1575 MHz < 1 m di ff. Unlimited $287,000
ISTAC, Inc. 1228 MHz Mode
Pasadena, CA
(818)793-6130
AEROSERVICE Global 1575 MHz < I m Multiple $235,000
Houston, TX 1228 MHz (1/2 hour)
(713)7 84 - 5800
-------
At best, the absol ute accuracy of Loran-C over short distances using
the ground wave varies from 185 to 460 m (607 to 1,509 ft), whereas repeatable
accuracy varies from 15 to 90 m (49 to 295 ft) depending on the vessel's
location within a given coverage area (Dungan 1979; U.S. Coast Guard 1974).
Over longer distances, the nominal ground wave mode accuracy is 0.1 percent
of the range, and sky wave accuracies vary from 5.6 to 9.3 km (3.5 to 5.8 mi)
(Umbach 1976). Achieving the short-range accuracies cited above requires
proper installation, maintenance, and operation of high-quality equipment
(Canadian Coast Guard 1981). Available equipment varies from simple receivers
and indicators to fully automated receivers with self-tracking capabilities
that can interface with a vessel's computer. Coverage in U.S. waters is
generally good, with the exceptions of Puerto Rico and some locations off
Alaska.
Although Loran-C is frequentl y proposed for coastal monitoring programs,
this application has potential problems. Inland location [up to 160 km
(100 mi)] of Loran-C chains requires overland transmission of signals.
This results in phase shifts that have been difficult to predict. Such
shifts can cause an erroneous position location fix. There are also anomalies
associated with land-water interfaces and large structures such as bridges
and tall buildings. Crossing-angles can also vary widely from one geographic
area to another. In some cases, lines of position are almost parallel,
making an accurate fix difficult to impossible. Noise and interference
(e.g., from engines and other electronic equipment) can also cause problems.
However, most Loran-C receivers are equipped with factory-set or tunable
notch filters to minimize such problems.
In an effort to improve navigational capability using the Loran-C
system, the U.S. Coast Guard completed a one-time survey of the east and
west U.S. coasts in which Loran-C positions were compared with those from
a cal ibrated microwave system. Corrections were obta ined for the Defense
Mapping Agency nautical charts, whose LOPs were based on theoretical trans-
mission over water paths only (Ryan, R., 11 January 1985, personal communi-
cation), but these corrections do not include seasonal or diurnal signal
effects.
8-24
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The land transmission-path effect (known as the additional secondary
phase factor) is currently being evaluated in a multiyear U.S. Coast Guard
study at approximately 100 locations where signal variations are being
recorded daily (Blizard and Slagle 1985). Study results indicate that
annual variations in positional minimum errors are highly dependent on
geographical location. The minimum 2 dnns error (normal rather than dif-
ferential Loran-C) for various U.S. coastal areas were found to be the
fo 11 owi ng :
Gulf of Mexico
2 drms Accuracy (m)
Lowe s t Highest Location
30 Boston
176 Miami
58 New Orleans
179 Corpus Christi
42 Seattle
194 San Diego
Area
East Coast
West Coast
Stated otherwise, repetitive measurements at a geographically known site
would vary these amounts over a 1-yr period. Thus, they represent the
minimum accuracy in absolute position achievable at each location indicated
with a calibrated Loran-C having no other error sources.
Loran-C units may be used in a differential mode, substantially reducing
the temporal and spatial variances frcm a predicted position (U.S. Department
of Defense and U.S. Department of Navigation 1984). In this mode, a Loran-C
receiver is located at a surveyed point onshore near the area to be monitored.
Signals observed at this location are compared to signals predicted for
this position to calculate offsets of differential corrections. The offsets
are transmitted to the monitoring vessel, enabling normally incoming signals
to be corrected according to transmission path anomolies. In examining
the adequacy of differential Loran-C to meet an 8-20 m (26-66 ft) accuracy
requirement of U.S. harbors and harbor approaches, the U.S. Coast Guard
8-25
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found that a 20 m (66 ft) 2 drms absolute position accuracy is feasible
in some U.S. coastal locations (Doughty and May 1985). Accuracies somewhat
higher than this may even be achievable using various methods to predict
time difference offsets (i.e., instead of a Sample Mean Method of the U.S. Coast
Guard, using Least Squared Error, Alpha-Beta Filter, or a Linear Regression
model) (Bruckner 1985).
Absolute and repeatable accuracies achievable with normal Loran-C
are not sufficiently accurate to meet the accuracy limits recommended herein.
Repeatabil ities of approximately +40 m (131 ft) appear optimal unless using
a differential mode when this figure might be reduced to +20 m (66 ft).
Thi s would be acceptabl e for sites beyond 100 m (328 ft) depths, where
+20 percent of the depth exceeds this value. However, almost all U.S. coastal
outfall sites are located in more shallow waters.
Viewnav
To improve upon the positioning accuracy achievable with standard
Loran-C receivers, Navigation Sciences has developed Viewnav, an interactive
computer system that uses differential Loran-C to position a vessel with
a cla imed repeatable accuracy of 4.6 m (15 ft). Absol ute accuracy of the
system is on the order of ~10 m (~32.8 ft) at the 90 percent confidence
level, and +5 m (+16.4 ft) at the 20 percent confidence level (Newcomber K.,
21 January 1985, personal communication). Loran-C offsets are obtained
by interrogating onshore monitors established by the company. In addition,
a land-based microwave system is used to calibrate a vessel's initial position
or track. The system is particularly effective in ports and harbor areas
where large buildings or the land-water interface may alter Loran-C readings.
A unique feature of the system is an electronic display of the survey
area based on digitized nautical charts. As the vessel moves, its position
on the display moves relative to depth contours and land boundaries. Other
waterborne radar images in the area are also indicated.
A full system presently costs approximately $40,000, with a supplementary
annual service fee of $2,000 for chart corrections and equipment maintenance.
8-26
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The base price includes a mainframe Ai-M16 computer with 512KB of main
memory, a flexible disk drive with 1MB capacity, and a Winchester hard
disk of 10MB capacity. The basic system provides 5MB of chart storage,
which equates to approximately 650 charts depending on scale selected.
Additional charts can be stored on floppy disks. The manufacturer was
to complete charts and install Loran monitors along the U.S. east coast
in 1985 and along remaining U.S. coastal areas by the end of 1986.
If all survey stations were beyond a 50-m (164-ft) depth, then the
+20 percent of depth positioning accuracy could be attained with Viewnav.
However, the need for gradient stations toward. shore or shallow stations
in special habitat or recreational areas will probably. require accuracies
beyond the system's present capability.
Lambda
The Decca Lambda is a low-frequency hyperbolic navigation system that
operates in the 100-200 kHz frequency range, up to 400-800 km (249-497 mi)
from shore stations. Lambda, an acronym for low ambiguity Decca, is a
version of the Decca Navigator System originally introduced in 1944 (Ingham
1975). It compares phases to determine distances from the transmitters,
with a master and three (purple, red, and green) or four (orange) slave
stations. A typical receiver includes an identical unit for each frequency
and internal circuits for signal phase comparison. Decometers provide
readings of position lines, whose intersections on Decca charts provide
the vessel's location. Lambda coverage includes Western Europe, the Persian
Gulf, the Indian subcontinent, the Far East, Australian waters, and the
Canadian maritime provinces. Because U.S. coastal water coverage is not
available, the system will not be further discussed in this report.
OMEGA
OMEGA is a time- shared, conti n uous wave, rad io-navigation system operating
in the very low frequency range from 10 to 14 kHz. The system offers global
coverage from eight stations having 10-kW transmitters on baselines of
approximately 9,660 km (6,000 mi). Because sky waves provide line-of-position
8-27
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data, corrections must be appl ied to observed values (e.g., from meteorolCX)ical
conditions and solar activity) for ionospheric disturbances. Although
the normal operating mode is hyperbolic with at least three shore stations,
a circular mode is possible with only two shore stations and a precision
o sc ill a to r. Receivers count 1 anes automati cally. However, it is necessary
to initialize them to within .::,0.5 lane width. If the signal is lost, an
external fix may be needed to re-establish the lane count.
System accuracy is directly related to the accuracy of propagation
correction constants, which are still being refined. The U.S. Navy designed
the system to achieve an rms accuracy of 1.6-3.2 km (1-2 mi). However,
without propagation predictions, rms accuracy is on the order of 8 km (5 mi).
Differential OMEGA uses signal corrections from a fixed receiver located
within 320 km (200 mi) to correct receiver readings. This mode theoretically
enables an accuracy of approximately 400 m (0.25 mi) within this range
(Kasper and Hutchison 1979). Composite OMEGA involves the analysis of
information from signals at two or more frequencies to minimize the effect
cif sudden ionospheric disturbances on position accuracy. However, this
approach has not yet been validated.
System costs vary due to the broad user spectrum and associated system
components. Prices range from a few thousand dollars for a manual single-
frequency receiver to over $25,000 for an automatic receiver that operates
at three frequencies and interfaces with complex avionics. The cost of
an automatic marine receiver is approximately $10,000. Due to OMEGA's
reasonable cost and global coverage, it is an attractive method of general
positioning and tracking at sea. However, its low achievable accuracies
preclude its use as a coastal monitoring navigational tool.
SATELLITE SYSTEMS
Characteristics of representative government and privately-owned satellite
systems are also presented in Table B-9. TRANSIT is the system currently
available, although daily coverage is highly dependent on location. Multiple
pass inputs are required to achieve the positional accuracies recommended
herein. Navstar GPS shaul d prov ide adequate coverage by 1~-89, but accuracies
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achievable will be dependent on code availability to canmercial users and
the extent to which the signal is detuned in the interest of national security.
Initial unit costs will be high, but will decrease rapidly. Some private
systems (using government satellite signals, or their own satellites) are
also described. In as much as the reported accuracies are adequate, further
development of such systems should be monitored carefully.
Transit (Navsat~
The U.S. Navy Navigation Satellite System (originally Project Transit)
consists of a group of satellites in 106-min circular polar orbits at altitudes
of approximately 1,411 km (877 mi). The system also includes ground tracking
stations, a ccmputing center, an injection station, U.S. Naval Observatory
time signals, and vessel receivers and ccmputers. Positional measurements
are based on the Doppler frequency shift that occurs when the relative
distance between the satellite transmitter and vessel receiver changes
(i .e., frequency. increase upon closure and frequency decrease upon separation).
Provided the satellite orbits are very accurately known, it is possible
to locate the receiver. The nature of the Doppler shift depends on the
exact location of the receiver relative to the satellite path (Maloney
1978). The systen operates at frequencies of 150 and 400 MHz so that ionspheric
correcti ons can be made through si gnal compar i son techn iques. Ground tracking
stations in Hawaii, California, Minnesota, and Maine continuously monitor
and provide orbital information to the computing center. Using these data,
best-fit orbital and time information are supplied to the injection station
for transmission to the satellites every 12 h. The vessel's position is
then determined based on II known II orbital pos it ions dur ing satell i te pas sage
and measured frequency shi fts..
As originally designed, at least one satellite would be within line-
of-site every 35 to 100 min. However, at U.S. east and west coast latitudes
the acceptable fix window is approximately every 90 min (Driscoll, C.,
15 January 1985, personal communication). This is caused, in part, by
the requirement that a satellite's maximum altitude be between 150 and.
750 of the horizon before a fix can be considered valid. Another problem
occurs when two satellites being tracked have approximately the same closest
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approach, whereupon it becomes difficult to know which one is being monitored.
Typically each satellite provides four fixes per day on two successive
orbits spaced by 12 h. Because one satellite is currently inoperative
and another has weakened batteries, it may take longer (e.g., several hours
at the equator) to gain a valid fix (Booda 1984).
A static position fix with Transit using single-channel equipment
can be made with an accuracy of approximately 90 m (295 ft). Dual-channel
receivers improve single-pass accuracy to 37-46 m (121-151 ft) (Hoerber
1981; Maloney 1978). With multiple passes, an rms accuracy of 3-5 m (9.8-
16.4 ft) is claimed by some equipment manufacturers. Using translocation
(i.e., simultaneous measurements at two fixed locations, one of which is
accurately known), estimated positional measurement accuracies are in the
range of 0.5-1.5 m (1.6-4.9 ft) (Driscoll, C., 15 January 1985, personal
communication; Moyer, C., 15 January 1985, personal communication). Magnavox
has developed a satellite surveying program (MAGNET) that is reported1Y
capable of simultaneous location of up to 10 sites with a point-to-point
accuracy of 30 cm (1 ft) or better. However, this system is designed for
coordinated geodetic studies over wide areas using 3-10 MX 1505 receivers
and recording data from ten or more satellite passes (Anonymous 1981).
Transit receiver costs range from $2,500 to $10,000 for basic single
frequency units (Murphy, W., 15 January 1985, personal communication).
More elaborate multiple-channel systems, sometimes in combination with
OMEGA, cost from $30,000 to $52,000 (Jolly, J., 15 January 1985, personal
communication; Driscoll, C., 15 January 1985, personal communication).
Use of the Transit system is generally not appropriate for coastal
monitoring programs due to the inaccuracy of the system for fixes from
either a moving vessel or from one occupying offshore stations for a relatively
short time. A fix must be based on a single pass. With satellite passes
at 1- or 2-h intervals, multiple-pass data acquisition is impractical.
Therefore, only the best single-pass accuracy of 37-46 m (121-151 ft) can
be achieved. Because translocation is designed for two fixed stations
rather than one on a moving sampling vessel, and because MAGNET applies
to land surveying only, neither method will provide the recommended minimum
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accuracy of !.3 m (!.9.8 ft) during the limited time the vessel will be at
anyone station. It is also noteworthy that the U.S. Navy intends to phase
out Transit (as well as Loran-C and OMEGA) over a 15-yr period (U.S. Department
of Defen"se and Department of Trans porta t ion 1984).
GEOSTAR
GEOSTAR is a pulse radio transmission system. It will provide satellite
information for positions within the continental U.S. and its coastal waters
by 1988. Three geosynchronous satellites (and a fourth as backup) will
orbit the earth at 37,000 km (22,991 mi) at 700, 1000, and 1300 W longitude.
System canponents include transceivers, satellites, and computers in a
ground station. The 1 inks between the ground station and the satellites
will operate at 5,117-5,183 MHz and 6,533 MHz, while user-satellite 1 inks
will be at 1,618 and 2,492 MHz (Whalen 1984). Should a satellite fail,
the backup would be moved into a proper orbit by telemetry command from
the ground canputer facility.
The system user will send a command through the transceiver, which
relays the message through the satellites to a central computer at the
ground station, reportedly in less than a second. The signal-arrival times
fran each satellite are used by the ground canputer to calculate the position
of the specially coded transceiver. The information is then transmitted
back to the satellites and relayed back to the transceiver in a similar
amount of time.
GEOSTAR will enable a typical single-shot positioning error of 2-7 m
(6-23 ft), according to the developer. When needed, accuracies down to
1 m (3.3 ft) reportedly will be achieveable using two-way interaction,
signal analysis, and averaging. Users at a known elevation (e.g., sea
l~vel) will be afforded the greater accuracy due to much smaller geometrical
dilution of precision where only two rather than three coordinates are
required. Continuous operations in a differential mode should also enable
correction inputs for such errors as ionospheric delays, satellite position
drift, and drifts in satellite electronic delays.
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System designers estimate that the cost of a basic hand-held transceiver
with a typewriter keyboard and LCD di spl ay will be well under $1,000 by
the time the system is operable. A monthly service charge in the range
of $10 to $30 is also anticipated (Howarth, C., 16 January 1985, personal
communication). At publication, FCC reportedly had completed its review
and approval of GEOSTAR Corporation's application for use of the requested
frequencies. Candidate users are urged to verify the latest satell~te/ground
station operating schedule, and obtain further information on transceiver
availabil ity.
Navstar GPS
The Navstar Global Positioning System (GPS) is a second-generation
satellite navigation system currently under development by the U.S. Department
of Defense. Its purpose is to provide precise, continuous, worldwide,
all-weather, three-dimensional navigation for land, sea, and air appl ications.
Under current pl ans, 18 satell ites will be launched into three co-planar
orbits 1200 apart to provide continuous transmission of time, three-dimensional
position, and velocity messages to system users. The GPS satellites transmit
at 1,227.6 MHz and 1,575.4 MHz to permit the measurement and correction
of ionospheric refraction error. Currently, five developmental satellites
are in orbit, providing approximately 4 h of coverage twice daily, separated
by a 12-h period. Continuous two-dimensional positioning information should
be available with 12 satellites by February, 1988, and three-dimensional
coverage is projected for late 1988 or early 1989 (DeGroot, L., 15 January
1985, personal communication; Stansell 1984). The system consists of the
satellites in 12-h, 20,200-km (12,552-mi) orbits, a U.S. master control
station, several monitoring stations, and user equipment in the form of
small, lightweight, relatively inexpensive receivers. Signals received
from any four Navstar satellites are demodulated, time-correlated, and
processed to obtain precise position information.
Two levels of positioning accuracy are achievable with the Navstar
GPS system. The lower level is obtained from the Standard Position Service
(SPS) using the coarse acquisition or "CIA code." When the system becomes
fully operational, navigational accuracy from these signals should be approxi-
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mately 100 m (328 ft) 2 drms, or a circular probable error (CEP) of 40 m
(131 ft) (Montgomery, B., 15 January 1985, personal communication). More
accuracy can be achieved using the Precise Positioning Service (PPS) or
"P-code" (i.e., 8-9 m two-dimensional CEP). Accuracy obtained during testing
with a multichannel, two-frequency, P-code receiver and with a single-channel,
one-frequency CIA code receiver is shown in Figure B-1. Accuracy available
to civil ian- users is indicated by curve C. Additional Rositioning accuracy
can be achieved by operating in a differential mode, in which receivers
on a vessel and at a known onshore location simultaneously receive the
satellite signals. The onshore receiver is calibrated. Bias corrections
. based on signals received at the fixed station are then transmitted to
the mobile receiver. These area-specific corrections yield more accurate
positional determinations. With differential GPS, a two-dimensional position
should be definable within a range of 2 to 5 m (6.6 to 16.4 ft) (Montgomery
1984; Stansell 1984).
Due to lack of full-time coverage at the present time, both SPS and
PPS are available to military and civil ian users. However, the government
intends to encrypt the P-codes, allowing use only by the military and other
authorized users [e.g., National Ocean Industries Association (NOlA) members].
NOlA has proposed that all P-code receivers be owned and operated by a
single service cOOlpany that would provide both the equipment and personnel
when national security conditions are met and no other reasonable navigational
alternatives are available (Stansell 1984). Under U.S. Department of Defense
policy, access to PPS is available to civilian users operating in the "national
interest" (Anonymous 1985). Equipment, cryptography, and support services
will be provided under contract to the user. However, it would appear
that general users in coastal areas will be limited to CIA code equipment.
I t wi 11 therefore be necessary to operate in the d ifferenti a 1 mode in order
to achieve the positional accuracies needed for coastal monitoring~
Because the GPS system is in a developmental stage, cost estimates
for the equipnent are difficult to make. Several major equipment manufacturers
are in the process of designing receivers with varying capabilities, and
a limited number of models are now available. SOOle manufacturers envision
that receivers with 100-m (328-ft) accuracy will cost less. than $500 when
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100
2 drms
90
w
::>
..J
~ 80
0
w
~
u 70
0
~
~ 1 drms
9
w
a:I
~
a:
0
a:
a:
w
w
:E
i= 30
u..
0
I-
Z
w 20
u
a:
w
Q.
10
0
0 10 20 30 40 50 60 70 80 90 100
HORIZONTAL ERROR IN METERS
SOURCE: Stansell 1984
Figure 8-1.
NAVSTAR/GPS test network accuracy.
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mass produced (e.g., for automobil es). At the other extreme, Texas Instruments
sells the TI4100 Navstar Navigator for $140,000. It is said to be capable
of slow dynamic positioning within a few meters, speed within tenths of
a knot, and time to the microsecond (Montgomery 1984; St. Pierre, R., 15
January 1985, personal canmunication). Motorola anticipates that the initial
cost of two stations needed to operate in the differential mode will be
in the $100,000 range (Sheard, S., 15 January 1985, personal communication).
Magnavox has a five-channel T-Set GPS Navigator, with real time differential
GPS operation as a planned option. A two-unit system, excluding communications
link, costs approximately $100,000 (Driscoll, C., 15 January 1985, personal
communication). Rockwell International sells a prototype CIA code receiver
for $17,500 and anticipates that GPS receivers will cost under $10,000
by 1988 (DeGroot, L., 15 January 1985, personal communication). Tracor
expects initial models to sell for under $10,000, fall ing to around $1,000
in 3-5 years (Murphy, W., 15 January 1985, personal communication).
Using the CIA code in a differential mode, the Navstar GPS positioning
system should provide the.!.3 m (.!.9.8 ft) accuracies recommend for coastal
monitoring near ZIDs. Errors as low as 1 m (3 ft) at a range of 200 km
(12.4 mi) from the reference station are predicted for daytime operation.
Smaller errors are expected at night (Kalafus 1985). However, testing
with the actual satellite signals modified to reduce achievable accuracy
will be required to confirm this capability. The two required stations
may be expensive at first, but competition and the large potential market
are expected to drop the price much lower.
SERIES
The SERIES positioning and navigation system (Satellite Emission Range
Inferred Earth Surveying) evolved from a technique originally developed
to measure movements in the earth's crust for tectonic studies and earthquake
predictions. The system enables use of signals from existing Navstar GPS
satellites with no knowledge of either the P or CIA codes by simultaneous
pseudoranging to multiple satellites in a differential mode. Accuracies
on the order of 2 m (6.6 ft) CEP or 5 m (16 ft) 2 dnns are said to be achievabl e
for vessels in a slow dynanic mode (0-10 kn) (ISTAC 1984). This is accomplished
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by comparing Navstar signal arrivals at one receiver to the same signals
received at another shore-based geodet ic reference mark. By expl icit differ-
encing of the frequency and phase measurements observed from each satellite,
it is apparently possible to eliminate the effects of Navstar signal variations
that secure the satellites from an unauthorized user in an autonomous,
single-receiver, real time mode. SERIES receivers are codeless spectral
compressors that simultaneously receive and incorporate spread-spectra
modulations of several Navstar satell i tes and then extract the frequency
and phase of each satell ite in view. Data are processed by a combination
of Doppler positioning and phase ranging.
System components include an unattended, fixed reference station,
a marine vessel station, and a reference link (via satellite or terrestrial
radio) to transfer the reference station signal to the marine vessel for
reaT time positioning. The reference station includes an SERIES GPS receiver
(MPS-1), a rubidium frequency reference, and a customer-supplied radio
transmitter (200 Hz analog signal). The marine station consists of the
GPS receiver with an integrated Navstar C/A-code-correlating subsystem
for broadcast satellite ephemerides, a rubidium frequency reference, a
microprocessor for real time position computations, and a customer-supplied
receiver for fixed reference station signal reception. A standardized
RS-432 interface is provided for conununication of the position information
to the vessel's navigation system.
The initial MPS-1 units are estimated to cost $205,000 for a ship
system and $82,000 for the fixed base system required for differential
mode operation (Whitcomb, J., 14 January 1985, personal communication).
The system will probably be available on a lease or service-contract basis,
as well.
When operational, the SERIES system may offer a high-accuracy alternative
to the Precise Positioning Service of Navstar GPS or to differential mode
operation with the CIA code. Although the projected cost for an initial
unit is high to recover research and develo~ent expenditures, cost shou ld
decrease with further production. Potential users should confirm price,
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verify accuracies achievable from a mobile marine vessel, and identify
specific data-link components that need to be suppl ied by the user.
Aero Service GPS
The Aero Service Division of Western Geophysical Company of America
is developing a codeless satellite navigation system for marine positioning.
The system will enable real time navigation in the slow dynamic mode to
less than 5 m (16.4 ft). For vessels in the semi-static mode (on station)
for 30 min, the system will reportedly yield a position fix accuracy of
less than 1 m (3.3 ft) (Mateker, E., 17 January 1985, personal canmunication).
The system will require one unit onshore at a fixed position, and one unit
on the survey vessel. Continuous real time communication between the two
systems mayor may not be necessary, depending on steps by the U.S. Department
of Defense to degrade certain elements of the GPS satellite signals. Prototype
testing was scheduled for September, 1985, with production systems planned
for early 1986.
This system is noteworthy, as it will probably be based on methods
now used for land surveying in the MACROMETER Interferometric Survey System.
The MACROMETER is designed for very precise positioning in a fixed-point
(static) mode, using GPS signals but no codes. Company tracking stations
(Phoenix, Arizona, and Woburn, Massachusetts) will follow up to six satellites
simultaneously. The MACROMETER is capable of providing position in terms
of latitude, longitude, and ellipsoidal height to within a few parts per
million of the distance from a reference point. In the continental U.S.,
1-3 h of observation will yield first-order (1:100,000) accuracy in all
three coonjinates using the GPS satellites. Data for a third-order (1:10,000)
survey point may be collected in less than 1 h (e.g., as little as 15 min
of observation).
The MACROMETER surveying system includes two or more field units and
an office data processer. Each V-1000 field unit includes an antenna,
receiver, tape drive, 30 m of cable, time receiver, power package, battery
chargers, generator, and DC/AC inverter. The P-1000 Office Data Processer
computer subsystem incl udes hard and floppy disk drives, video terminal,
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tape drive, printer, modem,
two-unit surveying package is
can be rented for $5,000 per
on each unit.
and all necessary software. A 1,000 series
currently priced at $235,000. The system
V-1000 unit per month, with a 2 month minimum
HYBRID POSITIONING TECHNIQUES
Characteristics of three systems which measure both range and azimuth
(therefore requiring a single onshore station rather than two or more)
are given in Table B-10. Modular units for use with an already-owned system
can be obtained for approximately $20,000. Semi- to fully-autOOlatic cOOlplete
systems are available for $70,000-$100,000. Angular and range accuracies
are adequate for the recommended coastal position requirements.
AZTRAC
The ODOM Offshore Survey AZTRAC is a sem iautOOlated opti cal angle-measuring
and transmitting system which can be used in conjunction with an independent
distance-measuring system to position a vessel. The AZTRAC system consists
of a modified Wild Heerbrugg Tl6 theodolite, an onshore transmitter, and
a vessel receiver. The theodolite has an infinite tangent drive and provides
information in a digital format. Typically, the separate distance-measuring
system consists of a microwave master receiver on the vessel and a remote
unit transmitter located at the theodolite.
For a survey, the AZTRAC theodolite and transmitter are setup at a
point of known position and the theodolite is zeroed on a known azimuth
line of backsight. The theodolite operator then sights and tracks the
survey vessel's ranging antenna or transponder. As the vessel moves, the
microwave ranging system continuously measures distance between the shore
station and the vessel. The tracking motion produces pulses that are decoded
by the AZTRAC transmitter and displayed to the theodolite operator as the
angle to the vessel from the reference azimuth 1 ine. The angle is al so
converted to binary code serial format and used to activate the transmitter,
which sends the information to the survey vessel. The AZTRAC receiver
converts the angle information to parallel format and displays it for manual
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TABLE B-10. RANGE-AZIMUTH POSITIONING SYSTEM CHARACTERISTICS
SYSTEMS FREQUENCY RANGE NOMINAL ACCURACY COST
AZIMUTH RANGE POSITION
AZTRAC Optical Visible 0.010 Both depend on accuracy of $22,500 plus
Odom Offshore Surveys and ranging system used ranging
Baton Rouge, Louisiana microwave system cost
(504}769-3051 (typical) cost
POLARFIX Laser 5000 m 0.010 !0.1m:!:0.1m/km :!:0.1m!u.2m/km $100,000
~ Krupp Atlas-Elektronik (904 nm)
~ Webster, Texas UHF Telemetry
(713 )338-6631 (406-470 MHz)
ARTEM IS 9.2-9.3 GHz 10-1400 m or 0.030 0.5 m short Not stated $70,000 to
Andrews Hydrographics 200-30,000 m 1.5 m long $75,000
Houston, Texas
(713) 558-2236
-------
recording. It simultaneously outputs the angle as serial data for automated
recording or processing by any available onboard computer or plotting system.
AZTRAC is designed to operate with most microwave ranging systems.
Two AZTRAC units working in an azimuth-azimuth mode can provide positioning
in survey areas where reflections from metal structures or electrical noise
from radar and other transmitters limit use of microwave ranging. The
.
Wild T16 theodol ite has a 30X magnification, 27-m (89-ft) field of view
at 1,000 m (3,280 ft), and an angular resolution of 0.010 (36 arc sec).
At a distance of 5 km (3.1 mi), this corresponds to an absolute arc length
error of 0.9 m (3.0 ft).
The National Ocean Survey recently examined range-azimuth positioning
of a vessel moving at a nominal speed of 6 kn (11.1 km/h) at ranges of
up to 3,000 m (9,842 ft). Instruments used included a Wild T2 theodolite,
from which angles were manually recorded onshore, and the AZTRAC, whose
angles were recorded on the vessel. Pointing errors (68 percent probability)
of these two instruments were found to be approximately' 1.3 m (4.3 ft),
independent of range when standard deviations of right and left movement
data were pooled (Waltz 1984).
The theodolite was sited on a white Del Norte Trisponder transponder
on the moving vessel. Visibility was good during the 2-day survey (off
Monterey, CA), with calm mornings giving way to afternoon winds of 15 kn
and 0.6-0.9 m (2-3 ft) seas. Apparently, range could have been extended
much farther under such conditions, particularly if color had been added
to the vessel target (Waltz, D.A., 18 January 1985, personal communication).
U.S. Army Corps of Engineer users have confirmed the effectiveness and
reliability of the AZTRAC system in conjunction with several different
microwave ranging systems (Ard, R., 18 January 1985, personal communication).
The system has also proven effective in port and harbor surveys when it
was impossible to achieve optimal shore station geometry for range-range
or hyperbolic operation.
For use in a coastal monitoring program, the theodolite could be set
at a predetermined angle, with the operator indicating when the vessel
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was in the sight path. Vessel range could then be adjusted until a pre-
determined distance was achieved, allowing a marker buoy to be released.
Anchors could then be set upwind and downwind, with line-out adjusted until
a properly buoyed position was achieved. Alternately, the vessel's bow
could be held into the sea and on the buoy, provided an offset correction
to the sampling point was allowed.
The current cost of the AZTRAC alone is $22,500 (Apsey, B., 10 January
1985, personal communication). A system consisting of the AZTRAC, a Motorola
Falcon 484 with one reference station (approximately $32,000) and an interface
unit ($10,000) would cost $64,500. Thus, provided the required range is
achievable under anticipated visibility conditions, the additional versatility
of the AZTRAC can be real ized for $25,000 above the $39,300 cost of a two-
station Falcon 484 range-range microwave system.
PO LA RF I X
POLARFIX is a dynamic range-azimuth positioning system by Krupp Atlas-
Elektronik. The system uses a scanning (300 horizontal) pulsed laser beam
from a single, fixed, onshore tracking station to follow the survey vessel
(up to lOa/see) and to transmit range and angle information via telemetry
link. The system incorporates a fully automated onshore tracking station,
which requires no attendance beyond initial station setup and azimuth refer-
encing. The shore station can locate the mast-mounted prism reflectors,
follow the vessel, and, if necessary, relocate the vessel by performing
a routine search pattern based on a record of tracking history. The shore
tracking station consists of a laser-sensing head mounted on a conventional
survey tripod, 1 inked by cable to an integrated control unit that houses
data control, transmission, and telemetry transceiver equipment. An integral
control unit (including a display and a second telemetry receiver), keyboard
terminal, printer, telemetry antenna, and prism reflector assembly are
onboard the vessel.
Under clear operating conditions, a 3-km (1.9-mi) range using a Class I
laser or a 5-km (3.1-mi) range using a Class IlIa laser may be selected.
Maximum range achievable varies with the prism assembly used to reflect
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the tracking station's laser beam. In foggy weather, range is said to
be 1.5 times visible range, due to use of the pulsed infrared laser. Si.ngle-,
dual-, and triple-ring omnidirectional prism assemblies can be stacked
on the vessel antenna. For average weather conditions, a two-prism assembly
(each with five reflectors) gives approximately 3.5 km (2.2 mi) of range.
This can be extended to approximately 5.0 km (3.1 mi) with the addition
of more assemblies.
Range accuracy is reported as 0.1 m +0.1 m/km (0.3 ft :,0.5 ft/mi) of
measured range. Azimuth accuracy is said to be 0.010 or better. The resulting
positional accuracy at 1, 3, and 5 km (0.6, 1.9, and 3.1 mi) is approximately
0.3, 0.6, and 1.0 m (1, 2, 3.2 ft), respectively. The positional algorithm
given is +0.1 m +0.2 m/km (+0.3 ft ~1.1 ft/mi). Current cost of the system
is $100,000 (Guillory, J., 28 January 1985, personal communication).
ARTEMIS
The ARTEMIS by the Christiaan Huygenslabortorium (Holland) is a distance-
bearing type of microwave positioning system capable of measurements at
ranges of 10 m (32.8 ft) to 30 km (18.6 mi), and angles from 00 to 3600
fran a single fixed shore station. Accuracies at the two-sigma or 95 percent
level are given as +1.5 m (4.9 ft) distance, and +0.030 azimuth, equivalent
to +0.5 m/km (2.6 ft/mi).
Angle measurements are based on automatic tracking antennas on the
vessel and at the shore station. Once locked, the two antennae move, always
pointing towards each other. A maximum canbined tracking speed of 30/sec
is allowable to achieve the angle error specified. The direction of the
fixed station antenna is measured very accurately with a precision shaft
coder, which is mechanically coupled to the main shaft of the antenna.
Measured angle data are transferred to the mobile station via the established
continuous microwave channel. The same microwave 1 ink is used to measure
distance by controlled interruption of the microwave signal. Both angle
and distance are nonnally displayed on the Mobile Control Data Unit, although
readout at the shore station is al so feasibl e. The microwave 1 ink is al so
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