United States
Environmental Protection
Agency
Environmental Sciences
Research Laboratory
Research Triangle Park NC 27711 V
Research and Development
EPA/600/S3-86/060 Jan 1987
SEPA Project Summary
LORAN-C Tetroon Transponder
and Tracking System
K. L. Clawson, E. F. Pound, C. R. Dickson, and
G. E. Start
An advanced system for tracking multi-
ple regional scale Lagrangian markers was
developed. The system consists of a
miniature tetroon-borne transponder and
a small computerized receiving station
capable of providing continuous real-time
data on tetroon location (latitude/
longitude) and altitude.
The transponder consists of a telemetry
subsystem, a LORAN-C receiver, and a
403 MHz transmitter. The telemetry sub-
system measures atmospheric tempera-
ture and pressure, converts the data into
frequencies, and then transmits these data
together with a transponder ID frequency
to a remote receiving station. Multiple
tetroons can be discriminated at a range
of up to 160 km. The transponder weighs
less than 365 g and can be flown on a
1 m3 tetroon. Battery life is 2 to 6 days.
The data acquisition system consists of an
FM receiver, a LORAN-C navigator, a fre-
quency to digital converter, and a micro-
computer. The receiving station can be
ground-based or mounted in a single
engine aircraft.
LORAN-C position fix uncertainty is 3.7
km. The system was tested both in the
laboratory and in regional-scale field tests.
The path and height uncertainties were for
those tests ±0.3 km and ±30 m,
respectively.
This Project Summary was developed
by EPA's Atmospheric Sciences Research
Laboratory, Research Triangle Park, NC, to
announce key findings of the research pro-
ject that is fully documented in a separate
report of the same title (see Project Report
ordering information at back).
Introduction
Monitoring the movement of air masses
is important in gaining the needed under-
standing of the origin and predicting the
fate of air pollution. One method of ob-
serving atmospheric trajectories and see-
ing the frequency and causes for erron-
eous trajectory estimates is the use of
Lagrangian balloon markers which have
been released into the air mass of interest.
These balloons, called tetroons (tefrahadral
balloons are superpressured to float at a
specific atmospheric density level. Tet-
roons, launched with expendable trans-
ponders, have traditionally been tracked by
ground-based radar deployed in a fixed or
mobile mode. A new methodology for
tracking tetroons on regional scales has
been developed and offers a relatively in-
expensive technique to document and im-
prove fundamental understanding of at-
mospheric transport processes.
Development History
Several different balloon tracking de-
vices were investigated for use in this pro-
ject. A tracking device based on the
LORAN-C navigational signals appears to
be the most readily adaptable navigational
system. LORAN-C or LOng flange >4id to
/Vavigation is a pulsed, low frequency
radio-navigation system used by ocean-
going vessels and commercial and private
aircraft. The navigational signals are emit-
ted on a carrier frequency of 100 kHz from
transmitter chains and are propagated as
both ground (which follow the surface of
the earth) or sky (which bounce off the
ionosphere) waves. Transmission of the
signal from each chain is coordinated from
a single master station within each chain.
The signals from each station in a given
chain are time-delayed at preset intervals.
The location of a giver receiver is deter-
mined by the difference in arrival times of
the various stations signals. Ground wave
coverage includes all coastal waters and
most of the inland area of the U.S. Within
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this area, location uncertainties are less
than ±38 m 95% of the time. Sky wave
propagation fields include all of the U.S.
but at position fix uncertainties of about
±3.7 km 95% of the time. This error is ac-
ceptable since in a large-scale trajectory
study on the order of 1000 km, the error
would be less than 0.5% of the overall
distance.
Three different prototypes of the
LORAN-C transponder and tracking sys-
tem were developed and tested. The final
acceptable transponder prototype used a
very efficient, low-powered, continuous
FM transmitter in conjunction with a very
sensitive tracking receiver. The FM trans-
mitter had an efficiency of 30% at 50 mW
output. The corresponding receiver pre-
amplifier system consisted of a gallium
arsenide field effect transistor to lower the
noise from 6 db to about 0.5 db. Thus the
relatively low transmitter power was easily
recovered by the sensitive receiving sta-
tion. The receiver system was also port-
able and capable of being operated in a
small single engine aircraft or the rear seat
of an automobile. Tests of the prototype
transponder and tracking station in the
CrossAppalachian Tracer Experiment long
range trajectory study (CAPTEX '83)
revealed areas for design improvement.
The most notable problem was with the
transponder transmitter. Transmitter car-
rier frequency drift caused by large
temperature changes was of such a mag-
nitude that, in some cases, the carrier fre-
quency drifted out of the reception range
of the base station receiver. Power output
of the transponder transmitter (50 mW)
also limited reception range to about 80
km. This design was modified to increase
the transmitter output to 250 mW which
doubled the reception range. Carrier fre-
quency drift was also reduced. The effi-
ciency of the base station receiver was
also updated to current state-of-the-art
technology with improved operating
characteristics.
Technical Description
Transponder
The transponder package consists of
three functional modules powered by three
3.4 V lithium batteries: (1) a LORAN-C
receiver, (2) a telemetry or house-keeping
section, and (3) an FM transmitter. Lithium
battery life expectancy is on the order of
2 days for AA size batteries and 6 days
for C size batteries. Each component is in-
expensive, making the cost of the total
unit about $200.00. The transponder and
AA batteries together weigh 260 g. Size
C batteries add an additional 115 g mass
to the transponder. The package is com-
pact (50 mm dia. and 235 mm long) and
can be flown on a 1 m3 tetroon. FAA
regulations governing the operation of un-
manned free balloons does not apply since
the weight to size ratio is 3.2 g cm~2.
Consequently there are no restrictions on
their use.
Base Station
The base or receiving station consists
of an antenna, a pre-amplifier section, a
400.15 - 406.00 MHz receiver, a LORAN-C
navigator, a frequency to digital converter,
and a microcomputer for data acquisition
and control. The most critical portion of
the base station is the series of pre-amps
which amplify the relatively small trans-
ponder signal. All of the base station com-
ponents are small enough to be mounted
either in a small van or single engine
airplane. The onetime acquistion cost of
these components is about $9000.00.
System Evaluation
All transponder prototypes and tracking
devices were tested to evaluate system
performance. These tests included trans-
ponder performance and calibration
checks in a pressure/temperature cham-
ber, transponder position accuracy checks,
transponder signal degradation checks,
and base station reception range and
system reliability. Transponder and receiv-
ing station prototypes which failed to per-
form satisfactorily were successively mod-
ified until adequate prototypes were de-
signed. Transponder prototype #3 and cor-
responding tracking system passed initial
tests and were subsequently tested dur-
ing CAPTEX '83.
Local Field Tests
Transponder-relayed LORAN-C signal
accuracy and discrimination capability of
the FM receiver were checked in two local
field tests. Reliability and accuracy of
the transponder relayed LORAN-C signals
were checked by lifting a transponder with
a tethered balloon to 100 m AGL and com-
paring the transponder LORAN-C output to
that of a second ground-based LORAN-C
receiver at the same location. The ground-
based receiver and the transponder
LORAN-C signals agreed with each other
and the known latitude and longitude of
the location to within 0.04 min latitude or
longitude. Repeated checks showed no
difference in compared LORAN-C
readings.
Discrimination capability of multiple
transponders by the FM receiving station
was checked by placing four transponders
on a tethered balloon. The carrier frequen-
cy of each transmitter was tuned such
that only 1 MHz separated each trans-
ponder from its nearest neighbor. Each
transponder was easily distinguished from
the other three in this mode regardless of
distance to the receiver, and LORAN-C and
telemetry signals were correctly acquired.
Hence, multiple targets launched simulta-
neously are expected to be easily
followed.
Operation Field Use
Following local evaluations, the proto-
type transponder was used during
CAPTEX '83. A total of 27 tetroons with
accompanying transponders were launch-
ed and tracked from a dual engine airplane
for various distances. Eighteen tetroons
were released from Dayton, Ohio (39°
54.03' N, 84° 11.86' W) during the period
25 September through 28 October. The
remaining tetroons were released from a
location near Sudbury, Ontario, Canada
(46° 38.60' N, 80° 54.90' W). The
tetroons were released in support of the
diverse aspects of CAPTEX '83 and were,
therefore, launched to various heights. On
occasion, multiple tetroons were launched
simultaneously. Single transponders were
tracked almost continuously, except for
aircraft refueling periods. Transponder
signals were detected and identified at
ranges up to 80 km from the tracking air-
craft. For multi-transponder tracking, the
aircraft tracked a single tetroon for 15 to
30 minutes, then located and tracked
another tetroon for another 15 to 30
minutes. Time required to lock-on to a
transponder and to obtain reliable readings
was from 5 to 20 minutes. Once locked
on, three to four readings were logged and
plotted during the 15 to 30 minute track-
ing period. After reviewing the signals,
those considered most reliable were
printed and stored by the microcomputer.
Ten of the eighteen transponders were
tracked for periods of greater than 10
hours. Five transponders were tracked for
distances over 500 km, with one for a
distance of nearly 1000 km. Some tet-
roons could have been tracked for longer
time periods but were abandoned due to
deteriorating weather conditions. Other
tetroons launched near the ground be-
came entangled in vegetation but their
transponders continued to transmit for
several days thereafter.
Supplementary Tests
Following CAPTEX '83 the transponder
was modified to minimize carrier frequen-
cy drift and increase transmission range.
The transponder and tracking system were
subsequently tested in 3 flights on 04
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April 1986 for signal degradation and
transmission range. The tracking system
was mounted in a single engine aircraft
while the transponder was flown from a
tethered balloon about 120 m above
ground level. The test was conducted from
1100 to 1830 hours MST and consisted of
a series of fly-aways from the transponder.
Transmission range was observed to be
about 160 km. All data collected within
the 160 km radius was found to be strong
and reliable. When the tracking station ex-
ceeded 160 km, the transmitted signal
rapidly degraded into the background
noise. The signal was easily reacquired
after the tracking station returned to the
160 km limit.
Precision and accuracy of the LORAN-C
navigator output was well within accept-
able error limits. The largest deviation from
true location was 6.82 km obtained at
1124 hours and was due to intermittent
interference of the LORAN-C signal by
some other local sourca All other location
errors were less than 2 km, even when the
tracking stations were located at 160 km
from the transponder. The average error,
excluding the 1124 data was 0.31 km with
a standard deviation of 0.44.
Conclusions and
Recommendations
The transponder and receiving station
show great potential for both long and
short range tetroon tracking projects but
are probably better suited for the former.
The system is now useable and is recom-
mended for operation in future trajectory
studies.
The system was developed using public
funding provided, in part, through the
sponsoring agency. Microcomputer pro-
grams and base receiving station sche-
matics excluding the antenna and pre-
amps are available from the Air Resources
Laboratory Field Research Division located
in Idaho Falls, ID. Transponder and base
receiving antenna schematics are held by
the E. F. P. Co. of Logan, Utah.
K. L Clawson, £. F. Pound. C. R. Dickson, and G. E. Start are with the National
Oceanic and Atmospheric Administration, Idaho Falls, ID 83402.
John f. Clarke is the EPA Project Officer (see below).
The complete report, entitled "LORAN-C Tetroon Transponder and Tracking
System," (Order No. PB 87-116 687/AS; Cost: $13.95, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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Penalty for Private Use $300
EPA/600/S3-86/060
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