o'Vx
United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-86/050 Jan. 1987
Project Summary
Development of an Adjustable
Buoyancy Balloon Tracer of
Atmospheric Motion:
Phase II. Development of an
Operational Prototype
B. D. Zak, H. W. Church, E. Vy. Lichfield, and M. D. Ivey
The adjustable buoyancy balloon tracer
of atmospheric motion is a research tool
designed to follow atmospheric flows in
both the horizontal and the vertical, in-
cluding the weak sustained vertical motion
associated with meso- and synoptic-scale
atmospheric disturbances. The design
goals for the tracer balloon specify a
lifetime >3 days, tracking range >100 km,
ceiling altitude >5.5 km (500 mb), and
capability to respond to mean vertical
flows as low as 1 cm/s. While the tracer
has applications throughout the atmos-
pheric sciences, the immediate motiva-
tions for this effort are to meet the need
to evaluate the accuracies of air pollution
transport models, to establish source-
receptor relationships to distances of the
order of 1000 km, and to assess the inher-
ent limits on the predictability of source
impacts at long distances. In Phase I of
this project, entitled "Systems Design and
Demonstration of Feasibility," the authors
proposed a generic design for such a sys-
tem, subjected the design to theoretical
analysis, constructed a testbed prototype,
and conducted a series of tests with that
prototype to evaluate the concept. In
Phase II of the project, the subject of this
summary, the authors developed an opera-
tional prototype designed to meet the
desired specifications. A limited number
of test flights of the operational prototype
were conducted south of Albuquerque,
New Mexico, in a nearly vacant 350
square mile area (900 km2) laced with
dirt roads. Flights were made using each
of the three currently available control
algorithms. Analysis of the data indicated
that, in each case, the control system
functioned properly. Phase III of the pro-
ject, system testing and refinement, is cur-
rently under way.
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
The adjustable buoyancy balloon tracer
of atmospheric motion is a physical
Lagrangian tracer (PLT) — an airborne in-
strumentation system that follows the
flow of air in its vicinity, and that can be
tracked electronically, for decades, re-
searchers have sought such a tracer to aid
in understanding the dynamics of the at-
mosphere, and more recently, to cast light
on long-range air pollution: acid deposi-
tion, regional haze and oxidant episodes,
and associated ecological effects. Ad-
vances in microelectronics, satellite com-
munications, and battery technology have
now made a PLT system feasible. The pre-
sent effort is motivated primarily by the
immediate programmatic requirements of
the U.S. Environmental Protection Agency.
In a more basic sense, however, it addres-
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ses a broad underlying need for a conven-
ient means of following atmospheric
flows.
PLTs are necessarily balloon-borne sys-
tems and, consequently, must operate
under Federal Aviation Regulations Part
101: Moored Balloons, Kites, Unmanned
Rockets, and Unmanned Free Balloons.
The purpose of this regulation is to strictly
limit the hazard to air navigation that such
systems might otherwise represent. FAR
101 divides balloons into two classes.
Those that offer little hazard to aircraft
because of their limited size, weight, and
density are explicitly exempted from most
of the other stipulations of the regulation.
Under this provision, the U.S. weather
services, together with the weather serv-
ices of other nations around the world,
routinely launch hundreds of radiosonde
balloons twice daily. Radiosondes measure
the meteorological conditions aloft from
the surface to beyond 20 km above many
major airports and certain other selected
sites. Balloons not satisfying the exemp-
tion provisions of FAR 101 are treated
much like other aircraft, and are subject
to strict regulation.
It is an explicit design goal that the
tracer balloon operate under the exemp-
tion clauses of FAR 101, as do radio-
sondes. If this goal were not met, FAR 101
would severely limit the usefulness of the
tracer balloon.
Because safety is a major consideration,
certain safety features beyond those spec-
ified by FAR 101 are incorporated in the
tracer balloon design. In particular, even
though the Federal Aviation Administration
(FAA) does not require it, when the tracer
balloon is flown at altitudes available to
other aircraft, it will carry an FAA tran-
sponder. Consequently, the FAA will see
the tracer on radar as prominently as a
commercial airliner, and can control air
traffic accordingly.
The intended uses also provide other
tracer balloon design goals:
• Lifetime >3 days.
• Tracking range ^1000 km.
• Telemetry of relative vertical air mo-
tion pressure, temperature, and
humidity.
• Ground system capable of handling
several tracer balloons at a time.
• Capable of establishing specified
ascent and descent rates under radio
command.
• Capable of reaching altitudes up to
5.5 km (500 mb).
• Capable of following mean vertical
flows as low as 1 cm/s with accept-
able fidelity.
• Sufficiently inexpensive to permit use
in significant numbers on an expend-
able basis.
As currently planned, the project is
divided into four phases:
Phase I: Systems Design and Dem-
onstration of Feasibility.
Phase II: Development of an Opera-
tional Prototype.
Phase III: Testing and Refinement of
the Operational Prototype
System.
Phase IV: Addition of Elements Nec-
essary to Create a Practi-
cal Research Tool.
Phase I was initiated in fall, 1983, and ran
through calendar year 1984. The results
were previously reported in the Phase I
report which gives the chain of reasoning
and the experimental data that led to the
operational prototype design described
here. Phase II spanned calendar year 1985.
The principal results are presented in this
summary. Phase III began in spring of
1986 and will extend into 1987. The tim-
ing and duration of Phase IV will be deter-
mined by budgetary considerations and
the needs of the National Acid Precipita-
tion Assessment Program.
Concept
Balloon systems obey Archimedes' Prin-
ciple: A body immersed in a fluid is buoyed
up by a force equal to the weight of the
fluid displaced. This implies that a balloon
system will be in equilibrium when the
weight of the air it displaces is equal to the
weight of the system.
Constant volume balloon (CVB) systems
have a unique characteristic. The equilib-
rium condition is met only at one well-
defined altitude, and the CVB seeks that
altitude. If a CVB should find itself above
its equilibrium altitude, it will experience
a net downward force due to gravity be-
cause the ambient air is less dense at the
higher altitude, and the volume of air dis-
placed is fixed. Likewise, if a CVB should
be below its equilibrium altitude, it will
experience a net upward force, since the
buoyancy force exceeds the gravitational
force. Thus, CVBs tend to oscillate around
their equilibrium altitude, the oscillations
driven by atmospheric turbulence. In at-
mospheric flows that have zero average
vertical velocity, CVBs naturally follow the
horizontal flow at their equilibrium altitude.
However, in flows in which the vertical
component is significant, a CVB will not
adequately follow the overall flow.
The tracer balloon is a modified CVB. It
is designed to sense its deviation from the
mean vertical flow, and to adjust its buoy-
ancy to keep its average vertical motion
relative to the air surrounding it near zero.
Thus, the tracer follows both horizontal
and vertical flows.
The buoyancy adjustment principle used
in the tracer was first put forward by V.E.
Lally of the National Center for Atmos-
pheric Research almost 20 years ago. He
proposed a CVB with an inner bladder, or
ballonet, to contain the lift gas (helium).
The remainder of the CVB was to be filled
with air. A system of pumps and valves
was included to allow air to be pumped in
or released, respectively increasing the
mean density of the balloon and thereby
decreasing its equilibrium altitude, or
decreasing the mean density and thus in-
creasing the equilibrium altitude.
Two basic approaches to the altitude
control problem were considered. The first
was to continuously measure the vertical
velocity of the air relative to the balloon,
and to adjust the buoyancy so that on
average, the relative velocity of the air is
zero — that is, so that the balloon and the
air move together. The second approach
is to take advantage of the very nearly
adiabatic nature of atmospheric flows.
When flows are adiabatic, the potential
temperature (or equivalent potential tem-
perature in the presence of liquid water)
is constant along each air parcel trajectory.
In this approach, the buoyancy is adjusted
so that the trajectory is isentropic. As long
as this condition is met, the balloon will
move along with the air surrounding it.
The approach to altitude control based
on relative vertical air motion is very direct,
but if it were to be used continuously for
three days, the air motion measurement
would have to be extraordinarily accurate.
Under most atmospheric conditions, the
approach based on potential temperature
is quite satisfactory, but in a layer of air
in which convective mixing is taking place,
potential temperature does not offer an
adequate guide to altitude control. Under
these conditions, the air surrounding the
balloon consists of turbulent flows moving
both up and down. The mixing makes the
potential temperature uniform with alti-
tude within the convective layer.
When convective mixing engulfs a par-
cel of air, the main effect is to disperse it
and to spread it out in the vertical, mixing
it with air from all the surrounding parcels.
If a tracer balloon is embedded in an air
parcel that is subjected to convective mix-
ing, as long as the balloon remains in the
mixed layer, it lies within the confines of
the now greatly expanded "parcel." A bal-
loon following the expanding parcel during
convective mixing makes less stringent
demands on the buoyancy control system
than does one following a parcel under
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stable conditions. Hence, a number of dif-
ferent control strategies are satisfactory
under convective conditions.
Thus, an approach that makes use of
different control algorithms under different
meteorological conditions will yield the
best results. On the basis of the sensor
data, the onboard microcomputer will
determine which control algorithm will be
implemented at any given time.
The operational prototype currently
incorporates three control algorithms
selectable by radio command. The control
parameters for these algorithms are rela-
tive vertical displacement potential
temperature, and ambient pressure. In a
stable (non-convective) atmosphere, in the
absence of liquid water, the potential
temperature control algorithm is most
appropriate. In a stable but saturated at-
mosphere, an algorithm yet to be written
using equivalent potential temperature
would be most effective. In a convective
atmosphere, relative vertical displacement
becomes the control parameter of choice.
Under convective conditions, over reason-
ably flat terrain, even the constant pres-
sure control algorithm may be satisfactory.
Operational Prototype
The operational prototype system con-
sists of the balloon envelope with its pay-
load and a ground support station. The bal-
loon is a sphere of 2.9 m nominal diameter
(12.5m3volume) made of 3.0 mil bilamin-
ated polyester with a 1.0 mil full-volume
polyethylene ballonet inside. The balloons
were made by Raven Industries. The pay-
loads consist of a buoyancy adjustment
subsystem, sensors, a microcomputer, a
telemetry subsyetem, a radio command
subsystem, a cutdown device, tracking
aids, and batteries.
An assembly drawing of the operational
prototype is given in Figure 1. Note that
the vertical anemometer and the sensor
package are mounted at opposite ends of
a long styrofoam boom which allows the
measurements to be made at positions
where the influence of the balloon itself
is negligible. A block diagram of the pay-
load is given in Figure 2.
The buoyancy adjustment subsystem
makes use of the same type of pumps and
valves used in the Phase I prototype. The
pumpdown speed — the rate at which the
equilibrium altitude can be lowered — is
determined both by the temperature lapse
rate and by the pressure head against
which the pumps must work, the "super-
pressure." With 30 mb superpressure, in
a standard atmosphere, the calculated
pumpdown speed with the three pumps
Styrofoam
Boom
Sensors
Vertical
Anemometer
Main
Electronics
Package
Batteries
Figure 1. System assembly drawing.
used is about 15 cm/s. The valves permit
a rise in equilibrium altitude about three
times faster.
The sensor subsystem consists of two
elements. The first is an aspirated sensor
assembly that measures ambient pressure,
temperature, and humidity. It is a modified
Atmospheric Instrumentation Research
Inc. prototype digital radiosonde. The
assembly outputs data in ASCII format.
The transmitter normally in place has been
deleted, and the assembly has been
housed in a styrofoam package normally
used for a tethersonde, rather than a radio-
sonde. The tethersonde package makes
provision for aspiration.
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+5V
Sensor
Assembly
ASCI ^
^>
Computer
Wake-Up
f/uv ^T-
Voltage
Regulator
-•-
Lithium
Battery
+5V
Figure 2. Block diagram of operational prototype pay load.
The second element of the sensor sub-
system is the vertical anemometer. It
makes use of a stock 22.9 cm (9 in) dia-
meter expanded styrofoam Gill propeller
from R.M. Young Inc., and a slightly modi-
fied Spaulding Instruments C1 rotation
sensor. It has a starting speed under 2
cm/s, and a measurement threshold of
under 3 cm/s. This starting speed and low
velocity performance should allow accu-
rate relative velocity measurements aver-
aged over minutes to be made down to 1
cm/s or less.
The heart of the control system is an
Intel 8052AH BASIC microcomputer. It
contains 4 K bytes of random access
memory (RAM) and 4 K bytes of program-
mable read-only memory. The control pro-
gram is written in BASIC and is entered
into the microcomputer from a terminal.
Both telemetry and tracking are handled
through the Argos satellite-based data col-
lection and platform location system. The
Argos platform transmitter terminal was
made by Telonics, of Mesa, AZ. It is
uniquely compatible with the flight con-
trol microcomputer. It is controlled by
ASCII input commands.
The radio command receiver was de-
signed and built for our application by
Hock Engineering of Boulder, Colorado. It
operates on 13.8035 MHz, a frequency to
which Sandia has access. It has a one
microvolt sensitivity. The command re-
ceiver antenna is a quarter wavelength
wire.
Commands are electronically encoded
at the ground station transmitter, and
decoded at the receiver. The encoder/
decoder circuit pair are commercially avail-
able components designed for use in tele-
vision remote control systems. Since they
are mass produced, they are quite inex-
pensive. The commands currently avail-
able are:
-activate cutdown
-reset cutdown timer
-initiate control in pressure mode
-initiate control in potential temperature
mode
-initiate control in relative vertical mo-
tion mode
-increment current value of control pa-
rameter
-decrement current value of control pa-
rameter
-turn on pumps.
The encoder/decoder pair is capable of in-
corporating many more commands with
minimal changes.
A cutdown device at the top of the bal-
loon releases the helium lift gas on com-
mand. The rate of venting is such that de
scent takes place at a safe velocity. A cut
down timer automatically actuates the
cutdown device after a preselected period
However, the timer can be reset to zero b\
radio command. In normal operation, th«
timer reset command is sent at frequem
intervals. As long as those commands are
received at intervals not exceeding the
preselected period, automatic cutdown is
avoided. If, on the other hand, radio com-
munication with the tracer is lost for a
period exceeding that which has been pre-
selected, the tracer is automatically re-
moved from the sky. This arrangement
avoids the possibility of the tracer becom-
ing a derelict in the event that radio com-
munication is lost.
The tracking aids (an FAA transponder
and a strobe light) have not been flown on
the operational prototype tracer during the
short flights conducted to date. The FAA
indicated that there was no need for track-
ing aids for flights under 300 m above
ground level in the test area.
The batteries, lithium thionyl chloride
units in AA, C, and D cell sizes procured
from Altus Inc., are reported to have excel-
lent low temperature characteristics. This
property will be necessary to meet the 5.5
km (500 mb) altitude design goal. At this
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altitude, low temperatures will be encoun-
tered. Preliminary tests with the batteries
have been conducted down to -18 °C.
The ground support system for the oper-
ational prototype tests, mounted in a 5-m
Airstream trailer, consists of a Handar
Argos downlink receiver and decoder, an
HP85 desktop computer, a command en-
coder, a Swan command transmitter, and
an antenna coupler. The HP85 converts
the Argos data stream from hexadecimal
to decimal form, formats it, and prints it
out on its integral thermal printer. It also
archives the data on its integral magnetic
tape cassette recorder. An Argos anten-
na and a vertical command antenna were
mounted on a crank-up tower on the
trailer.
Results and Discussion
Testing and refinement of the opera-
tional prototype system were assigned to
Phase III in the project plan. However, it
was felt that at least a minimal test pro-
gram was necessary as part of Phase II to
demonstrate that the operational proto-
type developed here was, in fact,
functional.
The site chosen for the initial flight
testing of the operational prototype tracer
was a large open area approximately 60
km south of Albuquerque known as the
Rio Communities. It is a reasonably flat
area of roughly 350 square miles (900
km2 laced with dirt roads. It slopes from
the western edge, which is near the Rio
Grande River, up to the base of the Man-
zano Mountains on the east.
On December 5,1985, the weather cri-
teria for a test flight of the Tracer Balloon
were met. Surface winds were extremely
light. The balloon itself had been inflated
in a shelter and prepared for launch over
the previous few days. On the morning of
the flight, final weigh off was done as part
of the launch procedures. The tracer bal-
loon was launched at 10:41 am (Figure 3).
The pressure control algorithm was used
for the entire flight and functioned proper-
ly. The tracer remained aloft for 2 h and
20 min, and was recovered about 6 km
from the launch site. The most relevant
data are given in graphical form in Figure 4.
On December 16, weather conditions
were again suitable for test flights. The
first launch took place at 7:38 am. Shortly
after launch, the system was put into the
constant potential temperature control
mode by radio command. As soon as the
tracer climbed above the surface inversion,
it was entrained in the surviving elevated
nocturnal jet and took off at high speed
to the southeast towards rising terrain.
The flight was terminated by the cutdown
timer at about 0.4 h elapsed time when
difficulty was experienced in getting the
timer reset command to the payload from
the ground station. The data indicates that
the potential temperature control algo-
rithm also functioned properly.
Figure 3. Tracer moments after launch.
Because the second flight was short and
was terminated early in the day, the deci-
sion was made to conduct another flight
as soon as preparations could be com-
pleted. The defect noted in the command
system was investigated, but in the time
available, could not be diagnosed and
cured. Hence, it was decided to conduct
a final flight with the intent of allowing the
cutdown timer to terminate the flight at
30 min after launch. This flight duration
was chosen to ensure that the tracer bal-
loon would not be carried by the winds
beyond the limits of the test area into
mountainous or otherwise inaccessible
terrain.
At 3:34 pm the same day, the third flight
was launched. After climbout, the tracer
was put under the control of the zero-
relative motion control algorithm by radio
command. It too functioned properly. The
cutdown timer terminated the flight as
planned. For the third time, the tracer was
brought down with no damage to either
the balloon or the payload.
Recommendations and
Conclusions
The experimental program revealed
some minor electronic flaws in the opera-
tional prototype, and made obvious the
desirability of certain improvements. The
most significant improvements proposed
for Phase III are expansion of the memory
available to the microcomputer from 4 to
16 K bytes and a major change in the pay-
load mechanical design.
In the present system, the 4 K byte
memory limits the entire data processing
and control program to about 200 lines of
BASIC code. With this constraint, the pro-
gram must be very simple. Expansion of
the memory to 16 K bytes will cost very
little in terms of dollars, weight, or power,
but will greatly enhance the capabilities of
the system.
The new payload mechanical design
concept is shown in Figure 5. Here, the
vertical anemometer and the sensor pack-
age are no longer mounted on a styrofoam
boom. Rather, they are mounted on much
smaller rods attached to the balloon skin
at the equator. The balloon itself is used
as a structural element to facilitate mount-
ing the sensors at least 50 cm out beyond
the balloon equator. In addition, with the
exception of the transponder and the
strobe, the other payload packages are at-
tached to the skin of the balloon with
Velcro. Here too, the balloon skin becomes
a structural element. The net effect of the
proposed changes is to simplify the de-
ployment of the payload. This in turn will
simplify the launch procedure and mini-
mize the probability of the Tracer Balloon
getting snared by trees or power lines.
At the end of the Phase I report, after
a detailed evaluation of the proposed
generic systems design, we concluded
that an adjustable buoyancy tracer balloon
meeting the design goals is both techni-
cally and economically feasible. At the end
of the Phase II report, a similar detailed
evaluation of the operational prototype
confirms that conclusion.
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810
Dashed Line is Control Pressure
Upper Line is Potential Temp.
Relative Vertical Air Motion
7 1.5
Elapsed Time - hrs
Figure 4. Data from Flight 1. Control parameter was pressure. The Tracer was launched with
positive buoyancy. In the top graph, note that it overshot the control pressure level,
and then pumped itself back down to it. Note also that the landing site was
significantly higher than the launch Site.
New payload packaging and
deployment concept. Systems
elements: A. vertical anemom-
eter; B. Argosantenna; C. sensor
package; D. buoyancy control
subsystem; E. main electronics
package; F. batteries and beacon
transmitter; G. command re-
ceiver and cutdown package; H.
drip skirt; I. FAA transponder
and strobe light.
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B. D. Zak and H. W. Church are with Sandia National Laboratories. Albuquerque,
NM 87185; E. W. Lichfield is with Spectra Research Institute, A Ibuquerque, NM
87185; andM. D. Iveyiswith Telemetries Southwest, Albuquerque, NM87185.
Thomas £. Pierce is the EPA Project Officer (see below).
The complete report, entitled "Development of an Adjustable Buoyancy Balloon
Tracer of Atmospheric Motion: Phase II. Development of an Operational
Prototype,"(Order No. PB87-100 525/AS; Cost: $ 16.95. 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
U G CFRCSAL
Official Business
Penalty for Private Use $300
EPA/600/S3-86/050
-0 ,*
/- *"*5i r. J ,-
. 0000329 PS
U S ENVI*PROTECTION
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