United States
Environmental Protection
Agency
Atmospheric Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/027 May 1985
Project Summary
Development of an Adjustable
Buoyancy Balloon Tracer of
Atmospheric Motion: Phase I.
Systems Design and
Demonstration of Feasibility
B. D. Zak, H. W. Church, A. L. Jensen, G. T. Gay, and M. D. Ivey
An adjustable buoyancy balloon trac-
er of atmospheric motion is a research
tool which allows one to electronically
track atmospheric flows in both the
horizontal and the vertical, including
the weak, sustained vertical motion
associated with meso- and synoptic-
scale atmospheric disturbances. The
design goals for the balloon tracer
specify a lifetime >3 days, tracking
range >1000 km, a ceiling altitude
>500 mbar (5.5 km), and the capability
to respond to mean vertical flows as low
as 1 cm/s. The balloon tracer is also to
measure and telemeter selected meteor-
ological variables, to be sufficiently
inexpensive to permit use in significant
numbers, and to be serviced by a ground
system capable of handling several
balloon tracers at a time. The balloon
tracer has applications throughout the
atmospheric sciences, but the immedi-
ate motivation for this effort is to
provide a means to evaluate the ac-
curacy of air pollution transport models
for the Eastern United States. The
authors have proposed a generic design
for such a system, have subjected that
design to theoretical analysis, have
constructed a prototype, and have con-
ducted a series of tests with the proto-
type to evaluate the concept. They
conclude without reservation that a
system meeting the design goals is
feasible, and are proceeding to build
that system in Phase II of this project.
This Project Summary was developed
by EPA's Atmospheric Sciences He-
search Laboratory, Research Triangle
Park. NC. to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
An adjustable buoyancy balloon tracer
of atmospheric motion is a physical
Lagrangian tracer (PLT), an airborne
instrumentation system that follows the
flow of air, and that can be tracked
electronically (Figure 1). Such a system
has been desired for decades by research-
ers in the atmospheric sciences to aid in
understanding the dynamics of the atmos-
phere, and to cast light on long-range air
pollution. The present effort, however, is
motivated primarily by the more immedi-
ate need to establish source-receptor
relationships to distances of order 100
km, to evaluate the accuracies of air
pollution transport models, and to assess
inherent limits on the predictability of
source impacts at long distances.
The adjustable buoyancy balloon sys-
tem must operate under Federal Aviation
Regulations Part 101, which covers un-
manned free balloons. FAR 101 divides
such balloons into two classes. Those
which offer little hazard to aircraft be-
cause of their limited size, weight, and
density are explicitly exempted from most
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Figure 1. Prototype Adjustable Buoyancy Balloon Tracer being readied for flight. Pay load
weighs 2.36 kg(5.2 Ibs) including batteries, and is constructed of styrofoam covered
with 0.6 oz fiberglass. It meets the exemption clauses of Federal Aviation
Regulations Pan 101.
of the other stipulations of the regulation.
So-called "weather balloons" (radio-
sondes) fall in this category. Hundreds of
such balloons are launched twice a day
from sites all over the US and around the
world to provide data on meteorological
conditions aloft. Balloons not meeting the
conditions contained in the exemption
clauses of FAR 101 are subject to strict
regulation, and are treated much like
other aircraft. It is highly desirable for the
balloon tracer to operate under the ex-
emption clauses, in that certain other
provisions of FAR 101 would seriously
limit the usefulness of a balloon tracer
was not exempt. Even though the adjust-
able buoyancy balloon system will be
exempt, it will nevertheless carry a radar
reflector and a Federal Aviation Admin-
istration transponder so the FAA can
independently keep track of its location.
In addition to meeting the exemption
conditions of FAR 101, the design goals
for the balloon tracer are:
Lifetime > 3 days
Tracking range > 1000 km
Telemetry of selected meteorological
parameters
Ground system capable of handling
several PLTs at a time
Ceiling altitude >500 mbar (5.5 km)
Ability to follow mean vertical flows as
low as 1 cm/s
Sufficiently low cost for use in signif-
icant numbers.
The project is divided into two phases:
Phase I. Systems Design and Demon-
stration of Feasibility.
Phase II. Development of an Opera-
tional Prototype.
This project summary and the associated
project report cover work on Phase I.
Phase II is now proceeding.
Concept
The design of the adjustable buoyancy
balloon tracer is based upon an idea put
forward by V. Lally of the National Center
for Atmospheric Research in 1967 (Figure
2). Here the outer skin of a spherical
balloon is made of a high modulus of
elasticity material which expands very
little as pressure in the balloon increases.
Hence, the volume of the balloon is very
nearly constant as long as the pressure of
the gas inside is greater than the ambient
pressure. A thin polyethylene bag, or
"ballonet," separates the interior into
two compartments. One of these compart-
ments is filled with helium and the lift
gas. The other is filled with air. The air
serves as ballast. A pump and valve
permit additional air to be taken into the
balloon or to be released. When the
balloon is at its equilibrium altitude and
more air is pumped in, the balloon
becomes heavier and sinks to a lower
altitude (pump-down). When air is re-
leased through the valve, the balloon A
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Figure 2. Schematic Diagram of the Ad-
justable Buoyancy Balloon. The
outer skin of the balloon is made
of a material which expands
very little as the internal pres-
sure increases. A thin polyeth-
ylene inner balloon, or "ballonet,"
keeps the helium lift gas separate
from the air ballast. A pump (P)
permits more ballast air to be
taken on. A valve (VJ permits
ballast air to be vented. The total
volume remains nearly constant,
so pumping or valving changes
the average density of the sys-
tem, and thus its altitude.
becomes lighter and rises to a higher
altitude (valve-up).
Expressions have been derived which
describe the rate at which pumping and
valving change the equilibrium altitude,
the behavior of the excess of internal over
ambient pressure (superpressure) as a
function of equilibrium altitude, the effect
of temperature changes on balloon pres-
sure, the energy required for pump-down,
how the ceiling altitude is determined by
system parameters, and a procedure for
properly filling the balloon to obtain the
desired characteristics. All of these calcu-
lations confirm that a properly designed
balloon system of the type originally
proposed by Lally can meet the design
goals.
Given a means of adjusting the buoy-
ancy of a constant volume balloon, the
balloon will become a tracer for atmos-
pheric motion if the buoyancy is period-
ically adjusted so that the balloon follows
the vertical motion of the air. The nature
of balloons is such that they naturally
..follow horizontal air motions. Hence, if a
palloon system is constructed to also
follow the vertical motions, that system
will follow the overall flow.
There are two basic approaches to the
altitude control problem. The first is 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 zerothat
is, so that on average, 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 tra-
jectory. In this approach, the buoyancy of
the balloon is adjusted so that the poten-
tial temperature is kept constant. As long
as this condition is met, the balloon will
move along with the air surrounding it.
Thus, isentropic trajectories are a good
approximation to actual air parcel tra-
jectories. These may differ dramatically
from the isobaric trajectories approxi-
mated by tetroons or other passive,
constant-volume balloons (Figure 3).
The approach to altitude control based
on relative vertical air motion is most
direct, but if it were to be used contin-
uously for 3 days, the air motion measure-
ments would have to be extraordinarily
accurate. Under most atmospheric condi-
tions, the approach based on potential
«_ .^-X
Figure 3. Comparison of calculated 12-hour isobaric and isentropic trajectories originating at
700 mb at 0300 GOT 28 March 1956, from a 1961 paper by E. Danielson. After 12
hours, the horizontal deviation is 1300 ±200 km. Tetroons and other passive constant
volume balloons approximate isobaric trajectories, whereas air parcel trajectories are
nearly isentropic, and hence much better represented by the Adjustable Buoyancy
Balloon Tracer.
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temperature is quite satisfactory; but
when the system is in a layer of air in
which active convective mixing is taking
place, potential temperature does not
offer an adequate guide for altitude
control. Under convective mixing condi-
tions, the air surrounding the balloon
consists of turbulent airflows moving up
and down. The mixing makes the poten-
tial temperature uniform with altitude
within the mixed layer.
When convective mixing engulfs a
"parcel" 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 balloon tracer is
embedded in an air parcel which is
subjectedto convective mixing, as long as
the balloon remains in the mixed layer, it
is within the confines of the now greatly
expanded "parcel." A balloon following
the expanding parcel during convective
mixing makes less stringent demands on
the buoyancy control system than does
one following a parcel in the absence of
convective activity. Hence, a number of
different control strategies are satisfac-
tory during these periods.
Thus it appears that a hybrid control
approach will yield best results. Different
control strategies will be employed under
different meteorological conditions. On
the basis of the data from the onboard
sensors, an onboard microcomputer will
determine which control strategy will be
implemented at any given time.
Systems Design
Having confirmed on paper that the
adjustable buoyancy balloon tracer is
viable in principle, we proceeded to
formulate a preliminary design for an
operational system. The major elements
of the design are the tracking and data
handling system, the balloon envelope
itself, the balloon payload, and the ground
support station.
Tracking and data reception will be
handled by the ARGOS satellite-based
data collection and platform location
system. ARGOS is a joint undertaking of
NASA, NOAA, and Centre National d'-
EtudesSpatiales(CNES, France). It makes
use of NOAA satellites and of both US and
French ground support facilities to serve
fixed and moving platforms collecting
environmental data. ARGOS has the
advantages that it is well-proven, has a
high data recovery rate, and provides
worldwide coverage, and that lightweight
hardware designed for use on balloons is
commercially available.
The balloon envelope design was under-
taken in consultation with NCAR and in
collaboration with Raven Industries, the
maker of NCAR's high altitude constant
volume balloons. The initial design adopt-
ed is for a spherical balloon of 2.9-m
diameter, 12.5-m3 volume, madeof3-mil
bilaminated polyester (mylar) film with a
1-mil polyethylene ballonet inside. The
balloon was designed to carry up to a 4.5-
kg (10-lb) payload to 600 mbar and to
have an operational superpressure limit
in excess of 80 mbar. With a lighter
payload, the ceiling altitude will exceed
the design goal of 500 mbar (5.5 km;
18,000 ft).
The payload consists of a buoyancy
adjustment subsystem, sensors, a micro-
processor or microcomputer, a telemetry
subsystem, a radio command subsystem,
tracking aids, and batteries. The buoyancy
adjustment subsystem consists of the
pumps, valves, and associated plumbing
mentioned earlier. The initial list of
sensors numbers 12, and includes those
necessary to follow either a zero relative
vertical velocity, or a constant potential
temperature altitude control strategy.
The microprocessor or microcomputer
processes all data, formats them for
ARGOS transmission, and uses them in a
control algorithm to determine what alti-
tude control measures should be taken to
follow the mean vertical airflow: no
action, vent air, or pump more air in. The
telemetry subsystem is, in ARGOS par-
lance, a platform transmitter terminal.
The radio command subsystem is a high-
frequency radio receiver and command
decoder enabling the user to override the
onboard computer control. The tracking
aids consist of an FAA transponder, a
radar reflector, and a strobe to aid in
visual tracking. The batteries are state-of-
the-art flexible ("paper") lithium batteries
with high power-to-weight ratio.
The ground support station (GSS) con-
sists of an ARGOS local user terminal
(LUT), an ARGOS uplink receiver, a
radiotheodolite or LORAN tracking sys-
tem, a command transmitter, and a
desktop computer with associated periph-
erals. The LUT allows one to receive data
from the balloon tracer in real time via
retransmission from the satellite when-
ever the satellite is within range of the
tracer and within range of the LUT. The
ARGOS uplink receiver allows one to
listen directly to the data system being
transmitted by the balloon tracer when it
is within radio range of the ground station.
The radiotheodolite or LORAN system
provides for local tracking of the tracer
when it is being used within radio range.
The command transmitter is a multi-band
transmitter capable of sending commands
to the balloon tracer over long distances.
The computer receives, formats, archives,
and displays the location and meteoro-
logical data as desired. It also includes a
modem to provide for data reception by
phone.
The GSS accommodates three modes
of use: (a) satellite/worldwide; (b) hybrid/
regional; and (c) ground-based/local. In
the satellite/worldwide mode, location
and meteorological data are received from
the tracer via the satellite either through
the LUT or through NOAA facilities ac-
cessed by phone. The LUT gives one the
data in real time, whereas the data are
available from NOAA approximately 6 h
later. Data are received by the satellite
only when it is within radio range of the
balloon tracer, about 10 min every 2 to 4
h. Depending upon the design of the
balloon tracer payload, the data transmit-
ted may be only the current values of the
measurements being made by the sen-
sors, or it may be all the values recorded
over the previous several hours.
In the hybrid/regional mode, the uplink
receiver and other remotely located uplink
receivers are strategically located so that
the balloon tracer is within radio range of
at least one everywhere within the region
of interest. Consequently, continuous
real-time data reception and archiving
are available over the region covered by '
the uplink receiver network. Tracking is
still accomplished by satellite.
In the ground-based/local mode, the
satellite link is not used at all. The balloon
tracer is locally tracked, and the data are
acquired directly by the uplink receiver.
This mode of use is limited by radio range.
Testbed Prototype
Demonstration of technical feasibility
was accomplished by fabrication and
evaluation of a "testbed prototype" (TP)
balloon tracer. The TP is sufficiently
similar to the flight system proposed in
the systems design so as to establish
feasibility, but does not meet all of the
design goals itself. The major differences
between the TP and an operational bal-
loon tracer are that the TP is designed for
local use only, and that it incorporates
elements which make changing the con-
trol algorithm easy.
The TP consists of a balloon much like
those for the operational tracer system,
and a payload consisting of a buoyancy
adjustment subsystem, an AIR (Atmos-
pheric Instrumentation Research, Inc.)
airsonde circuit board located externally,
a radio control command receiver, bat- .
teries, and a strobe (Figure 4). U
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Figure 4. Assembled Testbed Prototype Payload. Tubing is air ballast vent line. Wires are
airsonde andtethersonde antennae. Smaller diameter cylinder contains the aspirated
temperature and humidity probes. Payload is made of styrofoam covered with 0.6 oz
fiberglass.
The TP is flown under the control of a
prototype ground station, which consists
of an AIR ADAS (Atmospheric Data
Acquisition System) unit, an HP85 desk-
top computer, and HP3421 data acquisi-
tion and control unit, and a radio com-
\ mand transmitter. The ADAS receives the
data from the airsonde and tethersonde,
which give data on conditions inside the
balloon and in the ambient atmosphere,
respectively. The HP85 processes, ar-
chives, and analyzes the data. The control
algorithm is resident in the HP85. Altitude
control actions are transmitted back to
the TP via the HP342I and the command
transmitter. This arrangement allows the
control program to be written in a high-
level language, and to be altered on the
ground with a few keystrokes, even when
the TP is in flight. Almost exclusive use of
minimally modified, commercially avail-
able elements in the TP design made
demonstration of feasibility possible with-
in project time and resource constraints.
Experimental Program
The Phase I experimental program was
limited to the minimum necessary to
demonstrate that the concept of the
adjustable buoyancy balloon tracer is
viable. Initially, measurements were
made in the laboratory on individual
components to determine if their perform-
ance was satisfactory. Next, the testbed
prototype underwent tests in an enclosed
tower. Finally, testing began in the am-
bient atmosphere.
The most telling results were obtained
in the tower. The tower is part of the Solar
Central Receiver Test Facility at Sandia
National Laboratories. It offers an en-
closed volume roughly 10 m square by 52
m high. Since it is enclosed, it provides a
more controlled environment than does
the ambient atmosphere, which makes it
easier to interpret test results.
Measurements were made on several
pump-down and valve-up cycles in the
tower (Figure 5). The results made clear
that the theory developed does indeed
describe the behavior of the balloon
tracer. They also made clear that the
tracer's behavior is more complex than is
obvious from the expressions derived
under the assumption of dynamic equi-
librium. The equilibrium theory may be
thought of as describing the behavior of
the equilibrium altitude of the balloon
tracer, rather than its actual instantan-
eous position as a function of time. The
balloon tracer oscillates around its equi-
librium altitude, as its other parameters
oscillate around their equilibrium values.
The dynamic effects influence the details,
but not the gross features of tracer balloon
behavior. Testing in the ambient atmos-
phere confirmed the tower results.
Conclusion
The authors have examined the design
goals in light of theoretical analysis, their
experience in designing and building the
testbed prototype balloon tracer, and the
experimental results. They conclude with-
out reservation that an adjustable buoy-
ancy balloon tracer of atmospheric motion
meeting the design goals is feasible. They
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50
B. D. Zak. H. W. Church, A. L Jensen. G. T. Gay, and M. D. Ivey are with Sandia
National Laboratories, Albuquerque, NM 87185.
J. S. Irwin and P. G. Lamb are the EPA Project Officers (see below).
The complete report, entitled "Development of an Adjustable Buoyancy Balloon
Tracer of Atmospheric Motion: Phase I. Systems Design and Demonstration of
Feasibility," (Order No. PB 85-185817/AS; Cost: $ 16.00. 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 Officers can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC27711
500
Timefs)
Figure 5.
Results of a valve-up experiment
in the solar tower. Valve to
release ballast air was opened
at A. From B to C, the mean
vertical velocity was 8.1 cm/s;
from C to D, 24.9 cm/s. These
measured vertical velocities are
smaller than the actual rate of
change of the equilibrium alti-
tude because of drag forces and
other dynamic effects.
are proceeding in Phase \\ to turn this
conviction into operational hardware.
4
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