United States Environmental Protection Agency s>" Atmospheric Sciences ; Research Laboratory ~*f., « Research Triangle Park. NC 27711 / /,"- Research and Development EPA/600/S3-88/015 May 1988 Project Summary Development of an Adjustable Buoyancy Balloon Tracer of Atmospheric Motion: Phase III. Refinement of the Operational Prototype System B. D. Zak and E. W. Lichfield The adjustable buoyancy balloon tracer of atmospheric motion is a research tool designed to follow 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 tracer balloon specify a lifetime > 3 days, tracking range > 1000 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 atmospheric sciences, the immediate motivations 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 inherent 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 system, subjected the design to theoretical analysis, constructed a test-bed prototype, and conducted a series of tests with that prototype to evaluate the concept. In Phase II of the project, the authors developed an operational prototype designed to meet the desired specifications. A limited number of test flights of the operational prototype were conducted. Flights were made using each of the three currently available control algorithms: constant pressure, constant potential temperature, and zero relative motion. Analysis of the data indicated that, in each case, the control system functioned properly. In Phase III of the project, the subject of this summary, improvements were made in electronic design, packaging, and in the remote command and data reception system. Two test flights were made, and while the results from these flights were inadequate to demonstrate that the design goals were met, the results were not inconsistent with that conclusion. This Project Summary was developed by EPA's Atmospheric Sciences Research Laboratory, Research Triangle Park, NC, to announce key findings of the research project 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 instrumentation system that ------- follows the flow of air in its vicinity, and that can be tracked electronically. For decades, researchers have sought such a tracer to aid in understanding the dynamics of the atmosphere: and more recently, to cast light on long-range air pollution: acid deposition, regional haze and oxidant episodes, and associated ecological effects. Advances in microelectronics, satellite com- munications, and battery technology have now made a PLT system feasible. The present effort is motivated primarily by the immediate programmatic requirements of the U.S. Environmental Protection Agency. In a more basic sense, however, it addresses a broad underlying need for a convenient means of following atmospheric flows. PLTs are necessarily balloon-borne systems and, consequently, must operate under Federal Aviation Regulations Part 101: Moored Balloons, Kites, Vlnmanned Rockets, and Unmanned Free Balloons. The purpose of this regulation is to strictly limit the hazard ^o 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 services 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 exemption 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 exemption clauses of FAR 101, as do radiosondes. 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 specified 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 transponder. 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 motion, 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 acceptable fidelity. Sufficiently inexpensive to permit use in significant numbers on an expendable basis. The original project plan divided the work into four phases: Phase I: Systems Design and Demonstration of Feasibility. Phase II: Development of an Operational Prototype. Phase III: Testing and Refinement of the Operational Prototype System. Phase IV: Addition of Elements Necessary to Create a Practical 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. Phase III began in spring of 1986 and extended through 1987. The US EPA does not have resources to support Phase IV. Concept Balloon systems obey Archimedes' Principle: 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 equilibrium condition is met only at one well-defined altitude, and the CVB seeks that altitude. If a CVB should I itself above its equilibrium altitude, it experience a net downward force due gravity because the ambient air is li dense at the higher altitude, and volume of air displaced is fixed. Likewi if a CVB should be below its equilibrii altitude, it will experience a net upw. force, since the buoyancy force excee the gravitational force. Thus, CVBs te to oscillate around their equilibrii altitude, the oscillations driven atmospheric turbulence. In atmosphe flows that have zero average vertic velocity, CVBs naturally follow tl horizontal flow at their equilibria altitude. However, in flows in which tl vertical component is significant, a C\ will not adequately follow the overall flo\ The tracer balloon is a modified CV It is designed to sense its deviation fro the mean vertical flow, and to adjust i buoyancy to keep its average vertic motion relative to the air surrounding near zero. Thus, the tracer follows bo horizontal and vertical flows. The buoyancy adjustment princip used in the tracer was first put forward b V.E. Lally of the National Center f< Atmospheric Research almost 20 yeai ago. He proposed a CVB with an inn| bladder, or ballonet, to contain the lift ga (helium). The remainder of the CVB wa to be filled with air. A system of pump and valves was included to allow air to b< pumped in or released, respective! increasing the mean density of thi balloon and thereby decreasing it: equilibrium altitude, or decreasing th< mean density and thus increasing thf equilibrium altitude. Two basic approaches to the altituds control problem were considered. Th< first was to continuously measure th« vertical velocity of the air relative to the balloon, and to adjust the buoyancy sc that on average, the relative velocity ol the air is zero - that is, so that the balloon and the air move together. The second approach is to take advantage ol the very nearly adiabatic nature o1 atmospheric flows. When flows are adiabatic, the potential temperature (or equivalent potential temperature 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 basei on relative vertical air motion is very ------- ect, 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 altitude within the convective 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 tracer balloon is embedded in an air parcel that is subjected to convective mixing as long as the balloon remains in the mixed layer, it lies 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 arcel under stable conditions. Hence, a .umber of different 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 on-board microcomputer can be programmed to 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 relative 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 atmosphere, 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 reasonably 'lat terrain, even the constant pressure jontrol algorithm may be satisfactory. Operational Prototype The operational prototype system consists of the balloon envelope with its pay load and a ground support station. The balloon is a sphere of 2.9 m nominal diameter (12.5 m volume) made of 3.0 mil bilaminated polyester with a 1.0 mil full-volume polyethylene ballonet inside. The balloons were made by Raven Industries. The payloads consist of a buoyancy adjustment subsystem, sensors, a microcomputer, a telemetry subsystem, a radio command subsystem, a cutdown device, tracking aids, and batteries. An assembly drawing of the operational prototype is given in Figure 1. The inner balloon is attached to the outer balloon only at the helium fill fitting located at the top. The bottom fitting, a 19 cm diameter nylon plate, was modified to accept two fittings for air lines, and electrical leads to accommodate a pressure sensor assembly inside the balloon. A larger fitting, closed off in flight, was also installed on the bottom plate to permit rapid inflation and deflation. 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 *superpressure." With 30 mb superpressure, in a standard atmosphere, the calculated pumpdown speed with the three pumps used is about 15 cm/s. The valves permit a rise in equilibrium altitude about three times faster. The sensor subsystem consists of three 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 radiosonde. The tethersonde package makes provision for aspiration. The second element is a second modified digital AIR sensor assembly mounted on the inside of the baseplate to report pressure and temperature within the balloon. No aspiration is provided for this unit. The third element is the vertical anemometer. It makes use of a stock 22.9 cm (9 in) diameter expanded styrofoam Gill propeller from R. M. Young Inc., and a slightly modified Spaulding Instruments Cl 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 accurate relative vertical velocity measurements averaged over minutes to be made down to 1 cm/s or less. The heart of the control system is an Intel 8052AH BASIC microcomputer. For Phase III, the memory was increased from 4 K bytes of random access memory (RAM) and 4 K bytes of programmable read only memory (PROM) to 16 K bytes each of RAM and PROM. The control program is written in BASIC and is entered into the microcomputer from a terminal. To provide reliable polling of the sensor values, a Universal Syn- chronous/Asynchronous Receive and Transmit chip (USART) was added, along with a machine language subroutine to service the USART. A block diagram of the payload system is shown in Figure 2. Both telemetry and tracking are handled through the Argos satellite- based data collection and platform location system. The Argos platform transmitter terminal was made by Telonics, of Mesa, AZ. It is uniquely compatible with the flight control microcomputer. It is controlled by ASCII input commands. The radio command receiver was designed and built for application by Sandia National Laboratories 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 receiver antenna is a quarter wavelength wire which was attached to the skin of the outer balloon, rather than left to trail below. Commands are electronically encoded at the ground station transmitter, and decoded at the receiver. In Phase II, the encoder/decoder circuit pair were commercially available components designed for use in television remote control systems. This system was found to be highly vulnerable to electronic noise. The command system was ------- Figure 1. Assembly drawing of Phase III Tracer Balloon. A. Top fitting with cutdown device and helium fill line. B. Vertical anemometer. C. Cutdown timer. D. Recovery beacon (not flown on flight 2). E. External sensor assembly. F. Radar comer reflector. G. Radio command receiver. H. Independent airsonde. I. Internal sensor assembly. J. Bottom plate with valves and servo. K. Pump box. L Main electronics package. M. Argos antenna. N. FAA transponder package (not flown in flight 1). changed to be more noise resistant through the use of dual tone multifrequency coding. The same pair of tones must be present on multiple interrogations for the command to be considered valid. The commands currently available are: - activate cutdown - reset cutdown timer - turn on pumps (to lower altitude) - open valve (to increase altitude) - initiate control in pressure mode - initiate control in potential temperature mode - initiate control in relative vertical motion mode - increment current value of control parameter - decrement current value of control parameter The encoder/decoder pair is capable of incorporating many more commands with minimal changes. A cutdown device at the top of the balloon releases the helium lift gas on command. The rate of venting is such that descent takes place at a safe velocity. A cutdown timer automatically actuates the cutdown device after a preselected period. However, the timer can be reset to zero by radio commanc In normal operation, the timer rese command is sent at frequent intervals. A long as those commands are received <. intervals not exceeding the preselectei period, automatic cutdown is avoided. I1 on the other hand, radio communicatioi with the tracer is lost for a perioi exceeding that which has bee preselected, the tracer is automatical! removed from the sky. This arrangemer avoids the possibility of the trace becoming a derelict in the event tha radio communication is lost. For both test flights, a corner-typl radar reflector was flown, and on thi second flight, a FAA transponder witl ------- System Block Diagram +5v External Sensor Assembly Internal Sensor Assembly ^, ^ -T» USART ^ ? Computer +5v Wake-Up ^__ Voltage Regulator ^ Lithium Battery +5v Figure 2. Block diagram of Phase III Operational Prototype Pay load. +Sv encoding altimeter was flown. Thus far, no night flights have been conducted, hence a strobe light has not been needed. The batteries, lithium thionyl chloride units in AA, C, and D cell sizes procured from Altus Inc., are reported to have excellent low temperature characteristics. This property will be necessary to meet the 5.5 km (500 mb) altitude design goal. At this altitude, low temperatures will be encountered. Preliminary tests with the batteries have been conducted down to -18 C. The command and data reception system used in Phase III consists of a Handar Argos downlink receiver and decoder, an IBM compatible PC (NEC APCIII), a command encoder and interface package, an ICOM model 745 transceiver with FM option and an antenna coupler. A block diagram of the system is shown in Figure 3. For the first test flight, the system was mounted in ia 5-m Airstream trailer. For the second flight, planned to be much longer, the system was mounted in the Sandia de Havilland Twin Otter instrumented aircraft. The PC converts the Argos data stream from hexadecimal to decimal form, formats it, provides a hard copy listing of the results, and stores the results on the NEC internal hard disk. Inflation of the Tracer Balloon in the field without shelter would be very difficult. For Phase III, a conventional hangar at a conveniently located airport was used. Results and Discussion The Phase III test flights were launched from Sandia Air Park, a small airport about 40 km east of Albuquerque near the community of Edgewood. This site is east of the Sandia and Manzano Mountains, so flights made under the influence of the usual westerly winds would not immediately encounter rough terrain. The first flight was planned for an altitude of about 300 m. To aid recovery and tracking, a Telonics radio beacon designed for animal tracking was flown. Liftoff of flight 1, shown in Figure 4, occurred at 10:49 a.m. on August 13, 1987. Winds at the surface at launch time were estimated to be gusting to 15 kts. For this flight, the Argos antenna on the balloon was a high gain device designed by NCAR for use on high altitude balloons. The radiation pattern of this antenna is primarily vertical, thus making reception at the ground from a system at low altitude to the ground difficult. With the high winds encountered in this first flight, the Tracer Balloon was carried out of range of the data reception within a quarter of an hour. Data from the AIR sonde attached to the skin of the balloon was received for about a half hour, until a parked aircraft came between the ground antenna and the Tracer Balloon. The AIR signal was reacquired just before touchdown, but by that time the Tracer was no longer in sight. The Tracer Balloon appears to have responded to commands throughout the flight. It was decided, given the loss of reception of the Argos data at the ground station, to allow the automatic cutdown timer to terminate the flight. Touchdown occurred ------- Ground Station Components H.F. Antenna ARGOS Antenna (401.65 MHz) \ Keyboard] Figure 3. Block diagram ol Phase III command and data reception system. 14 km of the launch site, at about 11:59 a.m. The second flight was planned for an altitude of about 1000 m. The addition of the Terra FAA transponder added 1.8 kg to the balloon payload. To provide adequate lift, a zero pressure balloon was attached to the top of the adjustable buoyancy balloon. In this way, ignoring thermal heating of the balloon by solar radiation, the weight of the transponder is balanced by the lift of the zero pressure balloon independent of altitude. Launch of the second test flight occurred at 1:29 p.m. of January 13, 1988. About 12 minutes into the flight, the balloon had risen to about 110 m above ground and was about 1.5 km from the launch site. At that level, the Tracer encountered a violent wind shear. Below this level, the winds had been light and from the southeast. Above 110 m altitude, winds were much stronger and directly out of the west. The shear was so violent that the rigging holding the main electronics package and the transponder to the main balloon gave way. It is speculated that the coupled oscillations induced by the shear in the complicated tandem balloon system may have been responsible for whiplash of the payload packages resulting in high g-forces. The tandem balloon system, released from much of its burden, began to rise rapidly. With the loss of the electronics payload, automated control, remote command control, and the Argos data link were lost. The AIR sonde velcroed to the balloon skin remained and continued to send data. The main and auxiliary balloons remained together and rose to a maximum altitude of about 5200 m (17,100 ft MSL), and then came slowly back down. The chase aircraft caught up with the balloons as they passed through 3800 m on the way down. From the aircraft, there was no visual evidence of catastrophic failure of either balloon envelope. Rather it appears that the thin membrane which is part of the cutdown device on the main balloon served as a pressure relief device and blew out when the superpressure exceeded the strength of the membrane or the membrane holder. It is known from the cutdown timer setting that the cutdown device would not have fired before the balloon began descending. The aircraft followed the tandem balloon system until it touched down about 16 km west of the village of Anton Chico, NM at about 2:50 p.m., about 83 km from the launch site. Recommendations and Conclusions Problems with damage to the inner ballonet on installation in the outer balloon during the manufacturing process need to be overcome. Leaks at the top and bottom fixtures need to be eliminated by improved design. Failure of the circular top seam joining the gore structure to the top mylar piece must be eliminated by proper reinforcement at th< factory. These problems must bi eliminated if the technology is to set routine use. Clearly the rigging of the payloa< needs to be improved to assure that it i: not possible to lose the payload ii turbulence or wind shear. It is noted tha the packages velcroed to the skin of th< balloon appear to have survived withou difficulty. Attaching all of the packages ii this way may be a satisfactory solution. The control software still does no contain provision for either an "effectivi potential temperature" control algorithm or provision for self-selection of th< control algorithm. Both should b< implemented. The command and data receptioi system currently makes use of a Handa Argos receiver and decoder. It i obsolete, and in a cold environment does not function properly. It should b replaced. For long Tracer Balloon flight; use of a chase aircraft will prove to b very convenient - provided comman and data reception system are availabl both on the ground and in the aircraf This would require a second comman and data reception system. At the end of the Phase I and Phas II reports, the authors concluded that a adjustable buoyancy tracer balloo meeting the design goals is bot technically and economically feasibk ------- Figure 4. Tracer Balloon shortly after liftoff of flight 1. which occurred at 10:49 a.m. on August 13, 1987. Phase III continued to confirm that conclusion. It remains to be demonstrated, however, that those design goals have in fact been fully met. Nevertheless, the Tracer Balloon system is already a useful atmospheric research tool. ------- 8. D. Zak is with Sand/a National Laboratories, Albuquerque, NM 87185 and A. W. Uchfield is with Spectra Research Institute, Albuquerque, NM 87109. J. S. Irwln and R. G. Lamb are the EPA Project Officers (see below). The complete report, entitled "Development of an Adjustable Buoyancy Balloon Tracer of Atmospheric Motion: Phase III. Refinement of the Operational Prototype System," (Order No. PB 88-190 764/AS; Cost: $32.95, 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 Officers 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 AUG;l'88 rn q ,. U .-J Official Business Penalty for Private Use $300 EPA/600/S3-88/015 0000329 PS ------- |