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- ------- 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 ------- 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. ------- +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 ------- 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. ------- 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. ------- 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 ------- |