R. Anderson*
Basic Automation, Boulder, CO

C. Girz* and A. MacDonald
NOAA Forecast Systems Laboratory, Boulder, CO

T. Lachenmeier*
Global Solutions for Science and Learning, Inc., Hillsboro, OR

Abstract

This paper describes the Global Air-Ocean In-Situ System (GAINS) balloon instrumentation currently under development at NOAA's Forecast Systems Laboratory, including instrumentation requirements, the current design concept, flight hardware, and recent test flight results.

Introduction and Summary

This paper describes the PIII version of the GAINS balloon instrumentation. The GAINS project and the GAINS superpressure balloon are described in two companion papers by Girz, et al.¹ (1999) and Lachenmeier, et al.² (1999). An earlier version of the GAINS instrumentation is described in "Preliminary Results from an Experimental Balloon System Designed for Multiple Atmospheric Soundings" by Girz, et al.³ (1997).

The GAINS instrumentation described in this paper is a third generation prototype, called the PIII instrument. The GAINS instrument has been evolving over the last several years as the GAINS project definition has been refined and the required technologies have become available. The PIII instrument includes Low Earth Orbit (LEO) satellite communication which became feasible last year when ORBCOMM^TM launched the majority of the satellites required for its LEO constellation. As GAINS moves from test flights using 16-foot diameter superpressure balloons to 60-foot balloons, the instrument has added the additional safety features required. These include a backup control system, an aircraft transponder, an Argos transmitter, and a top-mounted helium controller. The PIII instrument also addresses the interests of the amateur radio community and the Global Learning and Observation to Benefit the Environment (GLOBE) educational program by providing telemetry in the two-meter ham band.

GAINS Instrument Requirements

The GAINS instrument design has been configured to respond to a variety of system requirements. These requirements are divided into the categories of mission definition, safety goals, redundancy, system reliability and the operational environment.

Mission

The primary mission of the GAINS instruments is to provide a stratospheric platform to release dropsondes, record data from the dropsondes, and forward the data to ground stations. To achieve these objectives the balloon must be controlled by remote commands to assure desired balloon spacing and drop locations. At all times the balloon must comply with international aircraft safety standards and be under ground control.

Safety Systems

Operations of the GAINS System must not present a hazard to persons or property in flight or on the ground. A number of safety systems have been included in the design to assure that the balloon system meets Federal Aviation Administration requirements of FAR 101 for balloon flight in US-controlled air space.

During the float segment of the flight the balloon floats above 60,000 feet and thereby presents minimum danger to civilian aircraft. However, ascent and descent through controlled air space demands that the balloon’s position and altitude be known and reported to Air Traffic Controllers.

The primary equipment for navigation is the Global Positioning System (GPS). Both the primary and secondary controllers contain GPS receivers and include the reported GPS positions and altitude in their telemetry. The helium controller has an Argos transmitter which operates continually to provide independent position fixes via e-mail from Service Argos to the GAINS operations center. Primary determination of balloon altitude is made using three independent altimeters. One altimeter is used for each of the primary, secondary and helium controllers. These altimeters use the same barometric pressure adjustment as aircraft operating above 20,000 feet. The GPS receiver also reports altitude, but with less resolution than the altimeters.

Additional position and altitude data are generated by an ORBCOMM™ Data Communicator and an Air Traffic Control Radio Beacon System (ATCRBS) transponder. The Data Communicator contains a built-in GPS receiver and operates continuously to provides GPS position information which is forwarded by e-mail to the GAINS operations center. ORBCOMM™ also uses Doppler measurements to locate the balloon. The aircraft transponder is enabled during ascent and prior to flight termination to provide Air Traffic Controllers with an independent way to locate the balloon. The ORBCOMM™ Data Communicator and the ATCRBS transponder have independent power supplies.

Communicating With GAINS Flight Controllers

The PIII instrument system relies on a combination of telemetry from line-of-sight communications and over-the-horizon communications from the ORBCOMM™ Data Communicator. The Argos transmitter provides an additional, albeit high latency, path for data from the balloon. On earlier flights using 16-foot balloons single channel line-of-sight telemetry was utilized. For flights using the 60-foot balloon, two channels of line-of-sight telemetry are used. On future longer duration flights, an Iridium data communicator will provide additional over-the-horizon communications redundancy.

The GAINS flight controller may communicate with Air Traffic Controllers using either the chase aircraft radio, land lines, cellular phone, or by satellite using an Iridium voice communicator.

Flight Termination

Normally, flight termination is initiated by mechanical rupture of the helium bladder. For shorter flights, terminate commands are relayed directly to the balloon using line-of-sight telemetry. For these flights, the ORBCOMM™ Data Communicator provides a back-up capability for flight termination. For longer flights, the ORBCOMM™ Data Communicator is the primary method to initiate flight termination. In the future, Iridium will be added as a second over-the-horizon control path to terminate flights.

A fail-safe flight termination system is built into the helium controller. The fail-safe system consists of an electronic timer which will open a top-mounted helium valve when the maximum allowable flight time has elapsed.

Flights using the 16-foot diameter balloons do not have the weight capacity to carry a motor-operated helium valve. Therefore, these short flights use a simple spring-loaded poppet valve for fail-safe helium release. At the predetermined time, a hot wire is powered from an internal battery. The hot wire cuts a nylon cord, freeing the mechanical spring and thereby releasing the helium bubble.

Future GAINS instruments will be able to automatically terminate the flight under several additional conditions: when the balloon crosses a specified latitude/longitude barrier (to prevent balloon intrusion into forbidden territory); when a loss of altitude allows the balloon to enter commercial flight lanes; or when a loss of performance of one or more critical flight control components is detected.

Redundancy

The fully deployed GAINS system will require 400 balloons at roughly uniform spacing around the northern hemisphere. The 30-year program requires 10⁸ hours of balloon operations. As a minimum goal the system mean time between failure (MTBF) should be greater than the projected hours of balloon operations. This is a level of safety which approaches the failure rate normally expected for the primary flight control system of a commercial jet liner⁴. A simple single channel control system will fail to meet this goal by several orders of magnitude⁵. The traditional solution to such a problem is to incorporate redundancy into the control system design.

A simple parallel redundant design having two independent control systems, with random failures and no common mode failures, will meet the desired reliability goal if each system has a MTBF of 10,000 hours⁶. If there are ten critical components in each system, then the average component must have a MTBF of 100,000 hours. This failure rate requirement would be very difficult to achieve using currently available aviation grade electronics. Therefore, another solution was sought.

Since three independent systems having a mean time between failure of just 500 hours will exceed the safety goal, the GAINS instrument design incorporates two fully redundant control systems and a third backup fail-safe system.

Adequate testing must be included in the development of the GAINS control systems to give assurance that the specific mean time between failure can be achieved. The testing must also demonstrate that the premise of no common mode failures is valid. Special attention must be paid to Electromagnetic Interference (EMI) issues, thermal environment, separation of power sources, and physical separation of the system’s components.

Diverse control system implementations have been used to mitigate the difficult issue of common mode failures. For example, various locating technologies such as GPS, ATCRBS transponder, ORBCOMM™ and Argos are used.

Experience with commercial aircraft primary controls has shown that the most difficult to solve failure modes in well designed systems tend to be in the areas of electrical interconnections and human errors in system maintenance.

Reliability

System reliability deals with the economic aspect of control system failure. For the GAINS system to be successful, the balloons must complete their missions with a high degree of reliability. A reasonable reliability goal may be that 90% of the dropsondes perform useful atmospheric observations.

The use of redundancy to achieve flight safety tends to work against the reliability goals. Since safe operation requires that flights be terminated soon after a major failure in one of the control systems, a greater number of parallel control systems may greatly improve safety at the expense of operational reliability.

Triple redundant control systems are required to achieve the safety goals. Minimum reliability requires that the average system will complete a one-year mission without early termination. The MTBF for the combined control systems must be much greater than one year. Each of the three control systems must have an MTBF of greater than 30,000 hours to achieve this goal. If ten components are assumed for each control system the typical MTBF for each component must be at least 300,000 hours. This is well beyond the present capability of today’s avionics.

A balance between the conflicting goals of safety and reliability may be achieved by dividing the operational aspects of the instrument and applying the principals of redundancy. For example, the payload of dropsondes may be divided into two or three compartments, each having its own release mechanism and a certain degree of cross-control between system components. Using this approach, some of the components may be allowed to fail without terminating the flight. Achievable component failure rates may then be assumed for the design.

Operational Environment

The primary and secondary controllers are mounted below the balloon, and are thereby protected from the incoming short-wave radiation during the day and outgoing long-wave radiation during the night. The helium controller is mounted atop the balloon and is similarly shielded from the sky by solar panels and radiation shields.

Major Systems

The GAINS instrument is built from several major components which are divided into categories of major systems and support systems. The major systems include the balloon density controls, primary controller, secondary controller, and dropsondes and their release mechanism.

Balloon Density Controls

The volumetric displacement of the superpressure balloon envelope is essentially fixed. Therefore the density of the balloon is adjusted by changing the composition of the gasses within the envelope. At a steady state operating point, the mass of the payload and helium can be assumed to remain constant as long as the envelope is fully inflated and operating under a slight superpressure. (For the moment ignore the elasticity of the shell.) The control variable is therefore the mass of air which is contained within the envelope. The operational balloon is 110 feet in diameter and displaces 750,000 cubic feet of atmosphere.

There are two mechanisms that will cause the steady state operating point of the balloon to change during the flight. One is the loss of helium due to diffusion through the helium containment bladder. The second is the loss in payload as the dropsondes are released. The intent of the balloon design is for the rate of helium diffusion to result in a loss of lift which is less than the loss in payload as the sondes are dropped. The operating set point may therefore be continuously adjusted by compensating releases of helium through a top-mounted helium valve. There is no plan to release ballast to control the balloon altitude.

Pump

Minimal functional altitude control requires at least a 10% change in balloon density. A displacement of approximately 75,000 cubic feet of air must be achieved to effect this level of control. For nighttime pumping at an altitude of 65,000 feet, the pump backpressure will range from 2-10% of the ambient air pressure. During daytime pumping, the intense solar radiation at flight altitude will cause the air and helium within the balloon to expand and increase the pump backpressure by an additional 8-12% of ambient pressure.

From the start of the GAINS project it was recognized that the air pump would present a major development challenge. Existing pump technologies do not deliver the performance required for the GAINS project. Positive displacement pumps were used on early development flights. Although these pumps could readily deliver into the backpressure required, they were heavy and had severely limited volumetric displacement. Pumping times in excess of two hours were required to achieve even a 2% change in balloon density.

High performance blowers were used on later flights. These pumps had significant flow rate capability, but were very limited in backpressure. While these pumps could deliver required volumetric displacements they could pump into a backpressure of only 3-4% of ambient pressure. This limitation prevented demonstration of the altitude control system during daylight hours and required testing after sundown.

A less commonly available pump technology, the scroll pump, appears to have potential for achieving the required pump performance. A scroll pump combines the flow rate capability of a turbopump with the pressure capability of positive displacement pumps. This technology depends on highly accurate machined spirals to achieve air compression without the use of mechanical valves. Scroll pump technology has found application in refrigeration compressors, but does not currently exist in a form useful to balloon systems.

An axial flow turbo pump is being developed for the PIII instrument package by a contractor. The specification for this pump is targeted at 3,000 cubic feet per minute into a back pressure which is 10% of ambient pressure. Initial environmental chamber testing of a prototype pump is planned for mid-summer of 1999.

Superpressure Valve

A superpressure valve must be used to trap the pressurized air within the balloon envelope when the pump is turned off. An off-the-shelf helium valve is being used for the PIII vehicle. The superpressure valve has a 14-inch diameter poppet that is opened and closed by a DC gear motor. During ascent the superpressure valve will be in the open position, allowing gasses to escape as the balloon moves to lower density air. Gasses escaping from the superpressure valve will pass through the pump to the atmosphere. The pump will enter a free-wheeling mode allowing gasses to pass with minimum restriction.

As the balloon approaches the selected float altitude the superpressure valve will be closed. As the balloon rises, the gasses within the balloon compress relative to the ambient atmosphere. The relative density of the balloon is increased thereby setting the float altitude. Superpressure in the balloon envelope will be continuously monitored and the valve will be opened to vent air in case of excessive pressure.

Helium Valve

As dropsondes are released, the total mass of the system is reduced, the balloon will tend to float at higher altitudes, and higher superpressure will be required for control. To a small degree the loss of payload mass will be offset by the natural diffusion of helium through the balloon bladder.

A valve is mounted atop the balloon to release helium as required for efficient pumping operations and to maintain a safe level of pressures in the balloon envelope during periods of high daytime solar radiation. Helium pressure within the balloon envelope will be monitored, and the helium valve will be opened if gas pressure approaches the design limits.

The helium valve may also be opened as a backup method of flight termination. In this case the rate of descent will be greatly reduced to an expected 300 to 400 feet per minute.

An EV-13 helium release valve was tested in the laboratory. Using a 24-volt battery the valve opened in 17 seconds and had a peak current of 5.5 amps under no-load conditions. The valve seal diameter was 14.5 inches, with a projected area of 165 inches². If it is assumed that the valve must open against a 20-mb superpressure, the load on the poppet is 47 pounds. The valve drive circuit was tested by loading the valve poppet with twice the required load. A motor current of 12 amps was measured. Significant electrical noise due to brush communication was noted. Electrical suppression circuits will be required to reduce potential EMI problems.

Primary Controller

Under normal flight conditions, all of the navigation and controls functions are performed by the primary controller. The primary controller is located below the balloon envelope and contains a microprocessor, GPS, line-of-sight and over-the-horizon telemetry, controls for dropsondes mechanism, flight termination hardware and a variety of sensors.

The primary controller synchronizes the operation of all transmitters carried on the balloon. Control and synchronization signals are issued from the primary controller to the secondary controller and the helium controller. The secondary controller is signaled by an opto-isolated data link. The helium controller is signaled by a low-powered radio link. Each of secondary controller and helium controllers has default scripts to execute in case of failure of the control links.

A pre-established transmission protocol is used to synchronize RF emissions from the various transmitters. This design assures that there are RF quiet periods in which to read analog sensors and to receive line-of-sight commands, GPS signals, and satellite communications. The transmission protocol is divided into four flight segments: ascent, float, terminate, and descent.

Microprocessor

The primary controller uses a RPC-320 embedded controller manufactured by Remote Processing Corporation. The controller has an Intel 8031 microprocessor (80C320) with an internal read only memory that has been programmed with RPBASIC-52, a BASIC interpreter which is a super set of Intel’s BASIC-52. The microprocessor instructions that control the balloon are stored in a 32k Electrically Erasable Programmable Read-Only Memory (EEPROM). An additional 32k of battery-backed RAM is used for data logging. The use of a microprocessor with built-in interpreter proved to be a real convenience when the balloon program needed to be updated at a launch site hundreds of miles from the laboratory.

Default control of the balloon is defined in a flight "script". The microprocessor program reads the time based script and automatically executes the instruction. The script contains the flight plan that will be followed in case of telemetry failure. The script is stored in the EEPROM and may be easily updated using a PC laptop computer. The script may also be modified in flight upon receipt of properly encoded commands from the ground.

The controller has 22 analog inputs that are used to measure atmospheric temperature, gas temperature within the envelope, instrument temperature, atmospheric pressure, balloon superpressure, relative humidity, battery voltages, solar panel charging current, and upwelling radiation.

The controller has two serial ports. One serial port is shared by the ORBCOMM™ Data Communicator (mostly outputs) and the GPS receiver (mostly inputs). The second serial port is used for the radio modem and 416.9 MHz line-of-sight telemetry.

Global Positioning System Receiver

The Global Positioning System is used to determine the geographic location of the balloon and to measure wind speeds. The PIII instrument uses a Rockwell "Jupiter", TU30-D140-221/231, GPS receiver. In normal operation the receiver is powered 100% of the time and contributes approximately one watt to the internal heating of the instrument package. A Matsushita Electric L1 passive antenna, PU21522GPS-ST, is mounted to the top of the instrument package and below the balloon, where it has a clear view of the sky above. Tests have shown that the balloon’s Spectra envelope does not affect GPS signal quality. Voltage level shifters are used to convert the 5-volt serial signal from the receiver to RS-232 voltage levels for the computer. The receiver is programmed to use National Marine Electronics Association (NMEA) data messages.

The Rockwell receiver is not approved for export from the United States. Therefore, it does not have the artificial 60,000-foot limitation required for receivers that are for approved export. Therefore, no modifications to the receiver are required for stratospheric flights.

Over-The-Horizon Communications

Low Earth Orbit (LEO) satellites are used to relay communications from the balloon to the GAINS operations center. The PIII instrument uses the ORBCOMM™ constellation of satellites for bi-directional over-the-horizon communications. Currently, the ORBCOMM™ constellation has 28 operational satellites in 6 orbital planes. The satellites orbit at an altitude of approximately 800 miles. Because of this low altitude, the balloon RF power requirements are very low and the transmitter is therefore small and light weight.

The PIII instrument uses the ORBCOMM™ satellites in a direct communications mode. In this mode, both the balloon and an Earth Station are within view of a single satellite. There are three Earth Stations in the United States. Telemetry from the balloon is queued as message packets in the ORBCOMM™ Data Communicator which is normally in a low-power mode, waiting for a satellite to come into view. The Data Communicator will "wake up" when a signal is received from a satellite. The queued messages are then forwarded to the satellite, and from the satellite to an ORBCOMM™ Earth Station. The Earth Station forwards the messages to the GAINS operations center by e-mail over the Internet. Commands to the balloon follow the same path in reverse. In recent tests, the ORBCOMM™ message latency has averaged 4 minutes, with a maximum latency of 13 minutes.

For future oceanic operations, the GAINS instrument will operate the ORBCOMM™ system in a message store and forwarder mode. In this mode, a message package is sent from the balloon to the satellite where it is stored on board the satellite. The messages are held by the satellite until the next pass over an Earth Station when the messages can be down loaded. The Earth Stations communicate with the balloon flight control center via e-mail over the Internet.

The balloon uses a Panasonic KX-G7101 full-duplex 138/148 MHz Data Communicator. For the GAINS system the Data Communicator is programmed to automatically and autonomously send GPS position messages every 15 minutes, even if the primary controller computer has failed. In addition to the built-in GPS, the ORBCOMM™ system has inherent position determination of the Data Communicator using Doppler measurements.

The GAINS project plans to add an Iridium communications link as the primary over-the-horizon communicator when datasets become commercially available. The Iridium system will provide very low latency communications for rapid processing of commands to the balloon. Iridium is well complemented by ORBCOMM™, which will provide low cost data transmission for the majority of communications that are not time critical.

Line-of-Sight Telemetry

For balloon launch and recovery operations, the GAINS instrument relies on line-of-sight telemetry for command and control. The 400-MHz band telemetry provides immediate bidirectional communications to the balloon when it is in line-of-sight with a ground station. In flight tests, line-of-sight telemetry has proven effective to a range of 200 miles.

The line-of-sight transmitters and receivers are Repco RLD series FM links. The transmitter operates at 416.9 MHz. A Tigertronics RTX-12 FSK radio modem operating at 1200 baud is used to interface the computer module to the transmitter. The data packets use a subset of ASCII characters and have a simple start and end character scheme to define messages.

No check sums are required for the data packets. Under normal conditions the ground station software will automatically extract the data from the message structure. For difficult telemetry conditions, the ground stations will display and save message fragments which may be used to reconstruct data packets. During flight tests this feature was very helpful because transmissions near the ground are often broken up by obstructions in the line of sight. Often the ground personnel are capable of extracting data directly from the message fragments in the real-time environment.

Within the data packet, the message is bracketed with five second bursts of RF carrier to provide a clean target for radio direction-finding equipment.

To provide maximum signal strength to receivers located below the balloon, a quarter-wave ground plane antenna is attached to the bottom of the instrument package.

The line-of-sight receiver operates on 408.4 MHz and uses Dual Tone Multi-Frequency (DTMF) signaling. The receiver requires only 0.2 watts of power and is power-on at all times. A MoTron Electronics AK-16 DTMF tone decoder uses password protection to authenticate commands from the ground. The decoder can decode 16 predefined commands that are initiated from a Radio Shack 43-146 DTMF tone encoder located in the chase and recovery vehicles. The line-of-sight receiver shares the quarter-wave ground plane antenna with the transmitter. An electronic radio frequency switch is used to add or drop the transmitter output.

Sensors

Instrumentation on the primary controller includes atmospheric, balloon envelope and internal sensors. The atmospheric sensors include temperature, humidity and absolute pressure. The balloon sensors include envelope internal superpressure, helium temperature, and superpressure valve position. Internal sensors include battery voltages, solar cell current, electronic compartment temperature, and battery temperature.

An HYCAL IH-3605B thin film polymer humidity sensor is placed in an external housing on the instrument package. The sensor contains monolithic signal-conditioning electronics which provide a linear voltage output proportional to relative humidity. During ascent and descent, this sensor is used to detect conditions which could adversely affect balloon control. Calibration data for each sensor are supplied by the manufacturer. Sensor ID is included in the data stream.

Eight additional channels of undedicated analog input are provided to accommodate signals from optional experiments to be defined in the future.

Low Power RF Link

A low powered RF link at 303 MHz is used to communicate with the helium controller mounted on top of the balloon, 110 feet from the primary controller. In addition, if video is flown on a test flight, the 303 link is also used to enable its transmitter in accordance with the pre-established transmission protocol.

Flight Termination System

Flight termination may be initiated by the primary controller, the secondary controller, or the helium controller. For the case of the primary controller, a password-protected terminate command is sent from a ground station on the dedicated 408.4 MHz channel. Once received by the instrument, the DTMF decoder will authenticate that the command then set the flight termination relay which connects a dedicated battery back to a hot wire on the balloon’s bladder destruct line.

Prototype Sonde Interface

Prototype sonde testing will be performed using a Vaisala RS-80-15 radio sondes. The sonde will be carried below the balloon and cut free after ascent. The sonde’s 401 MHz telemetry will be received by the primary controller and processed through a frequency-to-voltage converter. The output of the converter is fed into the microprocessor via an analog-to-digital converter and, from the microprocessor, the sonde data are included with the telemetry data stream.

Secondary Controller

The secondary controller duplicates many of the functions of the primary controller. In the event of failure of the primary controller, the secondary controller is capable of taking over control of the balloon, releasing the dropsondes, and processing telemetry. The controller has a microprocessor, GPS receiver, and flight termination hardware.

Two approaches are currently under consideration for the secondary controller. The first approach is to use a control package similar to the primary controller. The second is to use a dissimilar controller supplied by Physical Sciences Laboratories, New Mexico State University. The advantage of the second approach is in the robustness gained from use of a dissimilar control design. Both secondary controllers have been test flown on GAINS flights.

There are three major differences between the functions of the primary and secondary controllers as implemented in the PIII version of the instrument.

• The primary controller has an ORBCOMM™ Data Communicator and the secondary controller does not. However, in the next few years the primary controller will use an Iridium communicator and the secondary controller will have the ORBCOMM™ communicator.

• The secondary controller carries an Air Traffic Control Radio Beacon System (ATCRBS) transponder.

• The line-of-sight telemetry uses a different RF band assignment and command decoder technology.

Aircraft Transponder

The secondary controller carries an ATCRBS transponder. The Trimble TRT 250D transponder is connected to an Ameri-Kink Altitude Encoder/Reporter that provides uncorrected barometric pressure, consistent with air traffic control procedures. An ACPI GB300NA200 barometric activated switch is used to redundantly enable the transponder whenever the balloon is below 50,000 feet.

The transponder RF signal may need to be inhibited periodically to accommodate maximum sensitivity of the GPS receiver, destruct receiver, and analog signal input. The transponder may be deactivated by the secondary controller during the float segment of the flight.

When the instrument is in direct contact with ground operators, via line-of-sight telemetry or Iridium communicator, the secondary controller may activate the transponder "Ident" on request from air traffic control.

The transponder is enabled by the secondary controller during ascent, prior to flight termination, and during descent. Air Traffic Control assigns a unique code which is set into the transponder before the flight. Because the balloon is unmanned, the code can not be changed as the balloon is handed off to next flight control center.

Line-Of-Sight Telemetry

Telemetry in the two-meter U.S. amateur radio frequency band has been implemented for the PIII secondary controller. Telemetry in this band will use a commercial packet modem, known as a Terminal Node Controller (TNC), allowing radio amateurs to receive dropsonde data, track the balloons, participate in balloon recovery operations, and experiment with a near space radio relay. A number of ham clubs have expressed interest in participating in the program. In addition, educational programs such as NOAA’s GLOBE project could benefit from involvement in the GAINS project. By using inexpensive two-meter receivers, students may observe balloon data and perform rudimentary motion and meteorological calculations.

Dropsondes and Release Mechanism

The PIII instrument incorporates the prototype sonde interface described above. The sondes which will be dropped by an operational balloon exist only as a specification at this time. These dropsondes will incorporate standard temperature, humidity, and pressure sensors. In addition, they will need to have a scaled-down GPS receiver capable of measuring wind speeds. The sonde will transmit digital data to the balloon using a low power RF transmitter. The data format will be open architecture incorporating a flexible format to allow a variety of mission and sonde configurations.

The balloon will carry approximately 750 dropsondes, allowing two soundings per day. The dropsondes will be stored in three of the six compartments of the torus, 250 per compartment. Since these compartments will not be heated, the dropsonde to be released must be acquired from storage and transferred to a ready chamber prior to use. In the ready chamber, the sonde will be warmed up to operating temperature and the sonde battery activated before release.

Support Systems

Support systems include equipment that will fly on the balloon as well as equipment based on the ground. Flight support systems include the helium controller and power systems. Ground-based equipment includes the operations center and ground stations.

Helium Controller

The helium controller is a lightweight minimum function controller which rides on top of the GAINS balloon. This controller operates the helium vent valve either on command from the primary controller or at the end of the maximum allowed flight time. The helium controller also operates an Argos transmitter and recovery beacons.

A BASIC Stamp microprocessor board is used for the helium controller. It will process data from an altimeter, upper helium temperature sensor and an upward-looking radiation sensor. These data, along with housekeeping data, will be sent to the GAINS operations center by the Argos transmitter. Service Argos provides transmitter locating data which are independent from the GPS data. At a future date, Service Argos will provide for the delivery of simple one-bit commands to the balloon.

The helium controller is slaved to the primary controller by a low-powered 303 MHz radio link. Prior to flight, when the balloon is still on the ground, the microprocessor operates primarily in sleep mode. Every few minutes it wakes up to check for messages on the 303 link. If messages are present, it will power up its systems and prepare for flight. This process is necessary because, prior to launch, the helium controller is located 120 feet from ground level and must be installed one or more days prior to launch. If desired, the primary controller may put the helium controller "back to sleep". One of the commands from the primary controller will start the mission timer in the helium controller. Once started the timer cannot be reset and will terminate the flight on timeout.

Optional equipment considered for the helium controller includes two alternate flight termination modules. Since the helium controller is located 110 feet from the RF transmitters in the primary and secondary controllers, it is an ideal location to receive the DTMF commands sent at 408.4 MHz. These commands may be used to open the helium valve in case of failure of the bladder destruct mechanism and/or 303 local link. A backup helium release mechanism with poppet valve, spring, and hot wire, used on earlier GAINS flights, could also be incorporated into the helium controller.

Two balloon recovery aids are placed within the helium controller. The first recovery aid is a two-watt RF beacon operating at 408.4 MHz. This RF beacon may be used in conjunction with ground-based radio direction-finding equipment to locate the balloon during descent and landing. The second recovery aid is an audio device which emits a loud beeping tone. The primary purpose of the beeper is to draw the crew’s attention during balloon recovery after the flight. This is especially useful if the balloon lands in dense vegetation, such as a corn field. The microprocessor will monitor the altimeter, and upon descent through 15,000 feet, will activate the two beacons. The audio beacon is also used to communicate detected errors to the ground crew during launch operations.

Power Systems

Solar Cells

The solar array is passive; the panels are not pointed in the direction of the sun. Only the solar cells on the side of the balloon toward the sun collect energy. The payload geometry is that of a six section torus. Groups of solar panels are mounted on each of the six sections.

Batteries

Each controller uses multiple sets of rechargeable batteries for instruments, telemetry, terminate electronics, and mechanical loads such as heating and pumps. This separation of function prevents loss of control if the telemetry transmitters or pumps should drain the batteries. NiCd cell technology is used on the PIII instrument package. For now, this technology provides safe, reliable and relatively inexpensive energy storage. Two or three years into the future GAINS will evaluate available cell technologies and charging systems for the operational balloon. In addition to the NiCd rechargeable batteries, lithium primary cells are used for fail-safe backup and provide minimum control for flight termination if the main battery systems should fail.

• Various self-test systems have been built into the controllers to assist the ground crew in prelaunch system checkout.

• The balloon carries a low-power Telonics radio beacon that operates independently from the balloon instrument and power systems. This short-range beacon is used with radio direction-finding equipment to assist in locating the balloon on landing.

• The balloon safety system also includes a radar reflector, visible strobe for nighttime ascent/descent, and a 50-foot highly visible streamer.

• Video may be flown with downward and/or upward looking cameras. Video will be mechanically and electrically independent from the GAINS controllers. It will be limited to short time segments (snapshots) to reduce the potential for interference with GAINS telemetry. When the camera package is hung below the torus it will be attached with a line that will breakaway with less than 50 pounds of force. The weight-to-size ratio of the video package is less than three ounces per square inch.

Ground Stations

The ground stations receive line-of-sight telemetry from the balloon and issue commands to the controllers. One ground station is located at the base station, one in the chase vehicle, and one in the recovery vehicle. The base station is at a fixed location, preferably at a high point in the terrain. The chase vehicle is a light aircraft and the recovery vehicle is a passenger vehicle or van. The ground station has been designed as a compact unit to be operated in the confined spaces of these vehicles. The ground station has a real-time display for transmitter power level, receiver signal strength, and battery voltage. It also has an antenna disconnect alarm.

The ground station is controlled by a laptop PC running the Windows98 operating system. It has a graphical user interface with all important data readily available on the computer screen in common engineering units of measurement. The ground stations receive 416.9 MHz telemetry on serial port COM1. A second serial port is multiplexed between a GPS EarthMate receiver, two-meter telemetry, 303 local link, and, for the recovery vehicle, an ORBCOMM™ Data Communicator.

Commands to the balloon are sent using a hand-held Dual-Tone Multi-Frequency (DTMF) encoder. The ground station operator activates the DTMF in the quiet periods between balloon telemetry packets.

Ground station operators communicate directly with the GAINS operations center in Boulder, Colorado, for processing of over-the-horizon telemetry and flight path predictions.

The vehicles carry cell phones, radios for voice communications, various omni and directional antennas, and hand-held GPS receivers. The base station and recovery vehicles are equipped with two-meter receivers and command capability for the amateur band telemetry. A mapping program is used to track balloon position and may interface directly to the amateur telemetry using Automated Position Reporting Software (APRS).

Summary

The PIII prototype GAINS instrument incorporates redundant systems which provide the level of safety and reliability required for a large superpressure balloon. The PIII instrument also incorporates recently available Low Earth Orbit satellite communications. The interests of the amateur radio community and the GLOBE education program are addressed with the incorporation of inexpensive two-meter telemetry.

The PIII instruments are designed to demonstrate the feasibility of the GAINS concept for flight durations ranging from hours to days. This prototype incorporates all critical components of GAINS.

References