Description of the payload
A series of lidar experiments has been conducted using the Atmospheric Balloonborne Lidar Experiment payload (ABLE) as a demonstrator of the development of the needed technology for space lidar platforms, to be deployed in the future on the Space Shuttle or on a dedicated satellite.
The program was conceived in 1981, and successful flights occurred in 1984, 1987, and 1988, followed by groundbased measurements made in 1989 and 1990.
The ABLE lidar used a biaxial optical system with a multimode, frequency tripled, Nd:YAG laser transmitter and 50 cm Dall-Kirkham receiver optics. The flashlamp-pumped laser transmitter and power supplies were mounted in hermetically sealed pressure chambers which were maintained at an ambient pressure during flight.
The flashlamp thermal power was removed from the rod cavities by a primary cooling loop containing a mixture of deionized water and ethylene glycol, and transferred through a liquid-to-liquid heat exchanger to a secondary trichloroethylene cooling loop. This loop circulated the coolant through two outboard radiators where the laser system thermal load was dissipated by radiation and convection.
When the payload was configured for flight with batteries, balloon control electronics, telemetry electronics, ballast, and a recovery parachute, the launch weight was approximately 1270 kg.
The biaxial lidar system was mounted on an optical bench made of a rigid trussed structure of welded aluminum angle. Prior to payload fabrication, extensive structural and thermal analyses were performed to predict structural deflections.
An onboard microcomputer-controlled CAMAC system provided the lidar timing, current and pulse counting detector data digitization, analog housekeeping data acquisition, pointing mirror and filter wheel motor drive controls, uplink modem command interface, and downlink IRIG PCM data formatting. The unprocessed lidar PCM data were downlinked on an S-band telemetry link at 246 kbit/sec. The lidar data were recorded and graphically displayed in near real time at the ground station. A 300 baud command uplink provided real-time experiment control.
The lidar could be commanded to point 30° from zenith, in the horizontal direction, and 2° from the nadir. The pointing of the lidar in the azimuth direction was not controlled but was monitored by a magnetic compass. Lidar beam steering was provided by a small multilayer dielectric laser pointing mirror and a 50 by 72 cm plane receiver pointing mirror. These mirrors were mounted on a common rigid shaft so that thermal variations did not affect their co-planar alignment.
Lidar safety was a major consideration in the operational planning for each flight. Laser operation in flight required the simultaneous transmission of commands from the ground station to the payload over two independent RF links. During flight, nadir lidar operation was not permitted unless the payload was above a specific altitude and over White Sands Missile Range, NM.
Prior to each launch the flight configured payload was subjected to a simulated flight test, in an environmental chamber, at the thermal and pressure conditions to be encountered during ascent and float.
Details of the balloon flight and scientific outcome
Launch site: Roswell Industrial Air Center, New Mexico, US
Balloon launched by: Air Force Geophysics Laboratory (AFGL)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 8.740.000 cuft (1 mil. 2xCaps 1.0 Mils. Stratofilm)
Balloon serial number: SF 277.88-100-NSC-01 SN:8
Flight identification number: H84-22
Payload weight: 2700 lbs
The balloon was launched on August 23, 1984 at 21:30 local time, by dynamic method using the crane capture technique.
Shortly after launch, it was found that the balloon control system was unable to dump ballast. This resulted in a slower rate of ascent than planned, thus subjecting the payload to cold soaking longer than normally would have been encountered. During ascent, telemetry dropouts occurred at the Roswell site. This may have been caused by interuption of the telemetry antenna line-of-sight by the hangar structure and power transformer. The payload attained an altitude of 107,000 ft approximately 3 hours after launch.
At about 2230 hours, control of the payload and data recording/monitoring responsibility were transferred, as planned, to Building 850, at Holloman AFB.
After the scientific observations were completed, the flight was terminated to bring the payload down within the White Sands Missile Range.
The morning following the flight, a recovery crew, including four project experiment personnel, left Building 850 to locate and recover the payload. The payload was found by the serach aircraft adjacent to a dirt road. Once the recovery party recahed the zone, it was approached first by the eye-protected experiment crew whom initially safety interlocked the laser and turned off all electical power on the payload and then covered the receiver and transmitter optics. When it was concluded that the payload was in a safe condition, the Detachment I recovery crew was permitted to approach the payload to load it on the flatbed trailer and then trucked back to Building 850 at Holloman AFB.
This was the first flight of the ABLE payload. The objectives of the flight were to provide an experimental test of a balloonborne lidar and to make atmospheric backscatter measurements with 150 meter slant range resolution using the lidar.
The payload was controlled and monitored from launch to an altitude of approximately 5 km from two Air Force Geophysics Laboratory (AFGL) telemetry vans located at Roswell, NM. When reliable RF transmission was established, balloon and experiment controls were transferred to the AFGL Detachment 1, Telemetry and Control Center (Building 850), at Holloman AFB, NM.
During the flight,the ABLE lidar system operated successfully, and excellent backscatter data were acquired.
External references and bibliographical sources