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Point of Contact:

Dr. Jeffry Rothermel
Global Hydrology and Climate Center
NASA/ Marshall Space Flight Center / Code: SD60
Huntsville, Alabama 35805
Office Telephone: (256)961-7965
Fax: (256)961-7723
email: jeffry.rothermel@msfc.nasa.gov
AVAPS Homepage: http://linus.atd.ucar.edu/dir_off/facilities/laof.html

Brief Instrument Description of the
Airborne Vertical Atmosphere Profiling System- (AVAPS)

The Airborne Vertical Atmosphere Profiling System (AVAPS) uses dropwinsonde and Global Positioning System (GPS) receivers to measure the atmospheric state parameters during the sonde's descent. The AVAPS system is operational with the following aircraft: NASA DC-8, NCAR C-130, NOAA Gulfstream IV, NOAA P3, DLR Falcon, AES/Canada Condor, and the USA Air Force 53rd Hurricane Hunters WC-130.

Dropwinsondes work similar to rawinsondes, measuring vertical profiles of pressure, temperature, humidity, and wind during their descent. They can be released from a variety of aircraft over data-sparse regions such as oceans and remote mountain regions. In 1987, NCAR/Atmosphere Technology Division (ATD) developed dropwinsondes that employed LORAN navigation signals (L2D2) to estimate the sonde's motion relative to the aircraft, and used digital rather than analog circuitry to measure atmospheric state parameters. In 1993, in collaboration with NOAA/AOML and the German Aerospace Research Establishment (DLR), NCAR/ATD developed a third-generation dropsonde, using a new sensor module and a GPS receiver from Vaisala Inc. Together with a NCAR/ATD-developed, unique square-cone parachute that reduces the initial shock load and stabilizes the sonde, more accurate wind profiles are now available from the boundary layer anywhere in the world. The dropsondes are the responsibility of NCAR/ATD/Surface and Sounding Systems Facility.

The dropsonde incorporates a new pressure, temperature, humidity sensor module (RSS903) and a new GPS receiver module (GPS-111), both purchased from Vaisala, Inc. and designed for their new RS90 radiosonde so they are calibrated and produced in large quantities. The sensor specifications are shown in Table 1.


Table 1 Dropsonde Sensor Specifications

  Range Accuracy Resolution
Pressure 1060-20 hPa ± 0.5 hPa 0.1 hPa
Temperature -90 to +40 C ± 0.2 C 0.1 C
Humidity 0-100% ?.0% 1.0%
Horiz Wind 0-150 m/s ± 0.5 m/s 0.1 m/s

The winds are derived by the use of a low-cost codeless, 8 channel, GPS receiver in the dropsonde that tracks the relative Doppler frequency from the RF carrier of the GPS satellite signals containing the satellite and the dropsonde motion. These Doppler frequencies (i.e. 8 maximum) are digitized and sent back to the aircraft data system as a 1200 baud Frequency Shift Key modulation on the 400 MHz sonde telemetry transmitter. The aircraft data system has a Vaisala winds processing card (MWG-201) which contains a high-quality 12 channel GPS commercial full-up receiver (GPS engine) that measures the local carrier phase Doppler frequencies which are compared to the telemetered sonde Doppler frequencies. The GPS engine also generates GPS time, the satellite ephemerides and identifies the satellites and their Doppler frequencies so that the Doppler frequencies sent back from the sonde can be identified as coming from a particular satellite to make the wind calculations. The MWG-201 card uses this data to compute independent velocity measurements every 0.5 seconds. The MWG-201 card can operate in either the autonomous mode (less accurate) or the differential mode (more accurate by removing common errors).

In addition to the RSS903 sensor module and the GPS-111 receiver module, the dropsonde electronics board includes a microprocessor for measuring and controlling the sensor module and sending the measured data to the 100 milliwatt 400 MHZ telemetry transmitter and it also includes an 18 volt lithium battery pack for power. Surface mount technology is used on the electronics board to reduce size and increase the ease of manufacturability. In addition, the electronics board contains a connector that serves as an RS-232 link with the aircraft data system for test and checkout and for setting the telemetry transmitter frequency. The transmitter can be set anywhere in the 400 MHZ meteorological band in 20 kHz steps (about 300 channels) .

A unique square-cone parachute is used to reduce the initial shock load and slow and stabilize the sonde. The parachute is immediately deployed on exit from the launch chute and streamers for about five seconds until filled by ram-air. The stability of the square cone parachute is very good during the sonde's descent and reduces or eliminates any pendulum motion of the sonde.

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