Dropsonde
www.eol.ucar.edu
Dropwindsondes provide data back to their launch aircraft concerning winds. That data are transmitted by 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 (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 (MWG201) which contains a high-quality 12-channel GPS commercial full-up receiver (GPS engine) that measures the local carrier phase Doppler frequencies, which are then compared to the telemetered sonde Doppler frequencies. The GPS engine also generates GPS time and the satellite ephemeredes data, 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 MWG201 card uses this data to compute independent velocity measurements every 0.5 seconds.
In addition to the RSS903 sensor module and the GPS111 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 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 manufacture. 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 prior to deployment. The transmitter can be set anywhere in the 400-406 MHz meteorological band in 20 kHz steps, creating about 300 separate 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.
