AVAPS Dropsonde System

The Sounding Group in ISF operates the AVAPSTM (Airborne Vertical Atmospheric Profiling System) also known as the AVAPSTM Dropsonde System.

The AVAPS Dropsonde system, has dramatically extended the envelope of atmospheric profiling capabilities. Since its debut in 1996, it has flown on numerous missions in support of operational weather forecasting and atmospheric research, with impressive results.

The  AVAPS Dropsonde System is a key atmospheric instrument that measures high resolution vertical profiles of ambient temperature, pressure, humidity, wind speed and wind direction. Measurements are taken by a parachuted GPS dropsonde that is launched from the aircraft and descend to the surface. In-situ data collected from the sonde’s sensors are transmitted back in real time to an onboard aircraft data system via radio link.

Atmospheric soundings from dropsondes provide the ability for targeted observations over remote areas such as the oceans, Polar Regions and land masses; they also provide a means to obtain soundings in and around severe weather systems, such as hurricanes. Atmospheric soundings obtained from dropsondes during hurricane reconnaissance flights have improved the accuracy of forecasts of hurricane landfall by about 20 percent over the decade of the 1990’s. The use of aircraft released dropsondes have had a dramatic impact on the forecast track of hurricanes.


The sonde is a small electronic device which contains atmospheric sensors: pressure, temperature, humidity and a GPS receiver to derive winds. The sonde is launched from an aircraft where a parachute is deployed. As the sonde descends to the Earth’s surface it continuously measures the state of the atmosphere and telemetries this information to the research aircraft. The aircraft is equipped with dedicated hardware and software to process the signal from the sonde in real-time to display and archive the data. Once the sonde has reached the surface, all data collected during the descent is sent via satellite to atmospheric research centers or the National Hurricane Center or the World Meteorological Organization.

Dropsonde Description

The Dropsonde is composed of a small electronic circuit board, sensors and a battery housed in a cardboard tube with a parachute. The inner electronic components of the dropsonde consist of precision temperature, pressure and humidity sensors, low powered telemetry transmitter, GPS receiver, and a microprocessor. As the sonde descends it continuously measures the atmosphere from the release altitude to the Earth’s surface. Measurements are made every half second, which provides a detailed profile of the atmosphere. The parachute deploys from the top of the sonde within seconds of being released from the aircraft. The parachute is specially designed for high reliability and a very stable descent. As the sonde descends the GPS receiver tracks the position and velocity of the sondes, this change in motion corresponds to the atmospheric winds. The sensor data, GPS receiver 3D position, and 3D velocity along with engineering health of the sonde is all wirelessly sent via radio waves to the aircraft with a low powered transmitter operating in the 400-406 MHz Meteorological band.

NCAR has designed and built two different version of its dropsonde.

Dropsonde variants
  RD-41 NRD-41
Weight 323 g 169 g
Length 406 mm 305 mm
Diameter 70 mm 45 mm
Parachute size 300x300 mm2 200x200 mm2
Launcher Manual Manual and
remote controlled

The dropsonde incorporates a pressure, temperature, humidity sensor module (RSS421) designed by Vaisala, Inc. for their RS41 radiosonde, and a GPS receiver module independent of the sensor module. The specifications are shown in the following table.

Dropsonde Sensor Specifications

  Range  Uncertainty Resolution 
Pressure  1080-100 hPa ± 0.5 hPa 0.01 hPa
Temperature  -90 to +60 C ± 0.2 C 0.01 C
Humidity  0-100% ± 3% 0.01%
Horiz Wind  0-200 m/s ± 0.5 m/s 0.01 m/s


In addition to the sensor module and the GPS receiver module, the dropsonde electronics board includes a microprocessor for measuring and controlling the sensor module and sending the data to the 100 milliwatt 400 MHz telemetry transmitter, and a 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.

UCAR/Intellectual Property and NCAR/SSSF have licensed Vaisala Inc. of Woburn, Massachusetts to build the NCAR GPS Dropsonde, as Vaisala model RD41.

Users of the NCAR GPS Dropsonde system (AVAPS)

ISF has built a "double" 8-channel AVAPS system plus spares for the NOAA G-IV and two additional data systems plus spares for the two NOAA P-3's. These systems were delivered to NOAA in August 1996 for testing and use, and have since been used in numerous research programs as well as operational forecasting of tropical and winter storms.

An AVAPS system has also been built for DLR in their Falcon research aircraft.

USAF Reserve Hurricane Hunters
ISF has also built 10 AVAPS systems plus spares for the 53rd Weather Reconnaissance Squadron (aka Hurricane Hunters) at Keesler Air Force Base in Biloxi, Mississippi. These systems are operational in their WC-130J aircraft. During a typical hurricane season, the 53rd deploys 1000 to 1500 sondes on training and storm missions.

AES Canada
SSSF has also built and installed an AVAPS system in Atmospheric Environmental Services (AES) Canada-s Convair 580 research aircraft.

The Met. Office (UK)
UCAR/Intellectual property and EOL/SSSF has also licensed Vaisala to sell the AVAPS system to their customers worldwide. One such system has been installed in the United Kingdom Meteorological Office's BAE-146, operated by the Meteorological Research Flight (MRF) division.

NIPR (Japan)
Vaisala has sold another system to the National Institute for Polar Research (NIPR) in Japan.

NCAR has one AVAPS system, which can be used in NSF/NCAR's C-130 or G-V, or by the scientific research community in other aircraft, such as the NASA DC-8.


See Also

For more information contact Terry Hock