The NCAR GPS Dropsonde system, also known as AVAPS (Airborne Vertical Atmospheric Profiling System), has dramatically extended the envelope of atmospheric profiling capabilities. Since its debut in 1997, it has flown on numerous missions in support of operational weather forecasting and atmospheric research, with impressive results.

Previous NCAR Dropsonde Systems Current AVAPS Users
Development of AVAPS  Future Developments
Description and specifications:  AVAPS Images
 - GPS Dropsonde
 - Data System
 - Software
Interface Control Document

GPS dropsonde with NCAR's unique square-cone parachute.

Previous NCAR Dropsonde Systems

NCAR's Atmospheric Technology Division (ATD) developed the first Omega-based dropwindsonde (ODW) in the early 1970's for the Global Atmospheric Research Program's (GARP) Atlantic Tropical Experiment (GATE). NCAR continued development and improvements of the ODW for the Global Experiment in the late 1970's and these sondes were used successfully (about 7,000 dropped) to study the influence of the tropical oceans on Northern Hemisphere weather and climate. After the Global Experiment, the dropwindsonde continued to be used by NOAA's Aircraft Operations Center (AOC) for hurricane research. In 1982 the ODW was adopted by the U.S. Air Force Air Weather Service's Hurricane Hunters for their mission to support the National Hurricane Program.

In 1987, ATD's Surface and Sounding Systems Facility (SSSF) began development of an advanced digital dropsonde to support the Office of Naval Research's (ONR) 1988-1989 Experiment on Rapid Intensification of Cyclones over the Atlantic (ERICA). This development produced the Lightweight LORAN Digital Dropsonde (L2D2), a smart (microprocessor-based) sonde that was lightweight (300 grams vs. 1500 grams for the ODW), incorporated LORAN instead of Omega (Omega was added later in a redesign for the Air Force) for windfinding, and used digital instead of analog circuitry to measure the state parameters and telemeter the data back to the aircraft. This new digital sonde reduced or eliminated noisy data and also provided the capability (extra channels) for incorporation of other measurements. The Lightweight Digital Dropsondes (LD2) have supported numerous national and international field programs, including STORMFEST, TOGA COARE and CEPEX. They have been successfully launched from a variety of aircraft: the U.S. Air Force WC-130, the NCAR Electra, C-130, Sabreliner, and King Air, the University of Wyoming King Air, the NASA DC-8, NOAA P-3's, the Aeromet Learjet, and the DLR Falcon. In 1992 NCAR licensed the LD2 design to Radian Corp., Austin, Texas, which manufactured the sonde for use by the Air Force and the worldwide community.

  Development of the NCAR GPS Dropsonde System (AVAPS)

In 1993, SSSF performed a feasibility study for the German Aerospace Research Establishment, DLR, for an advanced lightweight digital dropsonde incorporating GPS, in support of DLR's development of a new high-altitude research aircraft. The German aircraft company, Grob, had been contracted to develop a manned  stratospheric research aircraft, the STRATO 2C, designed to fly at altitudes between 16 and 24 km for up to 48 hours. A manned aircraft flying at these altitudes would open up many research opportunities: studying the ozone deficit in the Antarctic and Arctic vortices, the effect of clouds and aerosols on radiation transfer, the global radiation budget, the distribution of climate-related trace gases, and the movement of tropical cyclones. Most of these research missions are aided by, or require, an atmospheric profile below the aircraft—a task best done, on a global basis, with a GPS dropsonde.

After completion of the DLR study, SSSF learned that NOAA's Aircraft Operations Center was purchasing a new Gulfstream IV (G-IV) jet for use in weather reconnaissance and research missions. A primary requirement in many of the new jet's mission profiles was that of measuring and recording the atmospheric conditions below the aircraft with a dropsonde system that could be operated worldwide.

The requirements for the DLR STRATO 2C and NOAA G-IV dropsonde systems were nearly identical, including the time schedule for delivery. In addition, NCAR's future support for national and international research programs with dropsonde systems both with the NCAR C-130 and Electra and in other research aircraft (e.g. NASA DC-8) also required a new system with worldwide operational capability to replace the existing Omega and LORAN systems. This need was primarily driven by the shutdown of the Omega navigation system in 1997 and the potential loss of the LORAN system around 2008. The marginal vertical wind resolution of Omega and LORAN was also a major consideration. Because of the required smoothing interval (60 sec), the vertical wind resolution is only on the order of 300 meters for LORAN and still worse with Omega. GPS promised far superior accuracy in wind profiling. As a result of the mutual requirements for a GPS dropsonde system, NCAR entered into a joint agreement with NOAA and DLR to develop the new GPS dropsonde and aircraft data system with the three organizations sharing the costs for the common portions of the program.

The following are the original NOAA/DLR/NCAR dropsonde system design requirements, now specifications of the operational system.

  • Global operation, at altitudes up to 24 km,
  • Deployment at indicated airspeeds up to 250 kts,
  • Simultaneous operation of up to four sondes (4-channel data system),
  • Sonde descent time approximately 12 minutes when released from 12 km,
  • 0.5-second sample rate for wind and thermodynamic data,
  • Sonde weight < 400 grams,
  • Sonde size 2.75 inches (6.98 cm) diameter by 16 inches (40.6 cm) long,
  • Sonde shelf life 3 years,
  • Operation out to a telemetry range of 325 km, and a
  • RF transmission bandwidth of < 20 kHz.

Description of the NCAR GPS Dropsonde

The dropsonde incorporates a new pressure, temperature, humidity sensor module (RSS903) and a new GPS receiver module (GPS111), both designed by Vaisala, Inc., for their RS90 radiosonde. The sensor specifications are shown in the following table.

Dropsonde Sensor Specifications

1080-100 hPa 
± 1.0 hPa 
0.1 hPa 
-90 to +60 C 
± 0.2 C 
0.1 C 
± 5% 
Horiz Wind 
0-200 m/s 
± 0.5 m/s 
0.1 m/s 

The winds are derived using 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.

UCAR/Intellectual Property and NCAR/SSSF have licensed Vaisala Inc. of Woburn, Massachusetts to build the NCAR GPS Dropsonde, as Vaisala model RD93.

Description of the GPS Dropsonde Aircraft Data System (AVAPS)

The aircraft data system is completely different, both in system software and hardware, from all previous dropsonde systems NCAR has developed. The most significant development in the hardware is the narrow-band 400 MHz telemetry receiver. The design requirements of the aircraft data system are:

  • Operation by one person
  • Receive and process data from four dropsondes simultaneously
  • Simple Graphical User Interface (GUI)
  • Continuously output real-time PTH and wind for each channel to the aircraft computer systems as the dropsondes are descending
  • Operation on aircraft with or without aircraft data systems.

The most significant feature of the AVAPS data system is its ability to receive and process data from up to 4 sondes simultaneously, essential for obtaining a fine horizontal distribution of soundings. Launch procedures are simple enough to allow releases less than 20 seconds apart, if desired.

The aircraft hardware is typically composed of a Personal Computer (PC) with a Pentium processor, color monitor, telemetry chassis, and printer. The hardware is designed for installation into a standard 19-inch rack. The system also requires a GPS antenna mounted on the top of the aircraft and a UHF antenna, for receiving the dropsonde signal, mounted on the bottom of the aircraft. If available, an RS-232 connection to the main aircraft data system for flight level information is highly desirable.

Telemetry Chassis

The telemetry chassis is built in a standard 13.5 cm high 3U Eurorack chassis. The chassis houses the following removable modules:

  • Power Supply Module. A single Power Supply module provides the DC voltage required for telemetry chassis operation.
  • Telemetry Receiver Module. The Telemetry Receiver module is a narrow-band 400 MHz receiver used to receive and demodulate the PTH and GPS telemetry data from the dropsonde. There are five receivers in the telemetry chassis, one for each of the four channels and a fifth to record engineering data for all of the deployed dropsondes.
  • PTH Buffer Module. The PTH Buffer module filters and synchronizes the PTH digital data stream from the telemetry receiver to ASCII   RS-232 data, which is routed to the PC for processing. There are four PTH buffers in the telemetry chassis, one for each of the four channels.
  • GPS Processing Card. The GPS processing card (Vaisala model MWG20 1) processes the 1200 baud FSK signal from the telemetry receiver to compute winds from the Doppler frequencies measured by the dropsonde. There are four MWG201's in the telemetry chassis, one for each of the four channels. Each MWG201 contains a high-quality 12-channel commercial GPS full-up (code-correlating) receiver that measures the local satellite RF carrier Doppler frequencies. The 12-channel receiver also generates GPS time and time satellite ephemerides, and identifies the satellites and their Doppler frequencies. The local Doppler frequencies are then compared with the telemetered dropsonde Doppler frequencies; this allows the frequencies sent back from the sonde to be identified as originating from a particular satellite. The MWG201 then removes the satellite component of motion. However, since the MWG201 is itself moving, the aircraft velocity must now be eliminated to obtain the actual motion of the sonde. The internally-computed GPS motion of the MWG201 is used for this purpose, although the aircraft's own flight data system can also be used. In the former configuration, the final wind estimate is susceptible to errors from Selective Availability (SA), the U.S. military's artificial degradation of GPS signals. Initial identification of the GPS satellites after launch typically takes about 30 s, although acquisition times of 60 s are not uncommon; during this interval no winds are computed. After initial acquisition, the MWG201 card computes independent velocity measurements every 0.5 s (about 5m in the vertical near the surface). Filtering to remove observational noise is generally not required, unlike with LORAN, which requires 60 s filtering, or Omega (180-240 s). Winds are obtained until the sonde telemetry fails at splashdown.
  • Dropsonde Interface Module. The dropsonde interface module provides the local interface required to prepare a dropsonde for launch. The dropsonde is connected to the module using an RS-232 cable. Through this interface, the dropsonde PTH sensor calibration coefficients and serial number are uploaded to the PC, and the dropsonde transmitter is set to the desired frequency.

Each of the above modules, with the exception of the power supply, contains a microprocessor and an RS-232 interface. The PC controls all functions and receives data from each module through I/O cards and displays the status of each module. All of the hardware in the telemetry chassis, except the MWG201 module, is designed and built by NCAR.

Additional information on the aircraft hardware is found in the AVAPS Interface Control Document.

Description of AVAPS Software

The AVAPS system software is written in LabVIEW, a graphic programming language developed by National Instruments that permits multi-tasking in the Windows 95 operating system. Multitasking capability is critical for a multi-channel dropsonde data system. The system software performs several functions prior to release of a dropwindsonde. The software logs the current system configuration and flight mission information, and initializes all electronic hardware for the release. A graphical spectral analysis of the 400-406 MHz meteorological RF band is provided so that the operator can select a transmitter frequency free of interfering signals. The system then performs a functional test of the sonde's PTH sensors and the 400 MHz transmitter by displaying cabin PTH measurements from the sonde transmitted through the 400 MHz telemetry link. When configured, the PC also communicates with the aircraft's sonde launch system, via the Dropsonde Telemetry Chassis, to launch a sonde at the operator's command. If the host aircraft has a data system to provide flight-level meteorological data (pressure, air temperature, dew point, wind speed, wind direction, altitude, etc.), these are automatically received by the AVAPS PC. However, the AVAPS does not require an interface with a flight-level data system in order to function.

After launch, the software processes PTH and GPS data from up to four dropsondes via the Dropsonde Telemetry Chassis and 16-port I/O panel. Both PTH and wind data are available every 0.5 s, although because they are obtained from independent modules, the PTH and wind measurements are not made at precisely the same times. The AVAPS system PC pairs the PTH and wind data that are most closely matched in time, and then displays, in real-time, selected parameters to the color monitor, including pressure, temperature, relative humidity, wind speed and direction, number of GPS satellites being tracked by the dropsonde, and geopotential altitude (computed hydrostatically from the PTH data). These and other parameters are also stored on the PC hard drive for archiving, and can be directed to other computers on the aircraft for further processing and transmission off the aircraft.

After a sounding is complete, the data can be analyzed and modified using a separate program, Aspen (Atmospheric Sounding Processing Environment). Aspen has the following capabilities:

  • Automatically apply quality control procedures to the sounding data
  • Present data in tabular and graphical forms
  • Automatically determine levels and code them in WMO message formats
  • Transmit the WMO messages to other systems
  • Save the raw and derived data products in various formats

Since Aspen can process data provided in the AVAPS “D” file, NCAR GLASS and NCAR CLASS formats, it is able to analyze both dropsonde and upsonde soundings. Aspen is designed to operate as automatically as possible, while allowing the user to have some control over the QC methods. For instance, as soon as the user selects a sounding file for processing, the data is brought into Aspen and automatically analyzed. In most cases this first pass will be the only one required. If the processing needs to be modified, the user can change the QC parameters and reprocess the data as many times as necessary. An extensive series of QC algorithms are applied to the data. These algorithms typically have one or two parameters that may be adjusted by the user if the default values are not suitable for a particular sounding. The user can save the modified options, so that when a new sounding is opened, the initial analysis will use the customized QC parameters. Aspen can have up to six sounding files open at the same time. This makes it convenient to compare soundings.

Additional information can be found on the Aspen web page.

Plots of typical Aspen output:



Users of the NCAR GPS Dropsonde system (AVAPS)


SSSF 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. (Due to increased costs and failure to meet design requirements, the STRATO 2C program was ultimately canceled.)

USAF Reserve Hurricane Hunters

SSSF 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-130H and 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 ATD/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 C-130, 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 NCAR's C-130 or Electra, or by the scientific research community in other aircraft, such as the NASA DC-8.

Future Developments

SSSF has begun development of a more autonomous dropsonde and aircraft data system that can be used in high altitude aircraft (e.g. WB-57, ER-2) or unmanned aerospace vehicles (UAV), and is working closely with NCAR/ATD/RAF's aeronautical engineers to design an automatic launcher for multiple sondes.  Automatic launching is necessitated by limited access by the observer/scientist and location of the launcher outside the cabin. Because its unpressurized and uncontrolled location, the launcher will also require some means of active temperature control so that the sondes do not cold-soak at the temperatures (-60 degrees C) found at these high altitudes. The aircraft data system will also be located in the same area as the launcher and therefore will require special packaging to control temperature and pressure for vulnerable components (e.g. the computer's hard disk drive).

For more information contact Terry Hock
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