Experiment Proves New Weather-Data Collecting Technique

September 19, 1996

Government, university, and private-sector partners who 18 months ago launched a small radio-signal receiver into space believe it proves that the existing network of 24 Global Positioning System (GPS) satellites offers tremendous opportunity for global weather prediction and climate change research.

Greatly improved weather forecasts could be realized within months after launch of a small fleet of microsatellites carrying radio receivers similar to the prototype instrument, say researchers collaborating in the Global Positioning System/Meteorological (GPS/MET) Satellite Program.

The GPS network is an advanced satellite system developed by the military for precise navigation. It is being used increasingly for scientific and commercial purposes. Researchers who envisioned using the system to measure properties of Earth's atmosphere formed an alliance that got their program off the ground 22 months after it was funded.

"Results of the experiment show that meteorologists, for the first time, could have weather data of unprecedented accuracy, every hour of the day, for anywhere on Earth -- land or sea," says Jay Fein, program director in NSF's division of atmospheric sciences, which funded GPS/MET, "from the troposphere where conditions drive weather and climate, up through the ionosphere, a region where space weather affects radio wave communications and occasionally produces major electrical power blackouts."

GPS/MET researchers piggybacked their eight-pound, laptop-size sensor on the MicroLab-1 Satellite. MicroLab-1 was launched on a Pegasus rocket flown from California's Vandenberg Air Force Base on April 3, 1995.

The receiver flies in a 450-mile Low Earth Orbit, picking up radio signals emitted by the GPS satellites orbiting at 12,500 miles. Radio signals are slowed and bent by refraction as they pass downward through increasingly dense layers of Earth's atmosphere. By measuring the angle of refraction, program scientists theorized, it should be possible to measure changes in air density and, given that, calculate temperature, pressure, and humidity with high precision for all layers of the atmosphere, top to bottom. It would be theoretically possible for a single GPS/MET instrument to get 500 "soundings" of the Earth's atmosphere every 24 hours. Voyager scientists in the 1970s and 1980s used the same radio "occultation" techniques to measure the properties of atmospheres of the outer solar system planets.

GPS/MET scientists reported initial results from their proof-of-concept project in January in the Bulletin of the American Meteorological Society. They have steadily been accumulating data since: As of late August, the team had cataloged approximately 45,000 radio occultations, says GPS/MET program manager Michael Exner of UCAR, the University Corporation for Atmospheric Research. The program goal originally had been to accumulate one thousand such measurements.

Conventional space-based techniques measure average temperature for layers of atmosphere 5-to-10 kilometers thick. GPS/MET produces an accurate temperature every 500-to-1,000 meters. Also, GPS/MET takes data above 25-to-30 kilometers, where radiosonde weather balloons burst. A single radiosonde station takes data only once or twice a day, and only from land. Land covers only a quarter of the Earth's surface, so there is a void of data for the remaining three-quarters of Earth's atmosphere over oceans, a void that GPS/MET fills.

University of Arizona

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