Research being done by the University of Arizona's atmospheric sciences department is promising improved weather predictions.

The UA is one of several research partners in the Global Positioning System/Meteorological Satellite Program, which promises weather data that is "more accurate than any current temperature measures in space," according to UA atmospheric sciences Professo r Benjamin Herman.

Michael Exner, GPS/MET program manager, said, "Conventional weather satellites use radiometers to look straight down at the Earth and measure emissions from the atmosphere, but these are very weak signals."

The GPS looks sideways through the atmosphere and the degree of bend in the radio signal is measured. The bend can be measured very precisely and converted mathematically into a very accurate temperature profile, he said.

In the 1970s, the military began launching a series of satellites into Earth's orbit to provide a global navigation system. The constellation of 24 satellites, called NAVSTAR, are the GPS.

Several years ago, scientists began speculating about using the satellites to gather weather data at high altitudes.

That speculation led to a $4.1 million National Science Foundation grant to develop the technology to convert the radio signals picked up by the satellites into medium and long-range weather forecasts.

Herman and Senior Researcher Dasheng Feng, from the UA atmospheric sciences department, created the computer software to allow for the conversions.

While the military has provided use of its NAVSTAR satellites, researchers had to provide their own radio receiver to collect the data.

In April 1995, GPS/MET researchers sent their radio receiver, about the size of a small notebook, into orbit on the Microlab-1 Satellite.

Orbital Sciences, the company launching the satellite, had extra room on board and donated it to the project.

Future receivers will probably not get a free ride into space, but the receivers are "cheaper and more accurate" than conventional weather-sensing satellite systems, Herman said.

The savings comes from using the transmitters already in orbit, he said.

GPS/MET could "save the taxpayers money in the long run," making the savings one of the key impacts of the program, Exner said.

Herman gave the following description of how the system works:

• The satellites collect radio waves and send them to the receiver, which is in low-Earth orbit.

• The radio waves are bent by the Earth's atmosphere as they travel to the receiver. The degree of bending is measured by the Doppler shift, which is the change observed in the wavelength of sound (and other waves) due to the relative motion between a wave source and receiver.

• Once the bend degree is known, the density of the atmos phere is calculated.

• Then a series of equations is worked and a temperature profile is the final result.

Plans are being discussed to put another 10 to 20 receivers into low Earth orbit. This would give scientists up to 5,000 temperature readings per day, compared to the 1,000 a day that weather balloons can offer.

In addition, the satellites take measurements over the entire globe, including the oceans, which currently go unmonitored, Herman said.

"It probably won't make much difference in 24-hour forecasts, but longer range - 48-hour to 3 to 5 days - should improve," he said.

Current funding for the project runs through December 1997, and the UA plans to stay involved as long as the government funds the project, Herman said.

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