Heavenly Weather Forecasting?

THE GLOBAL POSITIONING System's 24 satellites have already revolutionized navigation and mapping. Next, the GPS system may--indirectly--do the same for weather forecasting.

Since April, 1995, an eight-pound "bird" has been scooting around in low earth orbit, 450 miles up. It flies through the radio signals beaming down from the 12,500-mile-high GPS network. By measuring how the GPS signals are distorted as they pass through the upper atmosphere, this space sparrow makes it possible to calculate local variations in air temperature, pressure, and humidity. Integrating such data with regular measurements, including those from weather balloons that go pop before reaching 20 miles, could lead to around-the-clock forecasts of unprecedented accuracy, says Jay Fein, who serves as director of atmospheric sciences at the National Science Foundation.

Timely coverage of the whole earth "would require a constellation of 8 to 16 microsatellites," says Michael L. Exner, head of GPS meteorological research at the University Corp. for Atmospheric Research. Such a fleet, Exner adds, could be built and launched for less than $100 million.

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