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Double duty. GPS receivers could be used to monitor snowfall as well as track ground movements.

George Rosborough

GPS: Got Plenty of Snow?

A global positioning system (GPS) can help you find a campground or that new restaurant downtown, sure, but scientists have now discovered another use: When placed on platforms above the ground, GPS receivers can report the depth of the local snowpack, allowing researchers to compile maps of snowfall in real time. The discovery could help scientists improve weather forecasts and keep better track of changing snowfall patterns.

Researchers who study small movements of Earth's surface find GPS receivers a bit more troublesome than the rest of us do. That's because some of the satellite signals these receivers depend on strike the ground before they reach the antenna, forming echoes that create noise in the signal. These echoes make precise measurements--which in the case of plate tectonics, need to be down to the millimeter--more difficult. Painstaking analysis is required to factor out the noise. "They've been a nuisance for 20 years," geophysicist Kristine Larson of the University of Colorado, Boulder, says of the echoes.

Larson and colleagues had been trying to find a way to put these echoes to good use. Last spring they installed a GPS receiver in a field south of the university, checking if soil moisture affected the signal--something, she says, that could potentially benefit climate modelers and farmers. During the experiment, Boulder was hit with two late-season snowstorms.

That's when Larson's team discovered something that no one had apparently noticed before: The snow slowed the reflected GPS signals, and the delay corresponded to the snow's depth, which the team measured by hand and with ultrasonic sensors at the test site. Based on that data, Larson's team reports this month in Geophysical Research Letters, the GPS signal echoes reveal how much snow has fallen at the site of a receiver.

The discovery could be of great benefit to meteorologists and hydrologists, the team argues, because direct measurements of snowpack are sparse and difficult to obtain. (They are usually recorded with precipitation gauges and satellite surveys.) The U.S. National Science Foundation recently installed a network of 1100 GPS receivers to measure plate tectonics and volcanic activity across the western United States, including Alaska, Larson says. Most of those receivers have been installed in areas that receive snowfall. So, the same data being collected to measure plate tectonics can be used to measure snow, without the expense of installing any new equipment, she says.

The paper "is a classic case of one person's noise becoming another person's signal," says geophysicist Mike Jackson of UNAVCO in Boulder, the organization that operates the nation's largest GPS networks. Using GPS signal reflections to map snow depth "opens a broad range of research, hazard, and practical possibilities," he says. It could improve analyses of watersheds and potential floods based on snow depth, for example. And it could provide valuable feedback to help correct the noise in GPS signals, possibly influencing future antenna designs and improving measurements of plate tectonics, says Jackson.