Katzberg explained that a GPS signa

Katzberg explained that a GPS signal is made up of radio waves emitted by a GPS satellite. When the waves strike the surface of the ocean, about 60 percent of the signal bounces off.

The more wind that pushes across water, the rougher the surface of the sea gets. Bigger, more chaotic waves break up the GPS radio waves more as they reflect off the surface. A receiver can compare that disruption to the GPS signal coming from any particular satellite, and then use software to estimate the surface wind speed based on past data and calibration.

Katzberg says the technique has an accuracy better than 5 meters per second (about 11 miles per hour or 18 kilometers per hour), compared with an accuracy of about 0.5 meters per second (1.1 miles per hour or 1.8 kilometers per hour) for direct measurement by NOAA's dropsondes—tubes that are loaded with instruments and then dropped from a plane. They float down with a parachute, collecting data as they fall.

The GPS-based system works best over large expanses of water, Katzberg said. It cannot measure wind speed over land because it requires a change in the roughness of the surface, and that doesn't happen on a measurable scale as wind blows over the land. GPS can be used to measure whether something is ice or water, however, because there is a big difference between those two states of matter in terms of how they reflect signals.

For the receivers, Katzberg says, "We use very inexpensive systems, and no bigger than what is in a cell phone. It is basically throwaway, and we can glue it to the bottom of an airplane, a model airplane, or whatever can get up high."

For his studies, Katzberg placed the receivers on NOAA research planes, but he said there is no reason why they couldn't be put on commercial jets or drones. In fact, the GPS technology required to do these measurements is already in many consumer cell phones, representing an additional cost of "a few dollars" to implement such a receiver.

Katzberg pointed out that the technique is not free since it relies on GPS satellites putting out the required signals, but he added that they are already in place.

Why Is It Important?

"Our paper showed that you can make competitive measurements [of wind speed] with GPS," Katzberg said. He explained that the results stood up well to the traditional technique of instruments dropped from a research plane, when conditions were right.

GPS can only measure wind speed right above the ocean's surface. It also has difficulty with the eye of a storm, where wind speed is low but the water remains choppy.

To Katzberg, GPS is likely to prove a powerful new tool to meteorologists, giving them additional information about storms. They will still need to use dropsondes to measure high-altitude winds, but they may be able to decrease the number of drops—a boon considering each dropsonde costs several hundred dollars and can be used only once.

Katzberg said the beauty of using GPS to measure phenomena is that receivers are cheap and easy to obtain, so they will be available to many scientists. "This is unbroken ground in remote sensing research," he said. "The results for GPS planted a little flag in this ground, but the sky is the limit."

He added that the technology can be used to measure the amount of moisture in the first few inches of soil. The wetter the ground, the better it reflects the signal.