Space Lasers Take To The Winds

The Laser Radar (lidar) can fly on normal DC-8 Aircraft to detect wind direction and speed. An earth based lidar shooting up a sodium resonance beam over the night sky of Puerto Rico.

If this sounds a lot like Doppler radar, that's because it is. But there are some important differences between the two technologies.

"Radar excels at piercing bad weather, but it needs raindrops or hydrometeors (hail or snow) to get a signal," Kavaya said. "Lidar struggles to go through thick clouds or heavy rain, but it can get you wind (measurements) in clear air, because it relies on aerosols."

The reason for this difference is the frequency of the radiation that each technology uses. Both emit electromagnetic waves, but while radar typically uses frequencies in the range of microwaves, lidar uses higher frequencies in the visible or near-visible light range. Higher frequency radiation (light) will be reflected by smaller particles than lower frequency radiation (microwaves).

Lidar also emits a narrower beam than radar, which minimise interference from ground clutter and improves the resolution of the data. The disadvantage is that lidar has more trouble covering large areas than does radar.

Rivers are fair game, too

The narrower beam opens up another possible application for lidar: measuring water flow in rivers.

Scientists at the Marshall Space Flight Centre and the GHCC have been working with the U.S. Geological Survey to see if lidar may be able to replace the manual stream-flow measuring technique that the USGS currently uses.

"It turns out that some of their standard methods for measuring stream current involved an element of risk to personnel," said Dr. Jeff Rothermel, a NASA scientist at the GHCC. "In fact, one USGS employee lost his life in the line of duty while making measurements. So there is an interest there to determine whether lidar can be used to measure stream current."

Mounted either at the side of the stream or on a satellite in space, a lidar system would measure the speed of the water's surface at several points across the width of the river. Knowing the shape of the river's bottom would allow the volume of water flowing in the river to be calculated from those measurements.

"[In addition to US rivers] I imagine that we could contribute to the study of the Earth's hydrology greatly by having improved river flow (data) worldwide," Kavaya said.