Washington -- The National Aeronautics and Space Administration said it was evaluating a compact L-Band synthetic aperture radar system for use on unmanned U.S. aircraft.

NASA said the radar detects and measures small changes in the Earth's surface of geophysical interest, such as volcanoes, earthquake faults, landslides and glaciers.

NASA's Jet Propulsion Laboratory and Dryden Flight Research Center are developing the Unmanned Aerial Vehicle Synthetic Aperture Radar, or UAVSAR. A modified NASA Gulfstream III aircraft carries the radar in a custom-built pod under the aircraft's fuselage during its development phase.

The system uses microwaves to make an image, said Scott Hensley, chief scientist for the project at JPL. He said during the validation flights, the aircraft is using a technique known as repeat pass interferometry that requires it to fly each pass as close to the original flight line as possible.

For the experiment, two data passes, flown from minutes to months apart, are compared to examine changes in the Earth's surface.

Officials said the system will be extensively tested through 2008, after which it will become a community science tool for NASA.