Researchers at NASA's Jet Propulsion Laboratory are completing the first field tests of new, sophisticated radar system intended to fly eventually in space.

The system, multifrequency imaging radar polarimeter, was tested over the San Francisco Bay area in January and February aboard DC-8 airplane based at NASA's Ames Research Center, Moffett Field, Calif.

The latest in series of imaging radars developed at the laboratory over more than decade, the system offers researchers enhanced capabilities to identify objects on the ground by their radar polarization "signatures."

At aircraft altitudes, the system is able to detect objects as small as 10 meters (about 39 feet) across.

The system's capabilities are useful in such areas as determining forest types, measuring ages of geologic features and analyzing crop types.

The radar polarimeter is expected to pave the way for Shuttle Imaging Radar-C (SIR-C), an experiment planned for shuttle launch in May 1991. Previous JPL experiments in the SIR series were flown on shuttle missions in 1981 (SIR-A) and in 1984 (SIR-B).

Its design will also be used in radars on the Earth Observing System (Eos), freeflying space platform proposed by JPL as NASA mission in the late 1990s.

The new instrument replaces and upgrades one previously built at JPL that was destroyed when NASA Convair 990 plane carrying it caught fire in July 1985 during an aborted takeoff from March Air Force Base, Calif.

Like radars flown on the JPL-built Seasat satellite in 1978 and on the shuttle in the 1980s, the new system is side-looking, synthetic-aperture radar. These radars "illuminate" the Earth's surface with bursts of microwave energy. Signals reflected from the Earth are then processed to provide maps of ground features.

Both the Seasat and previous SIR radars, however, were limited to single frequency band and emitted energy that was polarized in single direction, horizontal. The new system actually consists of three radars operating in separate microwave frequency bands -- L-band (about 1250 MHz), C-band (5300 MHz) and P-band (440 MHz).

Each of the three radars is also capable of emitting and receiving energy either horizontally or vertically polarized. Each radar produces four maps of given target -- one with energy sent horizontally and received horizontally, second with energy sent horizontally and received vertically, third with energy sent vertically and received vertically, and fourth with energy sent vertically and received horizontally.

Using sophisticated data analysis methods, researchers can use technique called polarization synthesis to combine the four images. This in turn makes it possible to distinguish the "signatures" of various kinds of ground objects including vegetation types, geologic formations and manmade objects.

A particular feature of the polarimetric technique is the capability to "tune out" backgrounds and enhance desired natural or manmade target objects. It may be possible, for example, to distinguish various species of trees using the radar system.

With interferometric techniques, the system can measure velocities of slow-moving objects. The JPL radar destroyed in 1985, for example, was able to measure the speed of currents in San Francisco Bay moving as slow as one-tenth knot.

After additional engineering tests, the radar system will be flown in series of "ground truth" experiments in Alaska. In those experiments, radar data from the system will be compared with radiometer data from an orbiting meteorological satellite and with data collected on the ground.

The system will then be used in some 25 flights supporting variety of NASA-sponsored oceanographic and geological investigations.

The JPL research is funded by NASA's Office of Space Science and Applications.

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