Radar scientists and engineers at NASA's Jet Propulsion Laboratory have developed a new, more accurate, airborne radar system for topographic mapping of the Earth's surface.

The instrument, called TOPSAR for topographic synthetic aperture radar, has many potential commercial and scientific uses and will be about three times more accurate than any topographic mapper now readily available, said Dr. Howard Zebker of JPL.

TOPSAR is an interferometric radar mapper and is carried aboard NASA's DC-8 aircraft.

Radar interferometry measures the difference from each of the two antennas to a point on the ground to determine the height of that point by triangulation. The separation of the antennas, which forms the third side of the triangle, is called the baseline.

The instrument, developed in collaboration with an Italian consortium, is a prototype for a possible NASA satellite mission to map the entire globe at high topographic resolution, Zebker said.

The mission, TOPSAT, may be launched later in this decade.

JPL currently operates a multifrequency radar, called AIRSAR, aboard the NASA aircraft and TOPSAR uses much of the AIRSAR hardware. But several modifications were implemented to achieve optimum performance in topographic mapping, Zebker said.

"Our goal here is to provide an operational instrument capable of delivering digital elevation models at a height accuracy of 2 meters and a spatial resolution of 10 meters," he said.

The present instrument has an accuracy of only three meters in height, however, Zebker said, while the scientists continue to work eliminating phase errors that result from, among other things, aircraft motion.

The radar pulses are transmitted from a single antenna and received simultaneously at two different antennas. The best performance is achieved by minimizing errors in baseline length distance to each point in the image, and distortion in the data processor, Zebker said.

But the aircraft attitude is also important, he said, because the roll angle of the aircraft can be translated into an error in look angle of the radar.

Modifications in AIRSAR hardware and new computer software on board the plane are being used to get the accuracy the instrument is capable of achieving. TOPSAR was tested in topographic mapping of several sites in the United States and Europe.

Zebker said the instrument could be used scientifically to analyze geological processes expressed in surface topography. Commercially, he said, it could be used for land use and water drainage studies and to aid in management of disasters such as landslides and earthquakes.

"It's potentially a much cheaper way of mapping large areas than anything now available and is roughly three times more accurate than the U.S. Geological Survey's standard product," he said.

The instrument was described in a paper by Zebker and co-authors Soren Madsen and Jan Martin of JPL. The interferometric antennas mounted on the DC-8 were developed by Alenia S.p.A. under contract from the Italian Consortium for Research and Development of Advanced Remote Sensing Systems.

The research and development at JPL was under contact to NASA's Office of Space Science and Applications.

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