Brian Dunbar                                                                        
December 13, 1991
Headquarters, Washington, D.C.
(Phone:  202/453-1547)

Jim Doyle
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone:  818/354-5011)


RELEASE:  91-205

NASA DEVELOPS NEW RADAR FOR ENVIRONMENTAL RESEARCH


     Radar scientists and engineers at NASA's Jet Propulsion 
Laboratory, Pasadena, Calif., 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, an interferometric radar mapper, 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 satellite mission to map 
the entire globe at high topographic resolution, Zebker said.

     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 6.6 feet and a spatial resolution of 33 feet," he 
said.

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     The present instrument has an accuracy of only 9.9 feet in 
height, Zebker said, however, 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 
are 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 ascertain the accuracy of the 
instrument.  TOPSAR was tested in topographic mapping of several 
sites in the United States and Europe.

     Zebker said the instrument could be used to analyze 
geological processes expressed in surface topography to study land 
use and water drainage and to aid in management of disasters such 
as landslides and earthquakes.

     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.

     JPL managed the TOPSAR research and development for NASA's 
Office of Space Science and Applications.

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