A computer program introduces a predistortion into the chirp signal used to frequency-modulate a synthetic-aperture-radar (SAR) signal at the transmitter. The predistortion is intended to compensate for distortions introduced by nonideal performance of the various pieces of SAR equipment — especially the transmitter. More specifically, the predistortion is intended to make the signal in the receiver an ideal SAR signal in that it exhibits optimum correlation performance. The program is run as a calibration routine. It is used in conjunction with a closed calibration loop that includes the SAR transmitter and receiver, plus an arbitrary-waveform generator (AWG), which generates the predistorted chirp under computer control. The program causes the AWG to adjust the predistorted chirp iteratively until the chirp in the receiver differs from an optimum or reference chirp by less than a prescribed small measure of error. In a test in which a bad elliptical filter was used to simulate (with respect to distortion) the transmitting and receiving equipment of an SAR system, the program succeeded in generating a simulated receiver chirp within less than 1 dB of optimum.

This program was written by Andrew Berkun of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Software category.

This software is available for commercial licensing. Please contact Don Hart of the California Institute of Technology at (818) 393-3425. Refer to NPO-20420.

This Brief includes a Technical Support Package (TSP).
Software for Obtaining Ideal SAR Chirps

(reference NPO-20420) is currently available for download from the TSP library.

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This article first appeared in the October, 2000 issue of NASA Tech Briefs Magazine.

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