<title>Optical processor for SAR image formation: system development</title>

Abstract

We describe the current status of a hybrid optical processor being developed for real-time synthetic aperture radar (SAR) image formation. The processor is being developed for insertion into the ERIM spotlight mode SAR airborne data collection system under the ARPA TOPS program monitored by of the Army Research Lab. A 2-D Fourier transforming time-integrating interferometrically based optical processor is a key element of the system. The optical processor uses a modulated laser diode for radar signal insertion, crossed 1-D acousto-optic scanners for 2-D scanning, a modified Köster interferometer for fringe generation, and fast detector arrays for light detection and integration. The image space-bandwidth-product and dynamic range are enhanced by processing time-multiplexed interlaced image subpatches at real-time rates. Digital pre-and post-processing play essential roles in the system enhancement. The final image is a mosaic of the subpatch images. The optical processor design approach lends itself to the important attributes of high (real-time) data rates, multiple SAR mode processing capabilities, compact and rugged packaging, and power efficiency. INTRODUCTIONThe development and airborne demonstration of a compact real-time optical processor for synthetic aperture radar (SAR) signal processing is being performed under the ARPA TOPS Program through the Army Research Laboratory. Airborne insertion demonstration will be accomplished using the ERIM flight facilities and CV-580 aircraft with its state-of-art SAR system, the ERIM DCS (Data Collection System). The program includes development of the processor interfaces and instrumentation for monitoring flight operations and collecting test data and imagery.Synthetic aperture radar development began in the 1950's and it has evolved into an effective fine resolution imaging sensor. Real-time SAR image formation processing places very demanding requirements on processor performance. The required two-dimensional processing of large time bandwidth product (or space bandwidth product) data, generally requiring correlation and/or Fourier transform operations, provide the inspiration for applying optical techniques to address the computational as well as the quick access memory needs. Our optical processor development described here uses these important features with the attributes of power efficient real-time operation, small size, and adaptability.As indicated the optical processor is being designed for insertion into the ERIM DCS. This radar system has been described in a previous publication and only a short summary will be given here1. It is a fine-resolution spotlight mode SAR. As the aircraft flies along, the radar antenna is aimed to continually illuminate or spotlight a scene of interest during the desired integration time. See Fig. 1. Potentially, it could illuminate a spot with 90° of line-of-sight rotation. Typically, a much smaller angle is needed to obtain the desired resolution. After the required angular extent has been obtained the antenna is aimed to ill...

<title>Optical processor for SAR image formation: performance trades</title>

“…Thus, we can formally write that the complex image S(F) is given by (1) where s(f) is the PH signal after motion compensation and weighting has been applied, and f is the PH (spatial) frequency coordinate given by __ (2) where u is the instantaneous radar frequency, R0 is the instantaneous unit vector line-of-sight from the so called motion compensation point (MCP) to the radar, and c the speed of light constant. The detected image is S(i) I2• Thus, as implied by Eq.…”