15 - Synthetic Aperture Radar

Abstract

Synthetic aperture is a method used to improve radar resolution in azimuth or in the direction of the velocity vector of platform. This resolution is comparable with the one that is obtained by a very large physical antenna. This chapter focuses on the optimal aspect of the synthetic aperture processing and its uses in air-borne and space-borne imaging radar applications. The synthetic aperture effect is obtained by displacement of the platform or target. Focused synthetic aperture processing takes into account the variation of target angular position with radar velocity during illumination. The received signal is expressed in relation to the transmitted signal and target range. Unfocused synthetic aperture processing is performed by a simple spectral analysis that is matched to targets located at infinity. The processing of unfocused synthetic aperture is optimum when illumination time is very short. If the illumination time is sufficiently long, the received signal is linearly frequency modulated. Target illumination time depends on antenna aperture, platform velocity, and the distance between the radar and the target. A pulse waveform of low repetition frequency is generally used in synthetic aperture radar (SAR). The time and Doppler ambiguities are related to the value of the repetition period.