Synthetic-Aperture Radar Processing Using Fast Factorized Back-Projection

Synthetic aperture radar (SAR) is a general method for generating high-resolution radar maps from low-resolution aperture data which is based on using the relative motion between the radar antenna and the imaged scene. Originally conceived in the early 1950s [1], it is extensively used to image objects on the surface of the Earth and the planets [2]. A synthetic aperture is formed using electromagnetic signals from a physical aperture located at different space-time positions. The synthetic aperture may therefore observe the scene over a large angular sector by moving the physical aperture. Hence, the technique can give a significant improvement in resolution, in principle limited only by the stability of the wave field and other restrictions imposed on the movement of the physical aperture. A physical aperture, on the other hand, provides angular resolution inversely proportional to aperture size such that the spatial resolution degrades with increasing distance to the scene. SAR images of the ground are often generated from pulse echo data acquired by an antenna moving along a nominally linear track. It is well known that the spatial resolution can be made independent of distance to the ground since the antenna can be moved along correspondingly longer tracks [2]. It is therefore possible to produce radar maps with meteror decimeter-resolution from aircraft or spacecraft at very large distances. The resolution in these systems are limited by antenna illumination and system bandwidth but also by other factors, e.g. accuracy of antenna positioning, propagation perturbations, transmitter power, receiver sensitivity, clock stability, and dynamic range. The ultimate limit of SAR spatial resolution is proportional to the wavelength. The finest resolution is determined by the classical uncertainty principle applied to a band-limited wave packet. The area of a resolution cell can be shown to be related to radar system bandwidth B (= fmax fmin, where fmax and fmin are the maximum and minimum electromagnetic frequency, respectively) and aperture angle #2 #1 (the angle over which the antenna is moved and radiating as seen from the imaged ground) according to [3] ¢ASAR = ̧c 2(#2 #1) c