OCEAN IMAGING WITH 2-ANTENNA RADARS

Synthetic aperture radars (SAR's) have long been used to image ocean waves. The measurements are affected both by the radar backscattering cross section (sigma-0) and by the radial velocity (V(r)) of the surface. Velocity bunching can cause confusion between these two phenomena for single-antenna radars. We can use SAR's with two antennas to help separate the effects of sigma-0 and V(r) in situations where a single-antenna SAR would be impractical. We present an analysis of the capabilities of two-antenna radars, using a novel method for forming radial-velocity images, to provide a general framework for evaluating the performance of different possible instruments. We derive several key results. We can measure sigma-0 and V(r) separately, given a properly designed SAR. The error in V(r) depends on the thermal noise in the receivers, the spacing of the two antennas (2B), the coherence time of the surface (tau(c)) and the spatial resolution of the measurement. There is an optimal separation of the antennas proportional to upsilon-tau(c), where upsilon is the speed of the aircraft. In cases where radial velocities are unimportant, we can use two-antenna SAR's to image azimuth-traveling waves with wavenumbers larger than the usual azimuth cutoff. We show how phase errors affect the accuracy of the measurements of V(r). In this development, we also provide a simple explanation for velocity bunching, a subject that has caused much controversy.