Synthetic Aperture Radar imaging of ocean waves from an airborne platform: Focus and tracking issues

There has been a controversial issue of many years standing in airborne Synthetic Aperture Radar (SAR) ocean imaging which this paper addresses and resolves. Investigators have been strongly divided on the reasons for apparent improvement in wave contrast in response to processor focus adjustment. The dispute has centered on two parameters of wave dynamics: orbital velocity and phase velocity. This paper shows that both orbital velocity and phase velocity are of fundamental importance in the SAR wave imaging problem. The first affects the phase of the received signal, leads to velocity bunching, and is scaled by the ratio of sensor altitude to sensor velocity. The second affects the magnitude of the received signal, leads to translation of wave features during image formation (observed as blurring in the image), and is scaled by the ratio of wave phase velocity to sensor velocity, thus becoming significant for airborne radars. This treatment of the phase velocity parameter is new. It is shown that focus adjustment, as a side effect, shifts image position. This explains why experiments have appeared to “prove” that focus adjustment may be optimised for wave movement. The paper shows that better performance is obtained by compensating individual looks for wave movement before look summation, while using nominal perfect focus just as for static scatterers. The work is applicable to a full ocean wave spectrum and does not depend on the details of the scattering mechanism itself.