Processing Synthetic Aperture Radar Data of Ocean Waves

Abstract

The usual operation of a Synthetic Aperture Radar (SAR) assumes that the sensor platform moves at a constant velocity along a straight line and that objects to be imaged are stationary. The relative motion between the sensor and the objects produces a systematic set of Doppler frequencies which varies as the objects pass through the radar beam. These Doppler histories are recorded and processed to produce the fine resolution radar image of the objects. Propagating ocean waves as well as surface currents are not stationary and consequently perturb the Doppler histories in the SAR data. These perturbations can be troublesome when imaging gravity waves because the wave images are dispersed and thus defocused in the azimuth (along-track) direction. This defocusing can be compensated in the processor by readjusting the azimuth focus by an amount proportional to approximately the phase velocity of the wave. The perturbations of the Doppler histories can also be useful since they contain information pertaining to wave height and surface currents. Radial (line of sight) motion of ocean waves imaged by a SAR also creates problems in processing. Radial motion causes both an abnormal curvature in the recorded signal histories as well as a shift in the average Doppler frequencies. This effect can be partially compensated in the SAR processing by rotation of the cylindrical lenses by an amount proportional to the radial velocity. Perturbations of the SAR Doppler histories can also be used as a diagnostic tool to aid in the study of the SAR imaging mechanism of ocean surfaces. Adaptive manipulation of Doppler histories can be performed relatively easily in the SAR optical processor and the resulting wave imagery examined.