Imaging of ocean waves on both sides of an atmospheric front by the SIR‐C/X‐SAR multifrequency synthetic aperture radar

Abstract

Radar images of an ocean scene containing an atmospheric front and almost range‐propagating ocean waves which were acquired by the multifrequency/multipolarization synthetic aperture radar (SAR) aboard the space shuttle Endeavour during the spaceborne imaging radar‐C/X‐band synthetic aperture radar (SIR‐C/X‐SAR) mission over the North Atlantic in 1994 are analyzed. The L‐band SAR image spectra calculated from two areas located on opposite sides of the atmospheric front are quite similar, whereas the corresponding X‐ and C‐band SAR image spectra differ significantly. It is shown that this is a consequence of the SAR imaging mechanism; at L band the SAR imaging mechanism depends weakly on the local wind field, and at X and C band it depends strongly on the local wind field. This is in agreement with earlier results obtained from the analysis of airborne multifrequency/multipolarization SAR images acquired over the North Sea during the SAR and X‐Band Ocean Nonlinearities ‐ Forschungsplattform Nordsee (SAXON‐FPN) experiment. In this investigation it was found that at X and C band the phase of the real aperture radar modulation transfer function (RAR MTF) changes by almost 90°, when in the reference system moving with the group velocity of the dominant wave, the component of the wind velocity in the direction of the wave propagation changes sign. However, at L band such a change in local wind direction affects the phase of the RAR MTF only slightly. Using this phase behavior of the RAR MTF in simulations of the SIR‐C/X‐SAR image spectra, we show that the observed differences of the X‐ and C‐band SAR image spectra measured on both sides of the atmospheric front are consistent with a change in wind speed and direction across the front. From this we conclude that for inverting X‐ or C‐band SAR image spectra into ocean wave spectra that contain ocean waves propagating near the range direction, it is quite important to have a good knowledge of the local wind field, whereas for L‐band SAR image spectra this is of minor importance.