IMAGING OF OCEAN WAVES ON BOTH SIDES OF AN ATMOSPHERIC FRONT BY THE SIR-C X-SAR MULTIFREQUENCY SYNTHETIC-APERTURE RADAR/

Radar images of an ocean scene containing an atmospheric front and alm ost range-propagating ocean waves which were acquired by the multifreq uency/multipolarization synthetic aperture radar (SAR) aboard the spac e shuttle Endeavour during the spaceborne imaging radar-C/X-band synth etic aperture radar (SIR-C/X-SAR) mission over the North Atlantic in 1 994 are analyzed. The L-band SAR image spectra calculated from two are as located on opposite sides of the atmospheric front age quite simila r, whereas the corresponding X- and C-band SAR image spectra differ si gnificantly. It is shown that this is, a consequence of the SAR imagin g 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 loca l wind field. This is in agreement with earlier results obtained from the analysis of airborne multifrequency/multipolarization SAR images a cquired over the North Sea during the SAR and X-Band Ocean Nonlinearit ies - Forschungsplattform Nordsee (SAXON-FPN) experiment. In this inve stigation it was found that at X and C band the phase of the real aper ture radar modulation transfer function (RAR MTF) changes by almost 90 degrees, when in the reference system moving with the group velocity of the dominant wave, the component of the wind velocity in the direct ion of the wave propagation changes sign. However, at L band such a ch ange in local wind direction affects the phase of the RAR MTF only sli ghtly. 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 th e X- and C-band SAR image spectra measured on both sides of the atmosp heric front are consistent with a change in wind speed and direction a cross the front. From this we conclude that for inverting X- or C-band SAR image spectra into ocean wave spectra that contain ocean waves pr opagating near the range direction, it is quite important to have a go od knowledge of the local wind field, whereas for L-band SAR image spe ctra this is of minor importance.