COMPARISON OF ACTUAL AND SIMULATED SYNTHETIC-APERTURE RADAR IMAGE SPECTRA OF OCEAN WAVES

We have simulated synthetic aperture radar (SAR) image spectra of ocea n waves which were focused for stationary scenes using three popular f ormulations of SAR ocean imaging theory: the time-dependent, velocity- bunching, and quasi-linear models. All three models require functional forms for surface wave spectra, modulation transfer functions, and co rrelation times; these were obtained from data taken during the SAR an d X Band Ocean Nonlinearities (SAXON)-Forschungsplatform Nordsee (FPN) experiment of November 1990. These measurements were made on and near the German research platform Nordsee during the same period of time t hat SAR images of ocean waves at X, C, and L bands were obtained by ai rcraft near the tower. We compare the results of our simulations to th e actual SAR spectra for a variety of integration times, range-to-velo city ratios (R/V), and asimuth angles. We find that all three models r eproduce the observed image spectra well when integration times are sm all and R/V ratios are low to moderate. Some adjustments of the parame ters measured on the tower are occasionally necessary in order to prod uce this agreement, but in most cases these adjustments are small and within measurement errors; in all cases the same parameters suffice fo r all models. However, we reproduce a result previously obtained by Br uning et al. [1994] that measured modulated transfer functions do not properly account for imagery of range-traveling waves propagating down wind. We also find that our calculated velocity spreads seem to be too large to reproduce the observations when the significant wave height is above about 1.5 m and R/V is appreciable. For long integration time s and high R/V ratios, the velocity-bunching model smears the spectrum more than the time-dependent model and more than the observations. We find that the velocity-bunching formulation is viable up to an integr ation time that is a function of R/V; for lower ratios, velocity bunch ing is viable up to longer integration times, and we offer an explanat ion of why this is so. Finally, we find that for large R/V ratios the quasi-linear model fails to produce the low-frequency components obser ved in actual image spectra and produced by velocity-bunching and time -dependent models.