ESTIMATION OF THE REAL APERTURE RADAR MODULATION TRANSFER-FUNCTION DIRECTLY FROM SYNTHETIC-APERTURE RADAR OCEAN WAVE IMAGE SPECTRA WITHOUT A-PRIORI KNOWLEDGE OF THE OCEAN WAVE HEIGHT SPECTRUM

The phase and amplitude of the real aperture radar (RAR) modulation tr ansfer function (MTF) are, applying both simulated and real synthetic aperture radar (SAR) image spectra, shown to strongly influence the SA R ocean wave imaging of range- (or near-range) traveling wave systems. Conventionally, in situ measurement of the sea state has been used in connection with SAR estimation of the RAR MTF. In most cases, the SAR imaging has been simulated by varying the phase and amplitude of the transfer function until some criterium for best fit between the measur ed and simulated spectra is met. The main problem with this method is the need for in situ buoy measurements of the underlying ocean wave he ight spectrum. This paper proposes a new method for estimating the RAR MTF directly from the SAR ocean wave image spectrum. Hence the method differs from previously used methods in that it is independent of in situ measurements of the sea state. The only (weak) restriction is tha t the observed wave system is range- or near-range traveling. On the b asis of three range-going profiles the RAR MTF phase and amplitude are estimated. Investigations using synthetic data reveal that the SAR im age spectrum for realistic sea states is colored by the unknown transf er function to such an extent that the underlying wave spectral form i s not critical. Experimentally, the phase and amplitude of the RAR mod ulation are computed using the Norwegian Continental Shelf Experiment 1988 data. It is shown that the phase is most important for the SAR sp ectral distribution. Typically, the phase is observed to be in the int erval from 60-degrees to 110-degrees and the amplitude to be of the or der of 10(-18). Furthermore, it is shown from simulation studies that marked changes in real SAR image spectra crossing an atmospheric front are recreated when the measured MTF phase and amplitude are used. Eve ntually, the hydrodynamic modulation is also extracted from the RAR MT F data. Variations of the hydrodynamic MTF phase across the abovementi oned front are focused on. The estimates confirm a consistent wind dir ection induced modulation on each side of the front. No marked trends are observed for the amplitude. The overall conclusion of the study is that the conformity between simulated and measured spectra is improve d when measured RAR MTFs are incorporated in SAR imaging simulation pr ocedures.