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
S. Jacobsen et Ka. Hogda, 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, J GEO RES-O, 99(C7), 1994, pp. 14291-14302
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.