Applications of JERS-1 SAR data to the determination of the directional spectra of ocean waves using weighted cross-spectra

The JERS-1 synthetic aperture radar (SAR) was primarily designed for global land observation with its moderate incidence angle, and accordingly it is considered as being not suitable for oceanic applications. The authors show that the JERS-1 SAR can be applied to deriving ocean wave spectra and determining the wave propagation direction. Successive individual sub-images of ocean waves are first produced using split-look processing. This processing technique has an inherent property that the sequential sub-images are formed at different discretely delayed times; and hence wave propagation direction can be determined from these displaced sub-images. Although the images of ocean surfaces have low signal-to-noise ratio, the JERS-1 SAR has, being L-band, an advantage over C-band spaceborne SARs of having a longer integration time. Longer integration times imply longer inter-look time difference, suitable for the wave analysis using split-look processing. In order to demonstrate the capability of the JERS-1 SAR for wave imagery, the method of spectral phase shift, also known as weighted cross-spectra is applied to the wave data collected near the coasts of Japan. The method is compared with the standard Fourier transform spectra and the cross-correlation function of split-look images; and applications to improving wave forecast models are suggested using the ALOS-PALSAR (to be launched in 2002 by NASDA) which has even longer integration times than the JERS-1 SAR.