Study On The Retrieval Of Ocean Wave Spectrum From Complex SAR Data

The ocean wave is a significant small-scale phenomenon happening atthe sea surface.For its essential characteristics of complication andrandomness, the research on ocean waves relies on a great quantity ofmeasurements in order to have a knowledge of the statistical property ofwaves.For this reason, it is summarized that the fundamental theory andissues of ocean waves in the thesis.Synthetic Aperture Radar (SAR) is one of the most effective tools inocean wave measurement.Based on the high resolution images from SAR theocean wave spectra retrieved from SAR images have an irreplaceableadvantage over other approaches in spatial consistency and coverage. Itis presented that the physical mechanism of SAR imaging of ocean wavesand the principle of digital synthesizing aperture.The conventional SAR data or SAR image contains real data for eachpixel, which only offers statical information of the image. Therefore,the retrieval of ocean waves from conventional SAR images encounters aproblem of ambiguity in the wave propagation in classical solution ofretrieval of ocean waves. As a result, the retrieval can achieved onlythrough other information, such as the wind field or numerical wave model.Wave mode products apply the complex data of SAR image, which iscalled an imagette. The phase of the imagists embodies the motion of seasurface, namely the propagation of ocean waves, which means it is possiblefor the certainty of wave propagation. Through multi-looking processing,separate looks can be obtained from the imagette. Using the two looksseparated by a short interval, the cross spectrum is achieved. The crossspectrum can detect small difference in phase between the two looks andgive the azimuthal angle of waves in the imagette. By generating two separate looks with clear steaks and a short timeinterval, the properties of cross spectrum associated with the looks areinvestigated.The intensity distribution of the cross spectrum isanalysized in the real and imaginary part respectively.The anti-symmetryimaginary part solves the ambiguity problem due to the symmetric real part.Thus, the wave propagation is determined without180degree ambiguity.Engen proposed a nonlinear transformation from wave spectrum to thecross spectrum. Under the quasi-linear approximation, the crossspectrum of a given wave spectrum is simulated based on the transformfunction.The results of the simulation indicate the dominant wavenumberand propagation appearing in the given wave spectrum.The cross spectrum of the separate look pair is achieved bymulti-looking the imagette from ENVISAT-ASAR Wave Mode product. And thedominant wavenumber and propagation are retrieved according to thecorresponding wavenumber of peaks in the real and imaginary part of thecross spectrum.