C-band HH-polarized along-track interferometric synthetic aperture radar (ATInSAR) images of ocean waves from the Canada Centre for Remote Sensing (CCRS) CV-580 SAR are used to study ATInSAR wave imaging fidelity. ATInSAR phase spectra are derived using cross spectra between the interferogram phase generated from individual look complex SAR images. The resulting observed phase spectra are quantitatively compared with forward-mapped in situ directional wave spectra collocated with the ATInSAR observations. The forward mapping models include two quasilinear forward transforms that include varying degrees of velocity bunching nonlinearity, both of which include an azimuth null in the modulation transfer function, and an implementation of the fully nonlinear forward transform. The null arises from the cancellation of upward and downward orbital velocity components within a resolution cell. The second of the two quasilinear transforms and the nonlinear forward transform are shown to have agreement with the observed velocity spectra, as measured by correlation coefficients between the observed spectra or forward-mapped spectra and the ATInSAR spectra. ATInSAR can allow a direct measurement of the orbital velocity spectrum. There is, however, an added layer of complexity caused by the cancellation of upward and downward orbital velocity components within a resolution cell. Furthermore, the SAR complication of velocity bunching nonlinearity is not avoided by using an ATInSAR.
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