Comparison of SIR‐C SAR wavenumber spectra with WAM model predictions

Abstract

During April and October of 1994, the Space Radar Laboratory (SRL) flew on the space shuttle Endeavour at the relatively low altitude of 215 km. Using horizontal polarization, C‐band signal from the spaceborne imaging radar (SIR‐C), an onboard processor, designed and fabricated by the Johns Hopkins University Applied Physics Laboratory, formed synthetic aperture radar (SAR) images and computed over 100,000 corresponding image spectra. The low altitude, small look angle (23°–25°) and the use of horizontal polarization minimized both the loss of azimuth resolution caused by ocean surface motion and the relative contribution of the hydrodynamic component of the modulation transfer function. As a result, the SIR‐C SAR was able to image azimuth‐traveling waves with minimal distortion. After using linear inversions to convert image spectra to wave height‐variance spectra, the distributions of wavenumber and propagation direction from the processor‐derived spectra were consistent with wave model (WAM) predictions. Although the SAR wave height‐variance spectra underestimated significant wave height (SWH) at the higher SWHs, this error is compensated for by a simple linear correction. We collected 57 pairs of crossover spectra where the ground track pairs were nearly orthogonal. The crossovers were separated by 6 hours. Crossover comparisons show that the retrieved spectral parameters are independent of wave propagation direction. At this altitude and configuration, the SAR range and azimuth responses are nearly equal. The real‐time processing of spaceborne SAR data to produce accurate estimates of wavenumber, propagation direction, and SWH is clearly feasible with the orbital and instrument geometry of SIR‐C.