ERS‐1 and Almaz estimates of directional ocean wave spectra conditioned by simultaneous aircraft SAR and buoy measurements

Abstract

Abstract The European Space Agency ERS‐1 C‐band V‐V polarization synthetic aperture radar (SAR) and the Russian Almaz S‐band H‐H polarization SAR are compared for their wavenumber response to ocean wave fields existing on 23 November 1991 at the Grand Banks site of the North American ERS‐1 SAR Wave Spectra Validation Experiment. Two‐dimensional wave spectra from two Wavec heave, pitch and roll buoys and a Canadian CV‐580 aircraft C‐band V‐V polarization SAR are used to condition a linear modulation transfer model of wave imaging with SAR. A model of hydrodynamic modulation is included with the velocity bunching and tilt imaging mechanisms to better understand wind and wave interactions. Krogstad's quasi‐linear formulation of Hasselmann's ocean‐SAR integral transform is applied to model SAR velocity bunching and azimuth smearing. Narrow and broad bandwidth components of azimuth wavenumber response for the aircraft SAR are associated with, respectively, non‐linear and incoherent velocity smearing during Doppler resolution of the radar scene. The stationary resolutions of the SAR systems are compared for homogeneous scenes of wind‐roughened, but fetch‐limited, sea surfaces. This comparison is conducted in the Chesapeake Bay of Maryland using ERS‐1 and Almaz satellite imagery collected on 9 May 1992 and 14 May 1991, respectively. The results confirm that SAR imaging of ocean waves can be improved by flying platforms with low R/V (range/velocity) ratios to alleviate the azimuth velocity smear, and near‐nadir incidence angles to increase the effect of tilt modulation.