First results of the imaging of ocean waves by SIR-C/X-SAR over the North Sea and North Atlantic are presented. It is shown that the SAR imaging of ocean waves depends only weakly on the radar frequency and polarization, but depends strongly on wind speed and direction. Wind streaks are imaged best at high radar frequencies and VV polarization

For the inversion of SAR image spectra into ocean wave spectra the ocean wave-radar modulation transfer function (MTF) plays an important role for waves traveling in or near the range direction. MTFs can be measured from sea-based platforms or inferred from the comparison of measured and simulated SAR image spectra. The MTFs determined by the two experimental methods have comparable moduli, but different phases. Also they do not agree with the values derived from various modulation theories. A short summary of the present status is given and possible reasons for the observed discrepancies are discussed

Estimates of the ocean wave‐radar modulation transfer function (MTF) are derived from synthetic aperture radar (SAR) imagery acquired by the American Naval Air Development Center airborne three‐frequency SAR over the North Sea during the U.S./German SAR and X Band Ocean Nonlinearities‐Forschungsplattform Nordsee experiment in November 1990. This is achieved by comparing measured and simulated SAR image spectra. The simulated SAR image spectra are computed from ocean wave height spectra measured by a pitch and roll buoy and by applying the generalized velocity‐bunching model. First, SAR simulations are carried out by using the theoretical MTF which contains the tilt and range‐bunching MTFs as well as the hydrodynamic MTF calculated from the relaxation time model. Second, SAR simulations are carried out by taking the modulus and phase of the MTF as free parameters. For waves traveling toward the radar antenna, best agreement is achieved when using the following values for the modulus |M0| and phase η of the nondimensional MTF defined by (5) and (10): |M0| = 8 ‐ 13 for VV polarization and |M0| = 12 ‐ 15 for HH polarization; η = 60° ‐ 90° past the long‐wave crest when the wind is blowing downwave, and η = 0° ‐ 60° past the long‐wave crest when the wind is blowing upwave. The values derived for the modulus of the MTF are in good agreement with values obtained from tower‐based radar backscatter measurements. However, for the downwave case the phase of the MTF disagrees with the phase obtained from tower‐based measurements, where usually values between 20° and 60° are found.