Coherence scales for microwave Bragg scatter

The transfer functions used in most inversion schemes for calculating ocean surface directional wave spectra from SAR images are simple functions of k, the wave number, and are based on three modulation processes which are tilt modulation, hydrodynamical modulation and the velocity bunching effect. Alpers et al. (1981) indicated that the effect of velocity bunching on the SAR resolution on the sea surface was not included in his analysis. Heron and Amadon, (1992) showed that spatial resolution of the SAR system is adversely affected by velocity bunching. The present authors show that the tilt modulation mechanism has an impact on limiting the coherence scale of the Bragg scattering waves on the sea surface. For typical sea conditions they find that the coherence lengths are of the order of 1-10 metres. This calculation of coherence lengths gives a firm theoretical basis for the assumption that the coherence length is greater then the Bragg wavelengths but shorter than the SAR resolution scale.