Features identified by surface current signatures
Verification of the Forward SAR Imaging Model
The M4S model suite consists of a wave-current interaction module for the computation of modulated surface wave spectra in the presence of spatially varying current and wind fields and a SAR imaging module for the conversion of the wave spectra into Doppler spectra of the backscattered radar signal and, finally, the corresponding SAR image. Both modules are based on well-established physical theories and expressions and include only few tunable parameters. Furthermore, there are a number of intermediate results such as wind friction velocities, wave steepnesses, the shapes of wave spectra, intensity variations of individual wave components, normalized radar backscattering cross sections (NRCS), Doppler offsets and bandwidths, etc., which are supposed to be reproduced realistically by the model and which can be validated individually by comparison with experimental data and with model results from other sources. If a comprehensive set of reference data is available, shortcomings of the SAR imaging model suite can be identified quite precisely, and parameter modifications or an addition of new or improved model components can be performed in an efficient and physically adequate way. This has been demonstrated, for example, with the development of an optimized parameterization of the input wave spectrum for best reproduction of measured NRCS values (Romeiser et al., 1997), modifications of the source function of the action balance equation and the inclusion of a surface roughness - wind stress feedback mechanism for the reproduction of wave data from a buoy and airborne SAR imagery from the C-STAR experiment (Wensink, 1999), and the validation of Doppler spectra from M4S by results of a very fundamental scattering model and experimental data (Romeiser and Thompson, 2000).
Information on the quality and on known and possible shortcomings of various components of M4S as well as results of previous validation studies and applications in other contexts will be made available with the MARSAIS prototype and demonstrator. Within MARSAIS, new and improved elements, such as a modified parameterization of the wave spectrum adopted from Elfouhaily et al. (1997), which was expected to lead to more realistic absolute NRCS values, and the inclusion of wave breaking effects, have been implemented and tested. Finally, simulated radar signatures of oceanic fronts and internal waves have been validated as far as possible on the basis of available reference data.