|
In this paper we discuss the properties of two different methods for extracting information of non-linear mechanisms influencing the backscatter of microwaves from the ocean surface. In the first of these methods, denoted the Multi Frequency Technique (MFT), several frequencies, distributed in a narrow band around a carrier frequency, are simultaneously transmitted, and the non-linearities are detected as secondary peaks in their mutual cross-product spectrum. This technique has been extensively discussed in earlier papers as a proper method for extracting sea surface information (Alpers, W., and K. Hasselmann, 1978). The second method uses only the transmitted frequency and the non-linear effects are detected as secondary (or over-harmonic) frequency peaks in the bispectrum of the backscattered signal. This method has been successfully applied to studies of non-linear wave-wave iterations in experimental plasma physics, but has to the authors' knowledge not been used in studies of microwave scattering from the sea surface. We refer to this method as the Bispectral Analysis Technique (BAT). In order to correctly interpret the different signatures observed in microwave remote sensing of the ocean surface, it is important to fully understand how various physical phenomena influence the backscattered signal. The data used in this work, are generated by a numerical simulator. Based on the Holliday scattering model and a theoretical description of the power spectrum of the surface elevation, we are able to study in detail how various physical and geometrical conditions influence the backscattered signal. Specifically, we address the problem of detecting non-linear hydrodynamical phenomena induced by non-linear hydrodynamical phenomena (Stokes-type gravity waves) or non-linear modulation mechanisms (tilt and hydrodynamic modulation), using the MFT and BAT. We show in this paper that both methods are capable of detecting non-linear features, but that the performance is heavily dependent on the size of the foot-print relative the wave-length of the waves in question. The MFT can only be used with when the size of the foot-print is (much' greater than the waves, while the BAT applies to the opposite case.
|
