The scattering of high frequency electromagnetic radiation from the ocean surface : an analysis based on a bistatic ground wave radar configuration

The scattering of high frequency (HF) radiation from rough surfaces is addressed with a view to developing bistatic cross sections of the ocean surface. The analysis starts with an expression for the normal component of the electric field in the form of a two-dimensional spatial convolution involving the spatially Fourier transformed source field and the appropriate Green's function for the region of interest. This expression is reduced to integrals which are analyzed to second order in scatter. The reception point of the scattered field is kept general, and, initially, the good-conducting, slightly rough surface is chosen to be time invariant. The excitation of the assumed vertical dipole source is also kept general at the outset. Reduction of the resulting integrals is accomplished primarily via asymptotic techniques. -- The analysis leads to a first-order field component and a second-order solution consisting of three separate components. The latter account for (1) double scattering from a surface region remote from both the source and the receiver and (2) fields arising from single scattering near either the source or receiver which is followed or preceded, respectively, by a single remote scatter. These bistatic forms are shown to reduce to existing monostatic results with the introduction of the appropriate scattering configuration. -- Using the general field expressions, the source is next specified to be a vertical dipole with a pulsed sinusoidal excitation. This is done with a view to extending the analysis to obtain bistatic cross section expressions for the ocean surface when interrogation is carried out with a pulsed radar. Before this can be accomplished, time variation for the randomly rough surface is also introduced into the model. It is assumed that the surface varies much less slowly than the time necessary to obtain a single measurement of the scattered field. -- The HF bistatic Doppler cross section of the time varying surface is effected via Fourier transformation of the ensemble-averaged electric field and subsequent comparison with the radar range equation. This standard technique gives first- and second-order cross section models which are calculated and depicted by introducing an appropriate directional representation of the ocean spectrum. It is shown that all of the essential characteristics of the previous monostatic formulations are contained in these cross sections as a special case. -- Finally, a technique is developed for modelling the expected pulse radar spectrum when signal reception is externally noise limited. It is assumed that both the ocean echo and noise voltage may be represented as zero-mean Gaussian random variables. The resulting models are shown to compare very favourably with available monostatic spectra. -- The bistatic cross sections and the noise model for pulsed HF radar provide a means of setting the appropriate specifications of particular systems which may be used for ocean surface parameter estimation. Additionally, the properties of the scattering as manifested in the theory should be relevant to further developments of clutter suppression schemes for use in hard-target detection.