The validity of the composite surface model and its applications to the modulation of radar backscatter

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Abstract
The composite surface model is compared with the Kirchoff method and shown to be equivalent for radar backscatter at moderate incidence angles in most commonly occurring sea states. Based on this comparison, limits are determined for the validity of the composite surface model. The model is then utilized to study the modulation of the radar cross-section of the ocean surface by long surface waves and internal waves. It is shown that apart from the well understood tilt modulation and the direct hydrodynamic modulation of the short waves, there is also an additional indirect modulation of the radar cross-section due to the hydrodynamic modulation of the intermediate waves by the long waves. Computer simulations are used to investigae these modulation mechanisms for different sea states, incidence angles and radar frequencies. It is shown that this modulation is strongest at small incidence angles and high radar frequencies. The simulations show that internal waves may be observed by X-band and Ka-band radars due to the indirect modulation through the intermediate waves. It is also shown that, contrary to previous predictions, the indirect modulation process results in modulation of radar backscatter by surface waves that travel normal to the radar look direction.

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