Ship echo discrimination in H

HF radar can provide Over the Horizon detection of ships on very large oceanic areas, making use of the ionospheric refraction of radio waves. The Doppler spectrum of the sea clutter is composed of the first-order Bragg lines with a second-order continuum, as described in [1]. Normally, the Doppler shift produces by ships is small, of the same order of magnitude of the Doppler shift of the sea clutter. Consequently, it is not always easy to discriminate a ship line from a sea clutter line only on a velocity basis, though dual frequency operation can enhance ship detectability [2]. Different sources of ionospheric contamination often increase the difficulty by introducing smearing effects of the Doppler spectrum. Sometimes, these deleterious effects can be corrected by using adapted signal processing methods (see e.g. [3]-[5]). Using HF radar with large antenna arrays, it should be possible to find arrangement in radar data manipulation to obtain two signals coming back from the sea with low coherence, while the signal from a discrete target would stay relatively coherent. If verified, this property would allow the discrimination of discrete targets against sea clutter. In this paper the coherence of sea clutter is considered in the context of a radar interferometer. It is shown that the coherence function can be useful to discriminate between a ship line and sea clutter spectral peaks. The dependence of the coherence value with the signal to clutter ratio is also studied.