Use of maximum entropy spectral analysis to improve ship detection by over‐the‐horizon radar

Abstract

The over‐the‐horizon radar is able to detect ships over very large oceanic areas. Detection performance is sometimes reduced because of smearing of the sea echo due to variations of the Doppler shift imposed by the ionosphere. In this paper we propose a decontamination procedure, based upon maximum entropy spectral analysis of sea scatter Bragg lines, to correct the phase of the signal before performing classical fast Fourier transform processing. The improvement in quality of the signal after phase correction results in a sharpening of spectral peaks. In the example presented the signal was chosen to contain spectral lines of a transponder installed on board a ship, to be used as reference lines. The decrease in the spectral width is significant for the sea scatter Bragg lines and for the transponder lines. As the Bragg lines sharpen, the ship echo is more easily detected in the spectrum. The proposed decontamination procedure involves more computation load than the simpler frequency shift method used for sea state monitoring, but, on the other hand, it can handle Doppler shift variations during coherent integration time.