Theoretical limitation of the sea on the detection of low Doppler targets by over-the-horizon radar

Abstract

A theoretical model for the Doppler spectrum of high frequency (HF) backscatter from ocean waves (sea clutter) is used to determine the limitations that this clutter could impose on the detection of ships by skywave over-the-horizon radar (OTHR). Sea clutter spectra were computed as functions of radio frequency, sea state, radar-to-wind angle, and the amount of coherent integration employed in the processing of these spectra. Target-ship radar cross sections of 30 dBm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 50 dBm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> were utilized, and a radar antenna aperture of 2.5 km (yielding a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5\deg</tex> azimuthal resolution at 15 MHz) and a range resolution of 1.5 km were assumed for this study. The results of this analysis are presented in graphical form in terms of target ship detectability as a function of radial velocity, assuming that a minimum signal-to-clutter ratio of 10 dB is required. It is generally concluded that target detection is better at lower sea states, higher radio frequencies, and when coherent integration time is increased. The best computed capability exists at 25 MHz, where positive detection is predicted for both 30 dBm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 50 dBm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> targets with radial velocities either near zero or in excess of 10 knots, in any sea state. This capability is typical for daytime ionospheric propagation conditions. At a frequency of 7 MHz-and for small target ships-the detection capability is much more sensitive to the sea state; this capability is quite viable for stationary or transiting ships in sea state 2, but marginal for target ships in sea state 8.