Ship Classification using Multi-Frequency HF Radar

The ability to operate a multi-frequency HF radar allows one to classify ships and small boats. One makes use of the radar frequency dependence of the echo strength, or radar cross section (RCS), of individual ships. This occurs because for vertical polarization at HF frequencies, the echo strength is typically dominated by vertical structures of the order of a quarter radar wavelength, such as masts, antennas, stowed fishing lines, and, in the case of large ships, the bow and stern vertical rise from the water. Use of the 3-30 MHz range encompasses quarter wavelength ranges between 2.5 to 25 meters in height. An early paper on the measurement of the radar frequency of small boat RCS demonstrated this dependence, and is included here for reference.

The additional use of bistatic illumination, where a second (or more) transmitter is placed a few tens of miles from the receiver site, either on land or on an offshore platform, allows and additional dimension to the RCS dependence. When two or more vertical structures are illuminated using a bistatic geometry, they can produce RCS maxima and minima that vary with bistatic aspect angle, as well as ship aspect, or heading, relative to the receive site.

The first figure below shows the radar cross section frequency dependence of an ungrounded vertical mast 7.5 meters high. The shape of the region around the maximum is based on a mast width of about 10-cm, and will narrow/broaden with smaller/larger diameter masts. The solid line intersecting the Y-axis at ~37 dBm2 represents the locus along which this curve is slid for taller or shorter masts of length one quarter of the radar wavelength. More than one mast, or a tall radio antenna and a mast, can be considered as a pair of monopoles of corresponding lengths, and the RCS of the combination is a function of their spacing and aspect relative to the radar transmitter and receiver.

ISR Sensing: Ship Classification using Multi-Frequency HF Radar

Radar Measurements: The radar cross section for a small boat is shown in the figure below, and is seen to be well represented by a monopole resonant at 16.6 meters length, plus some additional monopole elements that are responsible for the other peaks. (See NRL Memo Report by Bogle and Trizna for more information and a drawing of the target boat).

ISR Sensing: Ship Classification using Multi-Frequency HF Radar

Radar Modeling: A model in which two masts resonant at 8 and 12 MHz, separated by 7.5 meters was used to calculate the RCS as a function of bistatic angle for a second transmitter. The ships heading is toward the receive antenna. The effects of the changing illumination angle for coherent addition of the scatter from the two monopoles is rather complex, but can be used to classify targets using such models.

ISR Sensing: Ship Classification using Multi-Frequency HF Radar

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