A Bistatic HF Radar for Current Mapping and Robust Ship Tracking

A bistatic High Frequency (HF, or 3-30 megahertz band) radar has been developed for dual-use application to ocean current mapping and ship vector tracking. The radar can be deployed in a multi-frequency version, allowing it to sample ocean currents at different depths between 20 centimeters and 3 meters, and thus map ocean current vertical shear. The multi-frequency feature allows for more robust tracking than single frequency HF radars by avoiding target fading due to echo nulls from frequency and azimuthal variations in ship radar cross section that occur using a single radar frequency. Modest ship classification can make use of these nulls and peaks. More details on the radar system can be found elsewhere, along with ocean current shear measurements. The bistatic capability is based on accurate system timing and radar frequency accurate to 2 millihertz, provided at each of two or more radar sites by rubidium clocks and GPS receivers. The primary site also transmits a monostatic signal and uses a 4 to 16element receive array, and all sites use a transmit monopole or a short log periodic array for multiple frequencies. This bistatic approach reduces the infrastructure and coastal site requirements because of the need for just one receive-antenna array per radar system, with as many as two remote transmit sites, either side of the receive site, to maintain optimal spatial coverage along the coast. These units could be staggered to create a system of radars, providing continuous coverage along a coastline, alternating transmit and receive sites. This type of arrangement could be used to provide robust ship tracking along a country’s coastline, and a modest estimate of type and tonnage of all vessel traffic based on target echo characteristics versus radar frequency. Due to its digital approach, the cost of these radars is substantially less than that of commercial HF radars currently available, none of which have a multi-frequency capability. The principle of how echo peaks and nulls can be used for classification is addressed here using the simple two mast ship model. However, the bulk cross section that dominate very large container ships requires a more complex scattering model that is not addressed here. .