A bistatic HF radar for current mapping and robust ship tracking

D. B. Trizna
2008 OCEANS 2008  
A bistatic HF radar has been developed for application to ocean current mapping and ship vector tracking. The radar can operate in a multi-frequency mode, so that it can map ocean current vertical shear and can provide more robust ship tracks than single frequency HF radars. This tracking robustness is achieved 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. The radar is fully digital in
more » ... equency generation and reception, and has no RF receiver components because the received antenna signals are digitized at the HF frequency directly. We use A/D conversion rates sufficiently high to maintain the 2 to 1 frequency ratio required for the highest radar frequency of interest to avoid frequency aliasing. The newly developed radar acquisition code provides real-time range compression, so that data files that are stored are in-phase and quadrature (I/Q) samples, at a much less dense rate than the original digitized signal time series. The bistatic capability is based on accurate system timing and radar frequency. These are provided at each of two or more radar sites by rubidium clocks and GPS timing, accurate for the first pulse to 50-ns to initiate data acquisition in the bistatic mode. Once acquisition is initiated, the rubidium clocks at each site maintain much more accurate frequency and time stability to allow Doppler velocity measurements accurate to 2 millihertz at 25 MHz operating frequency. The primary site requires an 8-or 16-element receive array, and both primary and satellite bistatic-illuminator sites have a modest 2 or 4-element monopole transmit antenna pair. This bistatic approach reduces the coastal space requirements because of the need for just one receive-antenna array per radar system. These systems can be operated with a pair of bistatic transmitters, either side of the receive site, to expand the spatial coverage. Using such an approach, 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 strength. Due to its digital approach, the cost of these radars is substantially less than that of existing coastal HF radars, none of which have a multi-frequency capability.
doi:10.1109/oceans.2008.5151982 fatcat:u6ovg4yepnhsxcswqduaouyhxi