Directional wave spectrum measurement with multistatic HF surface wave radar

Abstract

HF surface wave radars (HFSWR) measure sea surface currents and current profiles via the Doppler shift of the discrete (first-order scattering) line structure in the radar Doppler spectrum. They can also exploit the higher-order scattering contributions to the Doppler spectrum which yield information on the sea surface directional spectrum in the gravity wave band. Most HFSWR systems operate in a quasi-monostatic configuration, that is, they employ separate transmit and receive antennas but with a baseline d much less than the range R to the nearest resolution cell of interest. Under conditions where d<<R, the well-known monostatic second-order scattering formula of D. E. Barrick (1972) can be used without appreciable error. In order to determine an independent second component of the surface current vector, or to obtain additional wave Eeld information, or to resolve ambiguities, researchers have sometimes deployed two quasi-monostatic HFSWR systems which interrogate a region of interest from different directions. Alternatively, one such system may be used consecutively at two or more locations. This arrangement is sometimes referred to as stereoscopic radar. A third option, now reported, is to employ two spatially-separated receiver (or transmitter) systems in conjunction with a single transmitter (or receiver), that is, a multistatic radar. To be effective, at least one Tx-Rx combination will involve bistatic scattering for which the constraint d<<R is not satisfied. In May 1998 the authors deployed an experimental multistatic HFSWR system near Darwin, Australia, with a number of objectives including the validation of a multiple scattering theory of HF radiowave propagation across the sea surface and the inversion of multistatic sea clutter Doppler spectra to yield unambiguous directional wave spectra.