IMOS - ACORN - Direction finding ocean radar system (Seasonde)

Coastal ocean surface radar is a land-based technique which uses scattering from the rough sea surface to obtain echoes which are Doppler shifted by the dynamics of the sea. It has been shown (Crombie, 1955) that the scatter is predominantly a Bragg interaction between the radio wave and the sea surface gravity wave that is propagating in the same direction and with half the wavelength of the radio wave. This Bragg interaction gives two strong first-order lines in the echo spectrum; one from the resonant gravity wave moving radially away from the station, and one from the resonant gravity wave moving towards the station.

Direction-finding systems determine the azimuthal direction of a source point by comparing amplitude and phase signals on crossed loop receive antennas. The direction finding calculation is done on Doppler shift spectra for a specified range which includes return echoes from all azimuths. This means that the spectra are spread and second-order echoes from one direction are swamped by the first-order echoes from other directions. Consequently, parameter extraction using second-order echoes is not possible on a routine basis. Directional wave spectra at the location of each radar station are produced by analysing the full azimuthal spread of first-order echo amplitudes, and wind directions are derived across the grid points by taking the ratio of the positive and negative Bragg echoes.

SeaSonde HF ocean radar stations are used on the Turquoise Coast (Seabird and Cervantes), Western Australia, and on the Bonney Coast (Nora Creina and Blackfellows Cave), South Australia.