IMOS - ACORN - Phased array HF ocean radar system (WERA)

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.

Phased array systems determine the azimuthal angle between the station and a point on the sea surface by forming a narrow beam; the range to the point is set by the propagation delay for the electromagnetic wave. For the phased array systems, the two radar stations each operate for alternating periods of 5 minutes on and 5 minutes off, and out of phase, so that they can use the same frequency without interfering with each other. During each 5 minute of operation the radar measures the component of the surface current at each point projected onto the radial from the station to the point.

At the end of 10 minutes, the radial components of surface currents can be combined from the two stations to find the vector at each position on the grid. While surface currents are generally the data of most interest, the phased array systems can also produce significant wave heights and wind directions at each grid point and a calculation of the full directional wave spectrum at ranges of up to about half the maximum range (which is 150 km).

WERA HF ocean radar systems are used in the Capricorn Bunker Group (Tannum Sands and Lady Elliot Island), Queensland, at South Australian Gulf (Cape Spencer and Cape Wiles), South Australia, for Rottnest Shelf (Fremantle and Guilderton), Western Australia, and at Coffs Harbour (Red Rock and North Nambucca), New South Wales.