Articles

HF Radar Technology

The basic physical mechanism on which the technology of the HF Radar relies is the Bragg Scattering of electromagnetic radio waves from gravity waves travelling along the sea surface. The radars transmit a vertically polarised signal which propagates onto the water in a radial path away from the mast reaching well beyond the line-of-sight, that is further away than the point at which the more common microwave radars become blind. Salinity helps to make water an electrically conductive surface.  Rain or fog does not affect HF signals.

HF1The ocean is a rough surface, with water waves of many different periods. When the radar signal hits ocean waves, the radar signal is scattered in many directions with the sea surface acting like a large diffraction grating. By Bragg’s principle, the radar signal produces an echo returning directly to its source only when the signal scatters off a sea wave with a wavelength that is exactly half the wavelength of the transmitted radio signal, and when the sea wave is travelling in a radial path either directly away from or towards the radar. The scattered radar electromagnetic waves add coherently resulting in a strong return of energy at two sharp peaks in the Doppler Spectrum.

Due to the underlying ocean currents, the detected peak does not have a constant Doppler shift. Once the theoretical wave speed is computed from the dispersion relation, and subtracted from the Doppler frequency shift, the radial velocity component of the surface current can be found. By installing more than one radar at different locations with an overlapping beam pattern, the same patch of water can be viewed from different angles, and the surface current radial velocity components can be summed to determine the total surface current velocity vector. 

HF2

This data measured simultaneously a different sea points can be combined to produce hourly maps of current vectors within a regular grid.

A second derived measurement from each radar station is the significant wave height and the wave direction, which are extracted from the second order Doppler Spectrum. When there is an increase in wave height, there is no corresponding increase in the height of the first order spectrum (Bragg peaks) since these are generally fully developed; there is however an increase in the height of the second order peak energy which is proportional to the energy in the longer sea waves.

ERDFlogos