Propagation and scattering in MF/HF groundwave radar

Vertically polarised MF/HF groundwave radar has been used over the frequency range 2¿¿30 MHz for sea-state sensing, surface-current measurement and ship tracking. The paper reviews the propagation factors and noise environment which control the performance of such systems. The various definitions of antenna gain, propagation loss and sea backscattering coefficient are discussed and related to physical concepts. The various first-order and higher-order mechanisms of scattering are reviewed and related to the relevant electromagnetic and hydrodynamic processes involved. These processes, linked with the velocities of the water waves responsible, result in a Doppler spectrum of the radar echoes which may be used to infer the wave height and wave directional spectra on the sea surface. Particular attention is directed towards remaining problems in understanding these processes. In evolving radars for ship tracking, detectability depends on the knowledge of the Doppler spectra of the sea clutter and, in particular, on limitations in directional determination of ship echoes with practical antenna systems located on available coastal sites. In conclusion, experimental measurements of sea echo, ship tracks and antenna-array characterisations are compared with theoretical considerations.