The principle of HF surface-wave radar (HFSWR) is that a surface radio propagation mode can be utilised to make a radar signal follow the surface of the sea as it disappears over the horizon.
Over the Horizon HF Surface Wave Radar (HFSWR) works only for vertically polarised antennas in close proximity to salty conducting water; it is fairly ineffective over land or freshwater lakes. The basic system is dispersed over three sites, Transmit, Receive and an Operations Centre (although the Transmit and Receive sites can be co-located for monostatic operation). Separation between the Transmit and Receive Sites is approximately 80 km and these may be completely remote from the Operations Centre, which can be located at a command and control centre. Satellite communications can be used to convey system control, status and radar data between the sites, or local communications if available can be used. “Floodlight” illumination of a surveillance sector is provided by a log-periodic transmitting antenna, which has vertically radiating elements of varying length to enable it to operate over a wide bandwidth. The receiving antenna is an array of monopole doublets approximately parallel to the coast, which are used to form a number of beams simultaneously.
In the bistatic mode, the high power transmitter is a wideband linear amplifer which can deliver its rated peak power continuously. For monostatic systems, pulse waveforms are used. Modulation used in the SECAR bistatic radar is frequency modulated continuous wave or FMCW. FMCW involves sweeping the frequency of a CW tone to gain bandwidth and therefore range resolution. Bistatic operation allows energy to continuously radiate the target maximising the detection probility.
