High Frequency (HF) radars located on the coast or on an offshore platform measure radio waves backscattered from waves on the ocean surface out to around 200km from the radar. The main contribution to the signal at the radar receiver is Bragg scatter from ocean waves with half the radio wavelength. At HF (around 30MHz) these waves are nearly always locally wind-generated. The amplitude of the backscattered signal is related to wave height and direction; the Doppler shifted frequencies in the signal are related to surface current and wave propagation speeds.
The signal can be inverted using Seaview Realtime software to provide measurements of current, waves and winds on spatial and temporal scales suitable for a wide range of engineering and oceanographic applications. (Accuracy of HF radar measurements)
The maximum range of the measurement depends on the radio wavelength and the met-ocean parameter being measured. For example, low radio frequencies can provide current measurements to 200km, high radio frequencies can provide wave measurements to 20km.
Other factors that can impact on data availability are the radio interference environment and sea-state. In low seas, low radio frequencies are not as useful; optimum results are obtained in low seas with operation at higher radio frequencies. For applications where high seas are of particular importance, low radio frequencies are recommended. Low radio frequencies are more affected by interference particularly from radio signals propagated by the ionosphere. Flexible frequency management can minimise any difficulties. The spatial and temporal resolution of the measurements depend on radar operating parameters such as bandwidth and beamwidth. The ultimate limitation on this being the averaging time needed for accurate metocean measurements. (Accuracy of HF radar measurements)
The receive antenna array and associated signal processing of the HF system determines the directional resolution in met-ocean measurements. Systems such as WERA, Pisces and OSCR use long antenna arrays and the phase of signals at each antenna element to form beams in particular directions. Some compact radars use direction-finding to resolve the data in azimuth. The beam-forming approach (used with WERA, Pisces and OSCR) makes it easier to resolve the variation in wave parameters over a wide area. (Radar comparisons)
Methods for inverting the integral equation that describes the relationship between HF radar backscatter and the ocean wave directional spectrum have been developed at the University of Sheffield. With the benefit of the close relationship Seaview have with the University, we are uniquely able to provide cutting edge software (Seaview Realtime) which resolves parameters such as wave height, wave period, peak direction from the directional spectrum and provides surface current and wind data in real time. The software is available to existing users of phased-array radar systems and future customers with WERA, Pisces and other phased-array radars.