A summary of the HF ocean radar technology is given by Heron (2005) and in that paper there is a comparison of the phased array, and the direction finding genres. In both genres, the range resolution is determined by time delay, or its equivalent frequency offset in a chirp modulated radar. It is in determining the azimuthal resolution that the systems differ.
In a phased array a narrow beam is formed by adding phase offsets to antenna elements in the phased array. The width of the beam is determined (inversely) by the length of the antenna array. The beam formation is fundamentally robust and the azimuthal resolution of fine structure in the surface currents is determined by the antenna beam width. The phased array performs particularly well when the surface current field is complex, with eddies, jets and fronts. Also, with phased array methods, the second-order echoes may be used to determine directional wave spectra at ranges up to about half the range for surface currents. The long-range systems, operating at around 8-9 MHz maps surface currents to ranges of up to 200 km, and directional wave spectra up to 100 km. At higher operating frequencies the current values are more accurate, but the operating range is less. The phased array HF ocean radar is robust and has high precision.
For more details see the WERA website
In direction finding ocean radars the direction of arrival of a signal is determined by the amplitude response of orthogonal loop antennas. The small antenna footprint is the single most desirable feature. The direction finding procedure is critically dependent on the calibration of the polar patterns of the antennas. For simple flow patterns, the radial components of surface current are accurately determined. Direction-finding systems generally have wide azimuthal coverage. The long-range systems, operating in the 4-5 MHz band map surface currents up to 200 km. Directional wave spectra can be determined in waters close to each of the stations. The direction finding HF ocean radar is reliable and is particularly good for large-scale steady flows.
For more details see the SeaSonde website
ACORN adopts the position of working with researchers in the IMOS nodes to determine the best radar configuration to suit their needs and the nature of the surface flow dynamics in their region.
Heron, M.L., The role of HF ocean surface radar in Australia as a long-term coastal ocean monitoring facility, Coasts and Ports Conference, CDROM, IEAust., 2005