Measuring Ocean Currents with HF Radar

HF radar makes it possible to map ocean surface currents from shore without in-situ instruments. Currents are derived from the Doppler shift of echoes from ocean waves a few meters long, whose phase speed is altered by underlying currents. Accuracy of 10 cm/s has been demonstrated with a portable NOAA system called CODAR (Coastal Ocean Dynamics Radar) that illuminates the sea by radio waves that propagate along the ocean surface. With a radar frequency of 25.4 MHz, maps of surface current vectors out to 60 km from shore can be obtained. Longer ranges are possible with lower frequencies, and greater spatial resolution is possible with higher frequencies. Skywave radars, which illuminate the sea by way of ionospheric reflections, have measured ocean currents at much longer ranges (1000-2000 km), but only when very stable ionospheric layers exist, or when nearby stationary targets are available. Current-mapping radars have already proven useful in several oceanographic research exercises, providing new insights with their graphic displays of transport processes, estuarine flows and tidal oscillations. The CODAR system is now in a Transition Engineering Phase, in which this research tool developed by NOAA is being transformed into an operational instrument with broad scientific and commercial applications.