High frequency radios (HFR) are used to measure surface currents along some shores of Oʻahu and Big Island. They consist of a low-power transmitter sending radio waves and receivers that listen for the signal reflected back from the ocean. The return signal is Doppler-shifted by wave propagation (motion) and by surface currents, as well as wind direction and wave spectrum. This information can be used to aid in spill tracking, search and rescue, vessel routing and navigation, and forecast modeling. Maps of surface currents are available across the United States as part of the National HFRNet.
Google Earth KML:
http://pacioos.org/kml/hfradio_stations.kmz (station locations)