Real-time current measurements on U.S. Coast Guard navigation buoys

The National Oceanic and Atmospheric Administration, National Ocean Service\´s Center for Operational Oceanographic Products and Services (CO-OPS) operates Physical Oceanographic Real Time Systems (PORTS®) at many of our Nation\´s major sea ports to provide real-time information in support of safe and efficient maritime commerce. As part of on-going efforts to improve PORTS®, COOPS has partnered with the U.S. Coast Guard (USCG) to install current measurement systems on existing navigation buoys. These systems consist of a "clamparatus" that secures a current profiler and electronics box to the buoy. The entire package weighs less than 200 pounds and is easily deployed using a small boat and a block and tackle. Data are sent to shore via spread-spectrum radios. Deployments up to seven months have been achieved because of the low power consumption of both the profiler and radios. The biggest challenge to date has been correcting the compass error induced by the steel buoy; initial compass calibrations have removed much of the magnetic deviation. The results of comparisons with current directions from bottom-mounted acoustic Doppler current profilers (ADCPs) are encouraging, showing agreement to within +/-13° when all data are included. The difference improves to 11° when speeds less than 12.9 cm/s (.25 knots) are omitted. Recent experiments include the collection of global positioning system (GPS) compass measurements during calibration.