Comparison of Aanderaa RCM-9 and MAVS3 current meters measuring surface currents in the Gulf of Maine

The Gulf of Maine Ocean Observing System was created to provide the users of the Gulf with moored, modeled, and satellite and remotely observed conditions in the Gulf. As part of this effort the University of Maine and Woods Hole Oceanographic Institution are deploying moorings in the coastal and basin regions of the Gulf. ADCPs will measure current profiles, but surface current observations are also required. For this last task, the MAVS3 and Aanderaa RCM-9 current meters were considered, and on the Jordan Basin mooring both were deployed just below the buoy for evaluation. As a first test, tow-tank tests were run on the MAVS3 and its calibration and behavior adjusted. A preliminary comparison was conducted on a section of hourly telemetered data from June through August 2002. Both current meters sampled at 1 Hz for 1 minute each hour (dictated by limited Aanderaa sampling options). A visual comparison of the time series indicated that both current meters observed similar currents. The Aanderaa was mounted 1.7 m and the MAVS 2.7 m below the surface. A regression of the speeds showed that the Aanderaa readings were about 3 cm/sec higher on the average than the MAVS. The slope of the regression showed the Aanderaa speeds about 34% higher than the MAVS. The standard deviation of the difference was 5-6 cm/sec and the regression scatter appeared to be larger than would normally be expected in deep water current meter comparisons. The mean direction difference is 4.8°, and the standard deviation of the direction difference is 27.1°. That two current meters using different methodology agreed reasonably well in the complex wave and current field moored under a surface buoy indicates that both current meters are doing reasonable jobs of measuring currents.