Mooring motion bias of point-Doppler current meter measurements

Upper-ocean current measurements have been made for more than 20 years from taut-fine surface moorings deployed in the equatorial Pacific by NOAA´s Pacific Marine Environmental Laboratory (PMEL). Until 1998 the moorings were instrumented with mechanical current meters (MCMs, either Vector Averaging Current Meters (VACM) or Vector Measuring Current Meters (VMCM)). Comparison with nearby subsurface 150 kHz Acoustic Doppler Current Profilers (ADCP) indicated that differences between the two measurement systems were generally small (i.e., mean differences of 5 cm s^-1 or less). By the early-1990´s, maintenance of the aging MCMs (designed in the 1960s and 1970s) was difficult, time consuming and expensive. Early tests of the Sontek Argonaut-MD current meters by PMEL indicated that it was a good candidate for replacement of the MCMs. Subsequent comparisons between Argonaut-MD data and nearby ADCPs revealed significant bias between the two, with the Argonaut-MD reporting lower horizontal current speed. Further investigation, including the analysis of high-frequency output from the Argonaut-MD compass/tilt-sensor (Precision Navigation model TCM2), found that the source of the bias was the inability of the compass/tilt-sensor to function properly in response to extreme lateral and rotational accelerations experienced by the instruments in high current speed regimes. A solution to this problem was to reduce the acceleration of the current meters by attaching vanes to each instrument. Since PMEL introduced this modification, differences between Argonaut-MD and ADCP data are comparable to those found previously between MCM and ADCP.