An lntercomparison of current measurements from a Raytheon special moored system and a VACM on Georges Bank

The Raytheon special moored system, a realtime physical oceanographic measuring system, was developed for the Bureau of Land Management's New England Outer Continental Shelf studies on Georges Bank. Current measurements from the system are evaluated by intercomparison with a nearby VACM for a 3-month period in summer 1978, using data from the 75-meter level in about 85 meters of water. While the 3 km horizontal separation of the instruments is not ideal, the gently sloping bathymetry, strong tidal currents, and absence of local hydrographic structure suggest that the data should be comparable for tidal and subtidal frequencies. The low wind and wave conditions that prevailed, and the depth of the sensors, preclude the possibility of wave-frequency contamination of the measurements. The measured tidal currents were nearly indistinguishable, but the Raytheon system showed subtidal energies higher by a factor of 2 to 4 (typical low-frequency velocity difference magnitudes of 5 cm/sec). The character of the difference suggests that the Raytheon system was in error, although the specific cause cannot be determined. The magnitude of the error is greater than the expected system accuracy, suggesting an undiagnosed malfunction. The possibility of errors of this magnitude, which cannot be detected in the data, leads to the conclusion that the Raytheon system (as used on Georges Bank) is not more accurate than conventional current meters.