Global Tropical Moored Buoy Array: Wind Direction Accuracy Revisited

This work follows a previous analysis of ATLAS wind direction accuracy (Freitag et al., 2001). Wind direction is measured by two sensors (Table 1): a compass measuring the orientation of the anemometer base relative to magnetic north and a vane measuring the wind direction relative to the base of the anemometer. Two compass types had been used in NX-ATLAS systems at the time of the 2001 analysis, the EG&G model 63674 (referred to hereafter as the EG&G) and the KVH model LP101 (referred to hereafter as the KVH). At that time, the EG&G compass was no longer available from the manufacturer, but these compasses were being transferred from older, standard ATLAS systems to the NX-ATLAS. NX-ATLAS pre-deployment ensemble error statistics were found to be comparable for the two compass types, with mean errors of 0.04° for the EG&G (computed from 135 calibrations) and 0.33° for the KVH (106 calibrations). RMS errors were 1.42° and 1.45°, respectively, which were comparable to the ATLAS system resolution. The RMS error of the EG&G was well below the manufacturer’s specified accuracy (5°) and the KVH slightly higher than its specified accuracy (1°). While not always clearly stated, manufacturer’s specifications often refer to newly calibrated instruments and may not reflect drift over time. Post-recovery errors for 111 EG&G compass calibration checks were 0.61° in the mean and 2.38° RMS, within the specified accuracy. There were only a few post-recovery calibration checks available for KVH compasses in 2001, but it was noted that errors appeared to be somewhat larger than those of the EG&G. A third compass was added to the NX-ATLAS inventory in 2004, the KVH model C100 (referred to hereafter as the C100). Contribution 4903 from NOAA Pacific Marine Environmental Laboratory Contribution 2019-1032 from Joint Institute for the Study of the Atmosphere and Ocean (JISAO)