Towing Basin Speed Calibration of Acoustic Doppler Current Profiling Instruments

With new development in technologies, proliferation of manufacturers, and expanded applications of acoustic Doppler current profiling instruments, the need for proper sensor calibration procedures becomes increasingly important. This paper describes a towing basin speed calibration procedure that NOS has adopted for the past decade and a summary of calibration results of twenty-six sensors. These sensors include twelve RD Instruments ADCP's (seven 600 and five 1200 KHz units) and fourteen SonTek ADP's (three 500 and eleven 1500 KHz units). ADCP and ADP were calibrated repeatedly at same speed and on different dates. Tow carriage speeds varied from 5 cm/sec to 2 m/sec and were used as references for comparison. The speed measurement accuracies are expressed in terms of mean, standard deviation, root-mean-square (rms) of speed differences and percentage of reference speed. Overall, RDI 1200 KHz ADCP has smaller standard deviation and rms of speed differences and SonTek 500 KHz ADP has the largest. RDI 600 KHZ ADCP and SonTek 1500 KHz ADP are similar. Except for SonTek 500 KHZ, the standard deviation and percentage error of the tested sensors were slightly larger than manufacturer's specifications. The parameters affecting the calibration results, such as error sources and back scatter materials are also discussed.