Towing Basin Speed Verification of Acoustic Doppler Current Profiling Instruments.

Abstract

With new development in technologies, proliferation of manufacturers, and expanded applications of acoustic Doppler current profiling instruments, the need for proper sensor test procedures becomes increasingly important. This report documents a towing basin speed measurement verification procedure that NOS has adopted for the past decade and a summary of verification results of forty-one cases consisting of twenty-nine individual sensors. These sensors include fourteen RD Instruments acoustic Doppler current profiler (ADCP) units (seven 600 and seven 1200 KHz units) and fifteen SonTek acoustic Doppler profiler (ADP) units (three 500 and twelve 1500 KHz units). Some of these sensors were tested repeatedly on different dates prior to deployment. Tow carriage speeds varied from 5 cm/s to 200 cm/s and were used as references for comparison. The speed differences between sensor readings and carriage references are expressed in terms of mean, standard deviation, and percentage of reference speed. Overall, RDI ADCPs have smaller mean speed differences and standard deviations compared with SonTek ADPs of similar frequencies. There is a larger sensor-to-sensor variation among SonTek units, especially among the 500 KHz ADPs. Except for the SonTek 500 KHz units, the standard deviation and percentage error of the tested sensors were close to manufacturer’s specifications. Small zero offsets, on the order of 1 cm/s for ADCPs, 2 cm/s for 1500 KHz ADPs, and 5 cm/s for 500 KHz ADPs, were measured. The test results of both sensor types repeat well over time. To aid in the discussing of test procedure and results, a brief review of related ADCP and ADP design and operational parameters are also included in this report.