Correcting for slow platform movement in deep‐water ADCP measurements

Abstract

Water current measurements based on the Doppler principle have become a standard approach in aquatic sciences. An acoustic Doppler current profiler (ADCP) estimates the water velocity in a series of depth-integrated bins by measuring the Doppler shift in frequency between a transmitted pulse of high-frequency sound and the echo that is reflected from scatterers in the water. If the measurements are performed on a moving boat, then a measure of the boat velocity is needed so the measured ADCP velocity can be corrected by subtracting the boat movement. In waters that are shallow enough for the instrument to locate the bottom, the ADCP's "bottom-tracking" capability provides a good estimate of the boat velocity for the correction. However, in deeper water, navigational information is needed to correct for the boat movement. Shipmounted ADCPs, combined with accurate navigation systems such as the Global Positioning System (GPS), have been used successfully in the past for recording velocity transects (Joyce et al. 1982). The Differential Global Positioning System (DGPS) has provided accurate positioning for many oceanic expeditions (Pierce et al. 1999;Trump and Marmorino 1997). Oceanographic research vessels typically move at 10 kt (5 m s -1 ), and commercial ships of opportunity that are used for oceanographic measurements move at even greater speeds (20 to 30 kt). GPS accuracy is sufficient for many of these cruises, particularly when velocities are averaged over hundreds or even thousands of meters.When an ADCP is used on a near-stationary platform to measure relatively slow currents, navigation errors can significantly degrade the current data. Even if the platform were completely stationary, the collected GPS data would yield a non-zero "velocity" of the platform. A 24-h dockside test (a motionless platform) of a DGPS system showed that 5-minaveraged position data, clustered about their mean values with a standard deviation of 7.7 m, had apparent eastward and northward velocity standard deviations of 1. 55 AbstractWhen boat-mounted acoustic Doppler current profiler (ADCP) velocity measurements are taken in deep water where bottom-tracking capability no longer exists, navigational data via the Differential Global Positioning System (DGPS) are commonly collected to correct for the boat's contribution to the recorded ADCP velocity. The boat's cruise speed and the desired averaging interval for the water velocity are often great enough for navigational errors to become insignificant contributors to the uncertainty of the resulting water velocity. When the boat or other measurement platform is nearly stationary, navigational errors become significant and lead to an incorrect water velocity when navigational corrections are applied. Furthermore, when the water current is on the same order of magnitude as the slowly moving boat, the boat movement may not be easily distinguished from the fluid velocity. Factors that affect the quality of DGPS navigational estimates include the number of available satellites and thei...