Velocity Errors in Acoustic Doppler Current Profiler Measurements due to Platform Attitude Variations and Their Effect on Volume Transport Estimates

Abstract

Attitude (pitch, roll, and heading) variations of the platform on which acoustic Doppler current profilers (ADCPs) are mounted will affect the measurements of water velocity using ADCPs. The major correction required to the velocity vector is for heading. Here the authors concentrate on the magnitude of errors due to pitch and roll biases. Data used in this study were obtained using a 150-kHz RD Instruments ADCP fitted to the RRS James Clark Ross. In late November 1994 a hydrographic section was made across Drake Passage in the Southern Ocean along the World Ocean Circulation Experiment (WOCE) Special Repeat 1 section. The annual occupation of this line is a major contribution by the United Kingdom to the WOCE. A key element of the program is to determine the volume flux of the Antarctic Circumpolar Current as it flows through Drake Passage. One method of obtaining the total volume flux is by referencing relative geostrophic velocity profiles to the absolute ADCP velocity profiles, thereby obtaining estimates of the geostrophic reference velocity and hence the total geostrophic flow field; this is a technique used in several previous circulation studies. The authors find (i) use of pitch and roll corrections has no significant effect on heading misalignment and velocity amplitude calibrations; (ii) underway typical pitch and roll corrections amount to 0.005 m s?1 in the athwart ship water velocity; (iii) for this dataset, differences in the mean pitch and roll under way and on station explain half the geostrophic volume transport discrepancy obtained using underway and on-station ADCP velocities as a geostrophic reference; (iv) a pitch and roll bias of only 1° under way leads, in this dataset, to 10% errors in the estimate of the volume transport comparable to heading misalignment;and (v) hypothetically, pitch and roll variations at the same frequency and in phase can lead to much larger errors in volume transport estimates.