A Method for Calibrating Shipmounted Acoustic Doppler Profilers and the Limitations of Gyro Compasses

Abstract

Calibration of shipmounted acoustic Doppler profiles by a series of 90° turns during periods of GPS navigation provides estimates of misalignment angle φ and scaling factor A with standard deviations less than 0.2° and 0.3%. A varies by 1% with depth, and differs between bottom and water track modes, but φ is independent of these factors. Day to day variations in φ and A, however, are as large as 1° and 2%, the former because of long-period wander of the ship's gyro compass, the latter possibly because of variations in Doppler spectra in different conditions. The gyro compass also shows short-period bias of 2° after a 90° turn, with 0.5° bias persisting for over 20 minutes. All these errors indicate that the limit of current accuracy both along and athwartships is about 0.05 m s⁻¹ for a ship speed of 5 m s⁻¹.