Improved underway ADCP data quality through precise heading measurement

Acoustic Doppler current profilers provide valuable data on ocean currents from ships whilst underway, and they are a valuable tool when used in conjunction with high resolution hydrography. One of the main difficulties in using ADCP data to estimate mean cross-track currents (and hence transport) is the error in the conversion from ship to Earth coordinates brought about by errors in the ship´s gyrocompass. Errors in the cross-track velocity of tens of cm s^-1 may occur at moderate passage speeds. A relatively new tool - 3D GPS - provides precise heading measurements that can be used to correct the ADCP data. This paper uses 3D GPS, gyro, and ADCP data gathered during Cruise 198 of RRS Discovery to illustrate gyro errors dependent on latitude, speed and heading, and their effect on ADCP data quality