Introduction¶

The Acoustic Doppler Current Profiler (ADCP), on the NOAA ship Ronald H. Brown provides high-resolution ocean current measurements for science missions. Its accuracy depends on the accuracy of its heading inputs. Unfortunately, although there are 3 different heading sensors on the Brown, none of them is sufficiently accurate or functioning sufficiently well to conduct high quality current profiling.

This document uses the CLIVAR cruise RB1304 (from Iceland to Brazil) to review the heading devices on the Ron Brown and their impact on the ocean velocities calculated from the ADCP.

The main conclusions are:

1. The Mahrs has a calibration problem or some other reason to cause a latitude-dependent error at high latitudes (especially north of 45N).
2. The POSMV has many heading glitches, as indicated by the large number of “heading accuracy” excursions away from baseline. These glitches are difficult to remove correctly. Any reasonably effective treatment of the heading errors results in a loss of 25%-50% of the headings. This has been a consistent problem since the POSMV was installed.
3. Even after a reasonable effort at removing bad headings, the remaining POSMV data still show a latitude-dependent error of about 0.5deg in higher latitudes.
4. POSMVs can work well. For example, the unit on the Ka`imimoana degraded over several years, until it was upgraded in the fall of 2010. After some small adjustments, it worked flawlessly until the ship was retired.

The Ron Brown is about to head west and take on the maintainance of the TAO buoys. There is an existing body of shipboard ADCP data, mostly taken from the Ka`imimoana, which forms the foundation of a valuable time series of Equatorial Pacific zonal ocean currents. For this reason it would be very beneficial if the POSMV on the Brown could be fixed.

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