2015: Update on Ron Brown Headings

Introduction

Calculation of ocean currents from raw shipboard ADCP data requires ship speed and heading. The heading is used to orient the transducer (relative to north) for the calculations. An error in heading is multiplied by ship speed and results in an error in the calculated ocean currents, showing up primarily in the cross-track direction, i.e. a false current to port or starboard. The magnitude of the error is proportional to ship speed and is roughly 10cm/s for every degree of heading error. Since open ocean speeds are rarely in excess of 20cm/s, this represents a large error. Therefore we want the heading errors to be on the order of 0.1deg, so the calculated ocean speeds are not off by more than 1cm/s (on the order of other errors in the system).

Accurate heading devices are expensive and often difficult to maintain. The POSMV is no exception. When it works well, it is a highly accurate heading device. When it does not work well, it is usually still better than nothing, but the resulting ADCP data are compromised.

Past POSMV Quality on Ron Brown

The Ron Brown's POSMV was installed sometime in 2010 and its quality has been pretty consistent: poor. It provides useful headings only about 60% of the time, with many short gaps. On the Ron Brown, we have relied on the MAHRS, although it has a latitudinal heading bias and is not as accurate as a POSMV. We have used the POSMV to correct the MAHRS, but the POSMV is too gappy to be a good solution.

A field exists in the $PASHR message called "heading accuracy" (which should be thought of as "HEADING ACCURACY ERROR"), which is supposed to indicate the POSMV's own analysis of the accuracy of the heading. A low value means low error, a higher value means higher error. UHDAS uses a very strict criterion for judging whether the headings must be discarded, by throwing out any headings where the Heading Accuracy Error is greater than a (very low) threshold.

Previous reports and figures have illustrated the following points regarding the evaluation of a POSMV's heading accuracy:

  • if a POSMV is working (all GAMS==2 all the time) then the value is flatlined at it's minimum, tyically 0.018 (set by the distance between antennas?)
  • GAMS solution of 2 will give low (good) values of the Heading Accuracy Error. A mix of GAMS=1 and GAMS=2 may or may not have solid low values or may have excursions
  • if there are excursions of this Heading Accuracy Error value, then they are usually correlated with errors in heading
  • the Heading Accuracy Error is NOT in degrees
  • the error in heading is NOT proportional to the Heading Accuracy Error value
  • if the lowest value of the Heading Accuracy Error is not at its minimum, but is "floating" at a few degrees, there is no telling what the actual errors are.

Two Recent Examples of Characteristic POSMV Errors

The following are two examples of these "excursions", where the POSMV is compared to the ship's gyro. Although the ship's gyro also has errors, these deviations in heading are quite large in comparison, and very obviously associated with the period of time when the Heading Accuracy Error is above the baseline. A properly functioning POSMV will have heading errors around 0.1deg, but these are clearly more like 1-2deg.

Click on any image to enlarge.

excursions of heading accuracy error match heading deviations
excursions of heading accuracy error match heading deviations

GAMS Failure (late 2014/early 2015)

Something happened to the POSMV in late 2014 and the Heading Accuracy Error was more like 1deg, with small attempts to regain sanity. It looks like on about 2015/03/05 0500 UTC, the ship attempted to do a GAMS calibration, driving the ship around in a figure-8 pattern. Prior to that calibration there were multiple large defiations of heading during each day and GAMS was mostly 0. After that calibration, the POSMV returned to its mix of GAMS=1 and GAMS=2, and many small excursions of the Heading Accuracy Error.

before and after GAMS calibration zoom in, before GAMS calibration

MAHRS Failure, mid-2015

Because of the poor POSMV quality, the MAHRS has played a vital role in processing the shipboard ADCP data. Its quality is discussed in this report: http://currents.soest.hawaii.edu/reports/ship_reports/ron_brown_posmv_2013/index.html

The Mahrs suffered a failure of some kind around May 3, 2015, about 75% of the way through RB1503, the first of two GO-SHIP cruises. These cruises depend on high quality shipboard ADCP data for calibration of the Lowered ADCP and for the science of the project. Yet with the POSMV as bad as it is, even the poor MAHRS data is still helpful.

The following two figures show the onset of the MAHRS failure and the continued poor quality of the POSMV.

RB1503 Mahrs and POSMV quality RB1503L2 Mahrs and POSMV quality

Conclusion

It is imperative that there be a heading device with sufficient accuracy to give high-quality shipboard ADCP data. The Ron Brown is used by a variety of scientific programs that depend on this.

Options include:
  • fix the POSMV
  • fix the MAHRS
  • buy another device that will do the job.

One such possibility is the Ashtech ADU800. It has been vetted on two ships with UHDAS and shipboard ADCPs. It runs about $15K, which is probably cheaper than repairing either of the other two devices.