Examination of Gyrocompass Data

The gyrocompass information is essential for rotating the ADCP velocitiesfrom trans-ducer to earth coordinates. This rotation is done as the ADCP data are logged into the ping les by integrating the gyrocompass with the ADCP DAS. Thus, long-term or short-term drifts in the gyrocompass data lead to errors in the ADCP velocities.

A few sources of these errors and ideas for correction are given below.

A simplegyrocompass points not due north, but along the axis of the total rotation vector: the sum of the earth's rotation and the rotation of the compass due to its relative motion on the earth's surface. The dierence from due north is typically of order 1 for a ship, and is a function of latitude and of the north component of

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Figure 22: Sample^callrefpplots. In this instance, there seem to be a few bad xes that still need to be eliminated the x le was edited and the ^refabs and ^smoothr programs rerun.

63 the ship's velocity. Most gyrocompasses can compensate for this error, given correct settings of two dials, one for latitude, one for speed. Experience has shown that the latitude adjustment is most important and need only to have been correct to a few degrees. The ship captain usually is responsible for maintaining the compensation.

However, it can easily be neglected and in some cases, untouched throughout the cruise. At the processing stage, it is dicult to know how and when the gyrocompass was compensated.

Another potential source of error from the gyrocompass is the Schuler oscillation.

Literature indicates that the oscillation is excited mainlyby the meridionalcomponent of acceleration of the ship on the time scale of the oscillation (84-minute period). If so, then calibrations should be more accurate on zonal cruise tracks than on meridional tracks, and on the latter should be more accurate when the ship stops than when it gets underway. It is not yet clear that these eects are detectable in the data we have looked at. Also, it is not clear to us how the amplitude of the oscillation should vary from one compass to another the oscillations reported by 4] are larger than we seem to see on other ships and larger than we expect based on the calculations of

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The assumption underlying most discussions of ADCP calibration is that gyro-compass error may drift with time over a period of days to weeks, but at any time is independent of heading (apart from the short-term Schuler oscillation). This as-sumption has been demonstrated false in at least one case: a standard Sperry Mark 37 compass has been shown to have a dierence of 1.5 between the errors on north versus south headings. We do not know yet whether this dependence of the error on heading is a symptom of compass failure, how long it has been occurring, or how commonly it might occur on other compasses.

The best possible means of verifying and correcting the gyrocompass data is to utilize heading information from an independent source, such as a dual GPS system or an Ashtech system. The systems consist of pairs of antennas in xed locations aboard the ship. The heading data are relative to the geometry of the array, which in most cases is not exactly aligned with the keel of the ship. Thus, these supplementary heading data can monitor drifts in the gyrocompass data, but are not used for the absolute determination of ship heading.

The basic principles in using the independent GPS antenna array system is as follows. The gyrocompass data are compared to the antenna array system to obtain a le containing a correction for gyro drift as a function of ensemble time. This correction is applied to the ADCP velocities in the database with program ^rotate, which is explained later in this chapter¹². Thus, during the calibration stage, the phase coecient will be the angle between the transducer and the antenna system, which is a single constant for a cruise assuming neither was disturbed.

12The rotation of the database velocities can wait until after calibration. To see how the gyro correction aects the the calibration, one can choose to rotate only the output of^adcpsect: the ship velocities relative to the reference layer (for water track calibration) or the ship velocities relative to the bottom (for bottom track calibration).

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Another method of correcting gyrocompass errors is with a model. This model makes assumptions of how the compass was compensated for latitude and speed during the cruise. After each model run, the compensation versus heading data are plotted to see if they appear reasonable. If so, a le of time and oset angle is created and program ^rotate is used to correct the ADCP velocities. The model data are much less reliable than the antenna array system, but may be adequate for lling gaps if the latter is disrupted.

The software associated with the antenna array and gyro model have not been incorporated into the CODAS package at this time, but are available on a case-by-case basis. Please contact the authors for more information.