The two main causes of error, in this area, are:
• Wrong geodetic transformations being applied to the WGS-84 position derived from GPS.
• Erroneous coordinates for the Differential reference station.
8.2.2.2 Position Accuracy Determination Method 2
This method is most easily accomplished during the gyrocompass calibration. The antenna remains
mounted on the vessel. The surveyor will set up on the known survey benchmarks; using standard
land survey techniques, the exact absolute position of the antenna can be determined. During the
period that the surveyor is ‘shooting in’ the GPS antenna, the GPS position will be logged on board,
the averaging and statistical analysis will be as above.
The surveyor will need to take numerous shots to also obtain an average, due to the possible
movement of the vessel while alongside.
8.3 Gyrocompass
Utmost care is required for the installation of the gyrocompass. The gyrocompass is a sensor that
cannot be situated randomly. The purpose of the gyrocompass is to measure the vessel’s heading.
In order to do this, the gyrocompass should be placed on the centre line running from the bow stem
to the midpoint of the stern. If it is not possible to place the gyrocompass on the centreline of the
vessel, it can be mounted on a parallel to the centre line.
All survey grade gyrocompasses will be plainly marked for alignment on the centre line. This
marking may be an etched line fore and aft on the mounting plate, or possibly metal pins on the
front and the back of the housing that point down. If no marking exists, then measuring the fore
and aft faces and finding the centre may be sufficient.
No matter how well the gyrocompass is placed, there exists a possible error between the true
vessel’s heading and the gyrocompass derived heading. Any new installation of a gyrocompass
should include a gyrocompass calibration. There are various methods to perform a gyrocompass
calibration; the best method employed will be determined by the location of the vessel, the time
allotted for the calibration and the resources at hand.
8.3.1 Gyrocompass Calibration Methods
After the installation of gyrocompass (henceforth termed gyro) on a vessel, that gyro should be
calibrated to ensure that the heading it determines is the true heading of the vessel.
If the error is large, the gyro can be physically rotated to align itself with the true vessel heading.
Small errors can be corrected, either by internal adjustment to the gyro, or in the software that
receives the gyro reading.
Page 109 of 210
Version 5.0 Rev r002
Date 05-08-2014
To Next Page
Loading...
Need help?
General
