Applanix POS MV V5 Installation And Operation Manual page 272

POS MV V5 Installation and Operation Guide
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2. With the vessel tied alongside use survey techniques to determine the precise heading
against a known landmark. Record this value together with the heading displayed by the
gyrocompass.
3. Turn the vessel through 180° and again measure the precise heading of the vessel. Record
this value together with the heading displayed by the POS MV.
4. Note any offsets that may exist between the two values for surveyed heading and the
corresponding values for gyrocompass heading. Follow the instructions supplied by the sonar
manufacturer to configure the system with these gyrocompass offset values.
Sea Calibration
You can use the following procedure to calibrate the gyrocompass at sea. However, this method is
inherently less accurate than the preferred method detailed above and you should use it only where
there are no alternatives.
1. Sail a straight course while making a calibration survey chart of the seabed. Observe the
position of a target object on the chart which, ideally, should occur at about the halfway point
of the survey line.
2. At the end of the first survey line, turn the vessel through 180° and repeat the survey line in
the opposite direction. Again, note the position of the target object.
3. Any gyrocompass offset will appear as an apparent shift in the position of the target on the
seabed.
Survey Procedure
Each operator is likely to have a standard procedure that you should follow when performing a multibeam
survey. Similarly, the manufacturer of the multibeam system is likely to have a set of specific instructions
detailing how to configure and use their system within such a survey. You should give precedence to any
existing instructions and operating procedures over the procedures outlined in this appendix.
Generally, the following procedures should be included as part of any multibeam survey:
1. Verify the sound velocity profile.
2. Even if the system was previously calibrated, the first and the last survey lines should be sailed in
both directions to enable a post-processing calibration check.
3. Finally, an extra line should be sailed perpendicular to the other survey lines. This allows
detection of any changes in sound velocity or tidal action during post-processing.
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