Wrongly calibrated compasses common

There are some less well-known pitfalls in establishing deviation curves for magnetic compasses. On several occasions curves have proved to be completely wrong, even though personnel onboard believed they had been done correctly.
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There are some less well-known pitfalls in establishing deviation curves for magnetic compasses. On several occasions curves have proved to be completely wrong, even though personnel onboard believed they had been done correctly. Four ships were studied by an authorised compass calibrator. All ships had some form of up-to-date Compass Monitoring System. Ships A and B had traditional gyroscopes, fibre-optic attitude gyros, GPS compasses and magnetic compasses with sensing (through Flux-Gate Pick-off). Ships (C) and (D) had dual fibre-optic attitude gyros with north seeking and GPS support as well as magnetic compasses (with Flux-Gate Pick-off). Common to all four ships was a standard compass which could only be read on the top bridge (no mirror or video transmission). Crews read the magnetic compass value in the instruments from the control position using a Flux-Gate Pick-off. This sensor was wrongly calibrated on all of the ships. The values were noted in the deviation journal and log book, but because of the errors, there was no logic in the notes and the values were hardly useful. GPS navigators contain information about the amount of deviation for all areas on the Earth. A preprogrammed model is designed to take into account changes in the Earth’s magnetic field over time. However, the Earth’s magnetic field has changed faster than expected. If the model is not updated, the data will be false. In addition, there are incorrect ”deviation programs” in MS Excel that are used on Swedish ships. The program delivers deviation curves/values in the wrong direction, leading to the wrong use of tables. On ships (A) and (C) above, this occurred and the tables were accepted in spite of obvious errors. Contributing factors may have included the electronic charts, which have no compass rose. This error is sometimes detected when the wrongly calibrated sensor causes the alarm for gyro-diff to go off. This may in turn affect the functioning of the autopilot or trackholding system.
The following conclusions were drawn:
• Be thorough when working with a magnetic compass.
• Check the deviation curve to see if it is reasonable from time to time.
• Crewmembers need to know how to calibrate the sensor that registers the setting of the magnetic compass.
• On ships with electronic charts, it is especially important to ensure that the GPS has the latest geomagnetic model.

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