Gyro Wandering
The undesired movement of one of the spin axis of the gyro from its original position. Gyro wandering can be apparent or real. Apparent gyro wandering is the result of the movement of the earth. Real wander can result from mechanical and/or electrical reasons, usually caused by friction in the gimbal bearings.
Before we jump into the definition of Off Heading alarm & Off Course alarm of the auto pilot, let us understand something associated with these alarms.
The auto pilot takes the heading data from various sensors like the Gyro, Magnetic Compass &/or the satellite GPS compass, if fitted. Some vessels are fitted with 2 gyro compass instead of the satellite GPS compass. So, on the auto pilot panel, the user has to select 2 heading sensors, one will be used for the heading data input to the auto pilot and the second will NOT be used. Now the question arises, why to choose a heading sensor when we know that it will not be used for heading input.
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The auto pilot console uses the 2nd heading sensor selected to compare its heading input with the heading input of the heading sensor that is being used for steering. In this way, when the difference between these 2 sensors becomes higher than a set value, the auto pilot console gives an alarm. Let us understand this using an example:
Suppose your vessel has 3 heading sensor inputs to the auto pilot, namely, 1 from gyro compass, 1 from magnetic compass & 1 from satellite GPS compass. Let us assume that you select gyro as your heading input sensor. Now the auto pilot will use the gyro heading to steer the ship. And, assume that you select magnetic compass as your heading input sensor NOT being used for steering, then gyro will compare the headings of these 2 sensors at all times. If the difference between the headings of these 2 is larger than a set value, the auto knows that gyro wandering has happened and the vessel is not following its correct heading.
This is why the selection of 2 heading sensors is compulsory in an auto pilot. You cannot select the same heading input sensor for both, as this would defeat the whole purpose of selecting 2 sensors. Furthermore this is not allowed by the auto pilot system. Now that you know this information, let us get into the definition, which you will be able to understand fully now.
Off Heading Alarm
The off heading alarm feature monitors that the ship’s heading keeps the set heading. If the ship deviates a certain amount from the set heading, an alert is activated.
Off Course Alarm
The off course alarm feature monitors the difference between the heading information of the heading sensor NOT used for the steering and the set heading. An alert is activated if a certain amount of difference generates.
