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Rotating gyros that are not North-seeking are called "rate of turn gyros" Unlike North-seeking gyros these have two instead of three degrees of freedom. Autopilots use rate of turn gyros to automatically
steer the ship and show the rate of turn (ROT). They are only used over shorter distances as they don't keep their orientation in space over extended periods of time.
IMO accuracy demands for rate of turn instruments:
Rate of turn gyro with operator panel and a repeater that shows the rate of turn, ROT. (Raytheon Anschutz)
Ring Laser Gyro
In order to detect rotation of the instrument, the Ring Laser Gyros (RLG) uses mirrors to reflect laser light. The laser beam is split, so that one beam is directed clockwise and the other anticlockwise. They are then refleced by mirrors. When the device is not rotated the two beams will reach the comparator without phase difference. But when the device is rotated the laser beams has to travel different distances to travers the circuit. This difference is detected by comparing their phases and the rotation can be measured. This function is based on the so-called "Sagnac" effect.
A Fibre Optic Gyro (FOG) uses a similar concept to the Ring laser gyro, but a fiber-optic coil is used as a very sensitive rate sensor.
The input laser beam is split into two beams that travel the same path but in opposite directions: one clockwise and the other counter-clockwise.
When a rotation is applied, the optical path for one beam becomes slightly longer while that of the opposing beam becomes shorter producing Sagnac frequency shifts.
A combination of three such fiber-optic coils (gyroscopes) and a dual-axis electronic level sensor is able to determine the direction of true north.
This system has no gimbals, no moving parts. The Fibre optic gyro will not have any speed,- or ballistic error, but a small error that increases with increased latitude.
When the Gyro is not rotating, the distance travelled by the light are of equal length in both directions, and no phase difference will occur.
When the Gyro is rotating, the distance travelled the different ways are not equal, and the detector can detect a phase difference. The FOG translates this into a measurement of rotationThe satellite compass
The satellite compass measures the course of the vessel by comparing the phase difference between the carrier waves of the different satellites. This requires that the special antenna receives sufficient data
from satellites: some compasses need at least 6 satellites for this purpose. Satellite compasses may have up to four antennae. From the signals received the instrument can calculate several parameters:
The development of satelli compasses is aimed at presenting more and more data
Sources of errors
GPS compasses have not been approved by the IMO as a replacement for magnetic or gyro compasses.