The gyro will support a number of repeaters, including a repeater at the emergency steering position. Gyro repeaters on the bridge should be checked against the main gyro at least once a watch, and after excessive manoeuvring. Other repeaters should be checked frequently.

4.4.3Compass errors

As a safeguard against the gyro and gyro repeaters wandering, frequent checks should be made between the magnetic and gyro compasses.

Magnetic and gyro compass errors should be checked and recorded each watch, where possible, using either azimuth or transit bearings.

A record of magnetic and gyro compass courses to steer and compass errors should be maintained and available to the helmsman.

4.4.4Rate of turn

Rate of turn measurement is used by automatic track-keeping systems to perform controlled turns. When ships are manoeuvring, particularly large ships where the distance between the bow and the pivot point of the ship is considerable, rate of turn indication provides the ship handler with feedback on how quickly the ship is turning.

4.5 Speed and distance measuring log

Speed logs, depending upon their type, will provide either speed through the water or speed over the ground measurements.

4.5.1Types of speed measurement

In general terms, speed through the water is used for radar collision avoidance, and speed over the ground is used for navigation. Speed made good can also be measured on ships, and represents the speed that the ship has achieved over a period of time. Speed made good can be measured from the chart between position fixes, and is also calculated and transmitted by electronic position-fixing systems.

4.5.2Direction of speed measurement

Doppler-type logs can both be single-axis and measure speed in the fore and aft direction or dual-axis and measure,fore and aft and arthwartship movement. Coupled with rate of turn measurement, dual-axis logs are also able to calculate the speed and direction of movement of the bow and stern. Electro-magnetic logs provide single-axis measurement only.