The GS520 TMC uses a sensor coil that can “read” the compass it is attached to.

The resulting signal is in industry-standard NMEA for sharing with other equipment such as autopilots and VDRs. It can also be used as an input for heading repeaters such as the P1920 Steering repeater or P1251 UDDU multifunction display.

In addition to being a TMC, the GS820 has an independent off course alarm. This allows a heading to be set along with an alarm level. Should the heading deviate from the set course by the indicated amount an alarm will sound. This gives an automatic check on the autopilot and gyro.

The ability to set the alarm level allows the officer of the watch to determine the best tolerance for the current situation. For example, in heavy seas, you may wish to set a higher tolerance to avoid spurious alarms as the vessel yaws and rolls.

The main electronics unit in the GS520 also displays the compass heading in numerical form.