It is not the purpose of this paper to enter too deeply into the controversy between heading reference systems (combined with a velocity sensor) and full inertial systems. Rather it is intended to indicate what may be achieved at the present time, and in the next few years, using a heading reference system combined with a velocity sensor such as doppler radar. In conclusion the performance, weight, size, cost and reliability of such systems will be compared with similar figures for a full inertial system in the hope of stimulating thought and discussion as to what is the correct choice of system as well as to what the user may be able to afford.

In the early part of the last decade the most widely used heading reference was the gyro-magnetic compass—a flux-valve being used to give good long-term heading accuracy whilst short-term variations are smoothed out by the integrating action of a' directional gyroscope. The directional gyroscope was in fact slaved to the magnetic reference. Accuracies of 1°·0 (r.m.s.) are typically achieved by such a system, although higher accuracies are sometimes claimed under certain specified conditions.