The device described, which is in prototype form, accepts ship's speed from the Harrier electronic log and heading data from the Halcyon transmitting compass and continuously displays the north–south and east–west components of the water distance traversed from a starting position. It is a development of the Hadrian distance-offtrack computer which, with the Harrier log, has been described in a previous article published in this Journal.

The Sensors. Harrier is a through-hull impeller-type electronic log which records distance run in steps of 0·01 n.m. on an electromagnetic counter and indicates speed on a suitably calibrated milliammeter. Halcyon is a recent development, being a two-axis fluxgate transmitting compass which senses the Earth's magnetic field directly, thus avoiding the major problems associated with the use of pivoted magnetic systems in fast power vessels. The two rectilinear fluxgates are mutually perpendicular and are pendulously mounted with their axes in the horizontal plane. Viscous damping prevents their oscillating when subjected to vibration and shock. The outputs of the compass circuits are two d.c. analog signals which represent in magnitude and sign, the sine and cosine of the heading angle respectively.