In polar regions great reliance has to be placed on the directional information provided by a free azimuth gyro. For sufficient reliability the use of three such gyros is advocated. An analysis is made of ‘the best two out of three’ principle—taking into account the fact that in practice no two gyro readings will be identical.

It is demonstrated that for a given reliability attaching to an individual gyro, the reliability of a three-gyro system represents a vast improvement over a single instrument and that there is an optimum way of using the information presented. The advantages of middle signal selection are pointed out and the dangers associated with mean signal selection are explained. Attention is drawn to the extension of this principle to the problems of triple-signal selection in the automatic navigation system of the future.