When trials of civil aircraft doppler navigation systems are carried out over land the pattern of results generally show a small along-track error component and a large across-track error component, whereas when the basic doppler radar is assessed, the errors over land are about ±0·2 per cent of distance gone and ±0°.2 in drift (standard deviation). It is not possible to quote system errors for all permutations of doppler equipments and compass systems but it is suggested that a good working figure for the across-track system error component would be about ±3 per cent (standard deviation), i.e. a compass error of ±1°.7. Advantage can be taken of the inherent accuracy of the doppler radar equipment to determine the heading error from a knowledge of track made good and the mean doppler drift along that track.

The standard twin-channel civil doppler navigation computer combines aircraft heading with doppler drift and doppler distance gone to form a ‘computer track’ in coordinates of distance along and across a preset track and distance. The computer is, in effect, meaning the doppler drift between selected points and adding this value to an arbitrary datum (aircraft compass) to form the computer track. If we can determine the true track over the ground we can assume that the error between computer track and actual track is caused entirely by error in the arbitrary datum, i.e. heading error.