Differential-mode correction of skywave errors in low-frequency time-sequence position monitoring systems

It is possible to improve the accuracy of the position fixes made using low-frequency radio navigation systems by differentia I-mode operation, the position of the mobile being measured relative to that of a fixed monitor station, provided that the two stations experience errors which are substantially correlated. The recent development of high resolution automatic vehicle location systems which use radio positioning has led to proposals for the use of similar differential techniques to correct for the errors due to night skywave propagation. This paper describes and presents the results of a programme of trials designed to test the feasibility of the proposals and to determine the area over which the readings of each monitor station might be applied. The degree of correlation between the skywave errors received from certain transmitters of the Decca Navigator system at pairs of receivers at various separations and orientations is recorded. The results are interpreted in the light of a simple model of propagation and it is shown to be path difference effects, rather than ionospheric inhomogeneities which limit the region within which the data from a monitor station may be used. The implications of the results are discussed and should be of interest both to designers of new systems for vehicle location and to users of existing low frequency navigation systems.