Differential Omega as a Worldwide Navigation Aid

Abstract

i. I N T R O D U C T I O N . Navigation aids in use today have their origin in mainly military operational requirements for navigation accuracy, coverage and integrity. They have also been directed at the needs of air and sea operations.Within the next few years, satellite systems with global coverage will add significantly to our navigation capability. Furthermore, there is a rapidly developing civil market for land vehicle location and navigation which by the end of the century could more than surpass in numbers the aviation and marine usage.There is growing realization that ground-based radio navigation aids and satellite navigation aids are complementary and, furthermore, that it may be inadvisable to rely wholly on GPS, a national military system. Consequently, plans for keeping LORAN C in service and extending its coverage are flourishing, 1 and consideration is being given to civil satellite navigation systems. 2 Although Omega is the only worldwide ground-based aid, little is heard of its potential to complement satellites. This is due to a combination of factors:-the relatively poor accuracy (1-2 n.m.) of standard Omega -the range limitation (1^0-4^0 n.m.) of conventional differential Omega and the deterioration in accuracy at night -misunderstanding and doubts on the future status. The objective of this paper is to present recent work on improving the accuracy and extending the coverage of differential Omega 3 and then to illustrate how these techniques could be applied to give worldwide coverage.2. STANDARD OMEGA. The fundamental method of position finding by any radio aid is to measure the transmission time from a transmitter to a receiver and then to convert this to distance on the assumption that the transmission has been over the shortest path at a known velocity.With the Very Low Frequencies (VLF) of Omega, signal propagation is in the Earth-ionosphere waveguide and the path is assumed to be the geodetic between transmitter and receiver. The signal propagation characteristics (velocity and attenuation) are determined jointly by Earth and ionospheric boundary properties.A nominal, or reference phase, is calculated from the geodetic range between transmitter and receiver, and a nominal phase velocity of 1-0026 times that in free space.Sophisticated temporal and spatial models are then used to calculate propagation corrections that predict variations in phase from the reference phase, due to the geophysical properties of the Earth and ionosphere. 4 These corrections 88 N O . I