GPS/Loran-C Interoperability for Time and Frequency Applications - A Survey of the Times of Arrival of Loran-C Transmissions via GPS Common Mode/Common View Satellite Observations
Abstract:
Since the advent of GPS satellite based time and frequency transfer the role of Loran-C in this application has been greatly diminished. The capabilities of undegraded GPS are indeed superior to those of the Loran-C system in most respects including coverage, absolute timing accuracy, and ease of use. However potential drawbacks of GPS to the time and frequency user exist, such as higher cost, more complex hardware and non-civilian control of the system. This last has brought us the specter of Selective Availability SA, an on again-off again, intentional degradation of the accuracies obtainable from the GPS. Though not the focus of this report, it should be noted that the medium term r 780 seconds frequency stability of the Loran-C transmissions, for reasonably close transmitters, is almost two orders of magnitude better than that of the GPS transmissions observed under SA-reason enough to keep Loran-C in mind for frequency control purposes. This paper surveys the absolute time setting performance achievable from seven distinct transmitters in four North American chains via the Loran-C, Time of Coincidence TOC with UTC synchronization technique, Motivation for this undertaking consisted of both frustration with SA and strong curiousity about how well the propagation path of the Loran-C signals could be modeled and d how well the transmitters are synchronized to UTC as a result of the enactment in 1987 of Public Law 100-2232 requiring synchronization the 100 ns level. Navigation users desiring to combine GPS with Loran-C to enhance the overall reliability of their systems would prefer to treat the Loran-C signals analogously to those from the satellites, i.e. as pseudo-ranges rather than as time differences TDs, the input to the traditional Loran-C hyperbolic navigation solution. Multi-chain Loran-C navigation is also facilitated by absolute time synchronization as well.