Precise time and frequency dissemination via the Loran-C system

The Loran-C Navigation System is described in terms of its characteristics as a precise time and frequency dissemination system. The technique employed in timing a Loran-C chain is described, and user techniques are addressed including economic considerations. The accuracy available for both ground-wave and sky-wave transmission modes is shown to be a function of the low-frequency propagation factors and equipment delays involved. Phase and amplitude perturbations due to irregular terrain effects are shown to produce time difference discrepancies in inland service areas. Using broad assumptions, both ground-wave and sky-wave coverages are estimated. Experience shows that Loran-C provides an excellent medium for the dissemination of precise time and frequency on a continuous basis in both the ground-wave and sky-wave modes. The stability of the ground wave provides a submicrosecond precision capability, but the accuracy of the system is limited by propagation effects to about ± 1 µs. Loran-C sky-wave synchronization appears to be capable of ± 1 µs precision for one-hop daytime transmissions and an estimated ± 8 µs for one-hop nighttime transmissions. As distance is increased and more sky-wave hops are involved, nighttime synchronization precision is degraded to about ± 20 µs. Accuracy of sky-wave synchronization is limited to about ± 50 µs for both daytime and nighttime.