Timing a LORAN-C Chain

Abstract

Originally developed for marine navigation and now extensively utilized in offshore air navigation, the LOng Range Aid Navigation (LORAN) system has assumed considerable importance in the area of timing synchronization. LORAN-C chains are maintained by the U.S. Coast Guard. Wherever possible, the chains are monitored by the U.S. Naval Observatory (USNO) and differences, USNO Master Clock (USNO MC) minus the chains, are published in Time Service Announcements to enable users to establish and maintain traceability to the USNO MC. However, the USNO is within groundwave range of only four of the thirteen chains. Determining the timing relationship between the majority of LORAN-C chains and USNO MC, therefore, requires dependence upon monitoring stations within groundwave coverage of these chains. The methodology of forming and calibrating remote time scales for individual stations used in determining a time scale for a LORAN-C chain, in particular for LC/9970, is discussed. Portable clock measurements and satellite time transfers are used as the calibration devices. The maximum scatter in values of USNO MC minus LC/9970 determined by the individual stations forming the MEAN time scale amounts to about 0.15 microsecond.