Implementing a Wide Area High Accuracy UTC Service via eLoran

Gerard Offermans, Erik Johannessen, Charles Schue

Abstract:	The requirements for accurate synchronization of time and frequency are increasingly important in numerous applications that influence our daily life. Fifteen out of sixteen sectors of the Critical Infrastructure and Key Resources (CIKR) identified by the Department of Homeland Security (DHS) use GPS for timing and for eleven it is deemed essential. Systems are becoming effectively dependent on GPS or other GNSS for their precise position, timing, and frequency information. Holdover technologies at GPS-level accuracies to counter interference, jamming and spoofing are challenging to implement and expensive. The General Lighthouse Authorities of the UK and Ireland (GLAs) have deployed an Initial Operating Capability eLoran system along the east coast of the UK. Differential eLoran Reference Stations at seven harbors provide corrections to mariners using eLoran to navigate along the coast and enter a harbor area. The maritime receiver requires eLoran navigation signals from three or more transmitters; the differential corrections are broadcast using the Loran Data Channel (LDC) coming from one of these transmitters. The target horizontal positioning accuracy is less than 10 meters, 95%. In addition to being a high accuracy positioning service, eLoran is also a very effective UTC time and frequency distribution service by broadcasting UTC synchronization messages over the LDC. All eLoran transmissions are synchronized to UTC, traceable to a local national time standard. As is the case with navigation, timing users can benefit from further differential corrections to improve the time accuracy in their area to compensate for ground-wave propagation delays. A Differential eLoran UTC Reference Station can be installed which measures the difference between eLoran determined UTC and a reference UTC, which can be provided by any alternate means of UTC distribution. Corrections are then broadcast using the LDC to users in the region applicable to the reference station enabling UTC time to be delivered in a 100% GNSS denied environment. In the UK a consortium of the GLAs, Chronos Technology, and UrsaNav are implementing a trial differential service for UTC time distribution. The target is to provide synchronization to within 100 ns from UTC over large areas specifically in locations where the GNSS signal reception is difficult or under threat (e.g. indoors, urban or built-up areas etc.), or GNSS antenna installation costs are significant or vulnerable to vandalism. It is expected that the corrections needed to provide a 100 ns UTC synchronization service will only vary slowly and be valid over a large area. This limits the number of Reference Stations and can easily be accommodated within existing spare capacity of the LDC. Trials will be undertaken using the LDC broadcast from the UK eLoran transmitter in Anthorn, Cumbria. Application of differential UTC corrections over the LDC will make individual calibration of an eLoran timing receiver unnecessary and enable faster, less expensive and simpler field deployments. The paper discusses: intrinsic eLoran frequency stability and UTC recovery as compared with GNSS; our baseline set-up for trial service; zero baseline measurement results of the service; medium baseline testing with the Differential Reference Station providing on-air corrections and a Rover receiver using the corrections at fixed locations away from the reference station; and an assessment of coverage area, temporal and spatial decorrelation of the corrections.
Published in:	Proceedings of the 46th Annual Precise Time and Time Interval Systems and Applications Meeting December 1 - 4, 2014 Seaport Boston Hotel Boston, Massachusetts
Pages:	124 - 133
Cite this article:	Offermans, Gerard, Johannessen, Erik, Schue, Charles, "Implementing a Wide Area High Accuracy UTC Service via eLoran," Proceedings of the 46th Annual Precise Time and Time Interval Systems and Applications Meeting, Boston, Massachusetts, December 2014, pp. 124-133.
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