UTC Synchronization and Stratum-1 Frequency Recovery Using eLoran: The Alternate Basket for Your Eggs

摘要

Accurate timing and frequency is becoming increasingly important in many applications that influence our daily lives. Eleven 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 ten it is deemed essential. More and more systems are becoming solely dependent on GPS or other GNSS for their precise position, timing, and frequency information, especially as additional multi-constellation GNSS, i.e. Galileo, Compass, and GLONASS, and Regional Navigation Satellite Systems (RNSS) become fully operational and "fill the world's skies." Along with the explosive growth of systems and applications comes an increasing awareness of GNSS vulnerabilities. Interference, jamming and spoofing reduce availability and reliability of all GNSS. The General Lighthouse Authorities of the UK and Ireland have started the deployment of equipment for an Initial Operating Capability eLoran system along the east coast of the UK, and the Republic of Korea announced plans to deploy a nation-wide eLoran system. Other countries are likely to follow their example. eLoran is a High Power, Low Frequency (LF), Ground Wave radio broadcast system, capable of providing 10-meter positioning accuracy, Stratum-1 frequency distribution, and UTC timing within 100 ns across large areas. LF technology, including eLoran, is a well-established solution for providing services very similar to those delivered by GNSS, with characteristics and failure modes that are complementary to GNSS. UrsaNav has entered a Cooperative Research and Development Agreement (CRADA) with the U.S. Government, which allows using existing infrastructure to broadcast signals in the spectrum between 90-110 kHz in the U.S. UrsaNav broadcasts eLoran signals on a semiregular basis from the former USCG Loran Support Unit in Wildwood, NJ, using a 400 kW transmitter. Monitor receivers are set up in locations in Virginia, Washington, DC, and Massachusetts to monitor the transmissions and analyze the timing performance against GPS disciplined PRS, or better. One such monitor is installed at the U.S. Naval Observatory and is compared directly to the USNO master clock. These trials have been received with a great deal of interest in the U.S. and abroad, especially from telecommunications, power grid synchronization, and other timing application users that require alternatives or back-ups for GPS-based timing solutions. Included in the paper is a description of the transmitter and monitor receiver set-up, as well as system improvements to increase timing accuracy, such as differential eLoran. The data shows that eLoran is easily capable of sub 100 nanosecond accuracy and that further improvements can be made. This level of accuracy can be an important component in a national resilient position, navigation, and timing infrastructure.