Title

Loran Phase Codes, Revisited

Document Type

Conference Proceeding

Date of Original Version

11-10-2008

Abstract

The United States has a significant, strategic investment in the Loran system in its tower and monitor locations nationwide. Furthermore, many in the US and Europe have recognized that Loran (actually a developing modernized version called eLoran) is the best choice for a backup position, navigation, and timing (PNT) system to the global positioning system (GPS) in that it is not subject to the same vulnerabilities and failure modes. The current Loran-C signal, repeated groups of teardrop shaped pulses modulated to 100 kHz, has been operated by the U.S. Coast Guard since 1957. While recent years have brought numerous improvements to the Loran system, these have been technological upgrades to improve efficiency and reliability of the transmitter equipment; the details of the system's signaling have not changed and the system still operates essentially as it was designed and implemented 50 years ago. The design decisions made then were based upon the technology available at that time; the fact that the system is still operational and viable today is a tribute to the system design. However, now, with advances in technology, there are changes that could be made to the system that could improve performance. A natural question then, that should be part of the development of eLoran, is: "How should the system be configured to best serve PNT users given today's technologies?" In a paper at PLANS 2006, these authors began this discussion by re-opening some of the degrees of freedom in the system's design process. While certain hard constraints were kept (tower locations, power levels, spectrum, and it being an 8-pulse per group ranging system), that paper investigated the impact of several implementation options on navigation accuracy: time-of-transmission control of the transmitters, single-rating all stations, and chain/GRI realignment. The conclusion in that paper was that all three changes yielded a net gain in Loran navigation accuracy. This current paper continues the discussion by investigating possible changes to the Loran phase codes, and the potential improvements that such changes could provide. (The term "phase code" refers to a multiplier of ±1 on the envelope of each individual Loran pulse; the sequence of signs repeats every two groups or "phase code interval.") Currently, all Master signals use one specific phase code and all Secondary signals use another. The phase codes now in use allow for cancellation of sky wave interference (beyond 1 msec delay, that is) and for discrimination between Master and Secondary stations, an important consideration for legacy Loran receiver technology. The goal in this paper is to open up the discussion of phase code selection. Two importantissues considered are:•The codes themselves - could redesigned phase codes still provide sufficient sky wave protection, yet yield improved cross rate interference rejection?•The allocation of the codes - currently, all Secondaries share a common phase code (Master signals have a different code). Could different stations have different phase codes (e.g. implementing a form of code division multiplexing)? © 2008 IEEE.

Publication Title

Record - IEEE PLANS, Position Location and Navigation Symposium

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