Modernized eLoran: The Case for Completely Changing Chains, Rates, and Phase Codes (2015)

First deployed in the U.S. in 1957, Loran-C dominated radio-based navigation for many years. In 2000 the FAA began a significant recapitalization of Loran in the U.S.; the 2001 Volpe report on the vulnerability of the GPS reinforced the need for a revamped Loran. What emerged was an enhanced or evolved version, so called "eLoran," aiming to achieve, for example, 10- 20 meter absolute positioning accuracy, RNP 0.3 mile required navigation performance, and stratum 1 time. After 10 years of development, in 2010, this U.S. e ffort was halted and the U.S. transmitters were silenced; since that time, eLoran is still being developed in Europe and deployed in Asia. Earlier this year U.S. Government interest in eLoran has again stirred (evidenced by a U.S. Army request for information and a U.S. Dept. of Transportation request for public comment); the rest of these initiated much conversation at the 2015 ION ITM. The prior U.S. (and continuing European) development of eLoran kept many of the 1950's system design choices so as to be compatible with legacy Loran receivers. These include the pulse shape, groups, chains, rates, phase codes, emission delays, etc. Chosen to suit 1950's technology, many of these restrictions are no longer necessary given the advances in transmitter and receiver technology (e.g. software defined radio) over the last half century. It is the opinion of these authors that as Loran, per se, no longer exists in the U.S., any re-emergence of a low frequency radio navigation system need not be held to these performance limiting constraints. In prior work these authors have promoted more significant changes to eLoran to improve system performance; specifically, single-rating all stations, reconquering the chain/rate structure within the continental U.S., and changing the phase codes. The current paper expands on these prior e fforts. Specifically, we propose putting all of the eLoran transmitters on the same repetition period and employing unique phase codes for each transmitter. To effectively choose new phase codes for eLoran, and assess their performance, we rely on the auto- and cross-correlation metrics. These metrics describe how well a receiver can both acquire and track a specific signal when contaminated by multi- path interference, the existence of other signals, and noise. While a "perfect" auto-correlation function, large at zero lag corresponding to the actual arrival of the signal and zero elsewhere, and a "perfect" cross- correlation function, zero for all lags, are preferred, it is impossible to find such codes. However, limiting the size of the window for which we require perfect auto- and cross-correlations, such codes can be found. To create such codes for eLoran we adapt results from the CDMA literature on complementary sequences and Large Area Synchronized (LAS) codes. This paper begins with a brief review of the relevant characteristics of Loran-C, including a discussion of the effects of sky wave and cross rate interference. This is followed by a survey of previously published ideas/concepts on how elements of the system could be changed so as to improve performance. Finally, details on the proposed rate/chain/phase code structure are presented. The reader should recognize that these ideas and results are not intended to define what the best eLoran system would be; rather, if eLoran soars again in the U.S., we hope to initiate a dialogue that looks beyond the decisions made in the 1950's.