Performance analysis and potential improvements of the loran data channel (2007)

In 2001, the Volpe National Transportation Systems Center completed an evaluation of the Global Positioning System (GPS) vulnerabilities and the potential impacts to transportation systems in the United States. One of the recommendations of this study was for the operation of backup system(s) to GPS; Loran C was identified as one possible backup system. In a recently completed Navigation Transition Study, the FAA concluded that Loran-C, as an independent radionavigation system, is theoretically the best backup for GPS; however, in order for Loran-C to be considered a viable back-up system to GPS, it must be able to meet the requirements for non-precision approaches (NPA) for the aviation community and the Harbor Entrance and Approach (HEA) requirements for the maritime community. The current approach to HEA Loran navigation is to establish a spatial grid of ASF corrections for the harbor area and then supplement this with broadcast temporal corrections to the grid. The method being developed for disseminating these temporal corrections is to transmit the data on the Loran signal itself; frequently called the Loran Data Channel (LDC). Data transmission on the Loran signal is not a new idea; the Coast Guard experimented with a pulseposition modulated data communication system code named Clarinet Pilgrim in the 1960s. However, the use of advanced DSP-based techniques for receivers, combined with new equipment installations at the U.S. Loran transmitter sites now offers the opportunity for a reliable, higher rate data transmission system. During 2000-2003 a Loran Data Channel system that employed phase modulation on 6 of the 8 Loran pulses in a group, called Intrapulse Frequency Modulation (IFM) which achieved a data rate of 250 bps, was experimented with, but found to be difficult to implement in the existing transmitters. The method under development and proof-of-concept testing right now employs the pulse position modulation of an additional 9 pulse in each group. In our January 2006 paper (ION-NTM 2006) we examined some of the performance issues of this 9th pulse system. Specifically, after presenting a description of the signal space model for the system, we computed the theoretical worst case performance (as measured by symbol error rate) assuming that all Loran stations were broadcasting 9 pulse (both as a function of received signal to noise ratio and crossrate interference) and compared this to some limited (day only) on-air 9th pulses transmitted from the Loran Support Unit in Wildwood, NJ. Since that time, additional Loran stations have come on-air transmitting the 9th pulse 24x7, including Loran Station Seneca, NY. Further, we have also recently developed a 9th pulse receiver that demodulates the 9th pulse and decodes the messages in real-time. With this system we have been investigating the baseline performance of the LDC system using the signals from Seneca. In this paper we describe the demodulator, and report on the observed error rates both with and without erasure decoding in the Reed Solomon decoder. We examine the effect of crossrate interference and report on the percentage of symbol errors and decoder failures (and/or errors) caused by cross-rate vs. the percentage from other sources. We also look at the impact of skywave interference on system performance - especially at night.