Title

Improved signal processing methodologies for the loran data channel

Document Type

Conference Proceeding

Date of Original Version

12-22-2008

Abstract

In 2001, the Volpe National Transportation Systems Center completed an evaluation of the Global Positioning System's (GPS) vulnerabilities and their 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 pulse-position modulation 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. The method under development and proof-of-concept testing right now employs pulse position modulation of an additional 9th pulse in each group. In previous work (ION-NTM 2006 and ION-AM 2007) we examined some of the performance issues of this 9th pulse system and have been investigating the baseline performance of the LDC system using the signals from Loran station Seneca. We showed that the design and implementation details of the LDC demodulator have distinct impacts on performance. The performance is also impacted by how the tracking of the pulse template is implemented, by the declaration and use of erasures, and by the selection of cross-rate window size. This paper reports on continued analysis of the system to determine why the message error rates experienced were higher than expected. Some avenues explored are: better cross rate interference erasure declaration, the impacts of blanking, and improvements to the matched filter implementation. We have also explored the impact of skywave interference on 9th pulse demodulation.

Publication Title

Proceedings of the Institute of Navigation, National Technical Meeting

Volume

2

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