Title

Test methodologies for eloran transmitters

Document Type

Conference Proceeding

Date of Original Version

11-20-2009

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. This recommendation has taken a further step forward with the request in the President's FY09 budget to continue to operate the Loran-C system and to upgrade it to enhanced Loran or eLoran capabilities. To ensure that the Loran system consistently provides sufficiently "good" signal in space for navigation purposes, the US Coast Guard developed a detailed Signal Specification for the Loran-C Signal (COMDTINST 16562.4) in the early 1980's; the latest revision of this specification appeared in 1994. In parallel with the specification, the Coast Guard also developed a testing methodology to ensure that each transmitter met the specification, and developed hardware/software for transmitter certification, called the Loran-C Data Acquisition System, or LORDAC. Essentially, this was a system that sampled the Loran-C signal at the transmitter (via the antenna ground return current) and computed various statistical measures for comparison to the specification. Currently a revised Loran-C Signal Specification (to include eLoran) is the subject of discussion. Recognizing that eLoran is a multi-use system (timing and frequency users, data communications, along with the traditional navigation user), topics in a revised specification should include those changes to the signal deemed necessary to meet the more stringent navigation performance desired (harbor/entrance and approach), as well as to provide signals for timing/frequency users, and a data carrying component (the Loran Data Channel). In parallel, it is appropriate to simultaneously discuss a testing methodology for eLoran transmitters. This methodology should include new procedures to assess those changes to the signal that separate eLoran from Loran-C (such as the Loran Data Channel component), and also should be brought up to date, employing advanced features of modern data acquisition equipment, and modern ideas on statistical signal processing algorithms. This paper starts a public, written discussion of proposed testing methodology. Just as the eLoran Signal Specification builds on the existing Loran-C specification, any test methodology for eLoran should build on the prior test methodology for Loran-C, focusing on those parts of the signal most important for navigation, timing, and communications applications. Some procedures from the prior test methodology that are revisited in this paper include ECD measurement and zero-crossing jitter. New procedures discussed include assessing the correlation (repeatability) of pulses within a PCI since this is particularly relevant to the performance of the Loran Data Channel in the presence of skywave.

Publication Title

Proceedings of the Institute of Navigation, National Technical Meeting

Volume

2

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