Title

Temporal variation of Loran ASFs and their effects on HEA navigation

Document Type

Conference Proceeding

Date of Original Version

1-1-2008

Abstract

To fulfill the envisioned role of a backup navigation system to the GPS, Loran needs to satisfy the user requirements of accuracy, integrity, continuity, and availability. The biggest obstacle to accuracy lies in what are called Additional Secondary Factors (ASFs) which are changes in the time of arrival of the Loran signal due to the non-uniform propagation speed of the signal between the transmitter and the receiver. To better understand these ASFs, with the goal of bounding and/or mitigating their effect on navigation position accuracy, the US Coast Guard has established a network of Loran monitor sites that track and archive the ASFs at particular locations. One goal of this network is to monitor the total seasonal variation of the ASFs. This result is important to bounding the position error for aviation applications. For those locations near a port area, the monitor is also employed to provide real-time temporal corrections for Harbor Entrance and Approach applications. Of interest in this setting is the spatial correlation of the temporal corrections. Questions include: "What spacing of harbor monitors will provide sufficiently good temporal corrections to vessels? Is one monitor sufficient for a large harbor, such as New York, or is a second monitor needed? Can a temporal monitor at one harbor provide HEA performance at a nearby harbor?" The answers, obviously, determine the cost of implementing Loran for HEA. The ASF monitor installations began in early 2006; at recent meetings of the International Loran Association we have presented preliminary statistics on the annual range of ASFs observed and their spatial correlations. One significant result, described at the ILA's most recent meeting (Oct 2007), was that while spatial correlations (using standard statistical definitions) appear high, position accuracy is not directly related to correlation and is very sensitive to mismatch in the ASFs. By computing the impact of ASF temporal differences on position error (averaged over a year), we observed that, from the data available, HEA requirements will require monitor-to-vessel separation of less than 30 km. This paper documents our continued investigation into the navigation performance impact of the temporal portion of the ASF as a function of monitor separation distance. As part of this study we include data from two sets of closer spaced monitor sites (Point Allerton to the Volpe Center, both near Boston Harbor and at a distance of 15 km; Sandy Hook to Staten Island, both near New York Harbor and at a distance of 16 km). We also address the situation when a vessel is located between two monitors; specifically asking, "How do we interpolate the two temporal corrections?".

Publication Title

ENC-GNSS 2008 - European Navigation Conference

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