Navigating Using an ASF Grid For Harbor Entrance And Approach

Richard Hartnett, Gregory Johnson and Peter Swaszek

Abstract:	The Federal Aviation Administration (FAA) is currently leading a team consisting of members from Industry, Government, and Academia to provide guidance to the policy makers in their evaluation of the future of Loran- C in the United States. In a recently completed Navigation Transition Study, the FAA concluded that Loran-C, as an independent radionavigation (RNAV) system, is theoretically the best backup for the Global Positioning System (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. Through FAA sponsoring, the U.S. Coast Guard Academy (USCGA) is responsible for conducting some of the tests and evaluations to help determine whether Loran can provide the accuracy, availability, integrity, and continuity to meet these requirements. A major part of assessing the suitability of Loran is in understanding the nature of Loran ground wave propagation over paths of varying conductivities and terrain. Propagation time adjustments, called ""additional secondary factors. (ASFs), are used to adjust receiver times of arrival (TOAs) to account for propagation over non-seawater paths. These ASFs vary both spatially and temporally, and unless understood and/or modeled, we lose accuracy and may not be able to guarantee a hazardously misleading information (HMI) probability of less than 1x10-7. The Coast Guard Academy has been conducting a series of tests in the Thames River to measure ASF variations and assess the performance of navigating using an ASF grid. The intent is that this grid be used, along with temporal corrections from a ground reference station at the Coast Guard Academy, to model the ASF so as to navigate in the Thames River with high accuracy. The BALOR ASF modeling software produced by the University of Wales at Bangor has been used to create predicted ASF grids of the Thames River for comparison, analysis, and simulation purposes. In this paper we present preliminary comparisons of the BALOR predictions and measured ASF values. Using the predicted grids, analysis is presented on required grid spacing and interpolation techniques to reach the desired accuracy. To show expected performance, a simulation using BALOR predicted ASF values is presented. Finally, we present some ideas for using a Kalman-filtered integrated Loran-INS receiver in order to smooth out Loran position errors.
Published in:	Proceedings of the 60th Annual Meeting of The Institute of Navigation (2004) June 7 - 9, 2004 Dayton Marriott Hotel Dayton, OH
Pages:	200 - 210
Cite this article:	Hartnett, Richard, Johnson, Gregory, Swaszek, Peter, "Navigating Using an ASF Grid For Harbor Entrance And Approach," Proceedings of the 60th Annual Meeting of The Institute of Navigation (2004), Dayton, OH, June 2004, pp. 200-210.
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