Performance Trials of an Integrated Loran/GPS/IMU Navigation System, Part I

Gregory Johnson, Ruslan Shalaev, Peter Swaszek and Richard Hartnett

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’s) 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 path(s). 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 on a new integrated Loran/GPS/IMU receiver in the Thames River. This receiver integrates IMU information (velocity and acceleration) and ASF data from a stored grid into the Loran position solution to improve the accuracy and consistency of the resulting position. The density of the ASF grid used is based upon our previous study (ION AM June 2004); points in between the grid values are calculated by the receiver using a linear interpolation. The GPS information (position, time) is used to measure the ASF values in realtime to track deviations from the stored ASF grid. These grid differences are used to correct the grid values in the absence of a local ASF monitor station. Performance of the receiver using different ASF grids and interpolation techniques and corrected using the real-time calculated grid differences is shown. Finally, how all of these efforts lead towards meeting the accuracy requirements is shown.
Published in:	Proceedings of the 2005 National Technical Meeting of The Institute of Navigation January 24 - 26, 2005 The Catamaran Resort Hotel San Diego, CA
Pages:	840 - 848
Cite this article:	Johnson, Gregory, Shalaev, Ruslan, Swaszek, Peter, Hartnett, Richard, "Performance Trials of an Integrated Loran/GPS/IMU Navigation System, Part I," Proceedings of the 2005 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2005, pp. 840-848.
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