Can LORAN meet GPS backup requirements?

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 radio navigation (RNAV) system, is theoretically the best backup for the global positioning system (GPS). However, for Loran-C to be considered a viable backup system to GPS, it must be able to meet the requirements for non-precision approaches (NPAs) for the aviation community, and the Harbor Entrance and Approach (HEA) requirements for the maritime community. Through FAA sponsoring, the US 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 1times 10
^-7
. During the summer of 2003, the Coast Guard Academy, with flight support from the FAA Technical Center, conducted a series of tests to measure ASF variations in the vicinity of several selected airfields in Colorado, Arkansas, Florida, and California. In addition, approaches were flown at several airports in each of these areas. ASF and TOA data collected during these trials has been analyzed through post-processing to determine the Loran position accuracy during approaches. The accuracy of raw (uncorrected Loran) as well as ASF-corrected Loran positions are shown for a variety of ASF profiles. Using the BALOR ASF modeling software produced by the University of Wales at Bangor, we have also done some simulations of best-case performance analysis for receivers with little or no noise. In addition, we present some ideas for using a Kalman-filtered integrated Loran-INS receiver to smooth out Loran position errors. Finally, how all of these efforts lead toward meeting the accuracy requirements is shown