A methodology to map airport ASF\u27s for enhanced Loran

In 2001, the Volpe National Transportation Systems Center completed an evaluation of GPS vulnerabilities and the 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. The Federal Aviation Administration (FAA) has been leading a team consisting of members from industry, government, and academia to evaluate 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 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\u27s) for the aviation community, and the Harbor Entrance and Approach (HEA) requirements for the maritime community. A significant factor limiting the accuracy of a Loran system is the spatial and temporal variation in the times of arrival (TOAs) observed by the receiver. A significant portion of these variations are due to the signals propagating over paths of varying conductivity; these TOA corrections which compensate for propagating over non-seawater paths are called additional secondary factors (ASFs). Hence, a key component in evaluating the utility of Loran as a GPS backup is a better understanding of ASFs and a key goal is deciding how to mitigate the effects of ASFs to achieve more accurate Loran-C positions while ensuring that the possibility of providing hazardous and misleading information (HMI) will be no greater than 1×10-7. The future of Loran for aviation is based on multi-station, multi-chain, all-in-view, DSP-based receivers observing TOA measurements with H-field antenna technology. For an aviation receiver, the approach to mitigate propagation issues under study is to use a single set of ASF values (one for each Loran tower) for a given airport. This value may have seasonal adjustments applied to it. The Loran receiver will use this set of static ASF values to improve position accuracy when conducting a non-precision approach (NPA). A Working Group is currently developing the procedures to be used to map the ASF values for an airport. The output of the Working Group will be a set of tested and documented procedures for conducting an airport survey; these procedures can then be followed to survey airports nationwide. This paper discusses the procedures being envisioned and the testing methodology for the procedures. Equipment to be used in the surveys and the error budget for the survey equipment will be presented as well as a proposed error budget for the ASF methodology