BALOR model validity for the airport ASF mapping methodology

In 2001, the Volpe National Transportation Systems Center completed an evaluation of the Global Positioning System (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 GPS; however, in order for Loran-C to be considered a viable back-up system to GFS, 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 is 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. 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. The draft procedure has been tested during data collection efforts at airports in Maine, Ohio, and New Jersey. A key component of the proposed procedure is the use of the BALOR ASF prediction software to reduce the number of field measurements. ASF measurements made on the ground along the airport approaches and in the air on long baselines to and from several Loran towers are used to compare to the BALOR predictions to determine the validity of the BALOR model. This paper discusses the results of this data collection: how well the measured spatial variations match the BALOR model predictions, how well the proposed mapping procedure works, and results of the position accuracy obtained by the aircraft flying approaches when using the airport ASF values