Airport ASF Mapping Methodology Update

In 2001, the Volpe National Transportation Systems Center completed an evaluation of 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. 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 1x10-7. The future of Loran for aviation is based on multi-station, multi-chain, all-in-view, digital signal processing (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. The draft procedure has been tested during data collection at airports in Maine and Ohio. This paper discusses the results of this data collection: how well the spatial variation seen on the ground matches the BALOR model prediction and the implications of this on the proposed procedure, an analysis of how many ASFs should be required to meet Required Navigation Performance (RNP) 0.3 for each airport based on geometry and ASF variation in the area, and results of the position accuracy obtained by the aircraft flying approaches when using the airport ASF values.