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BALOR Model Validity for the Airport ASF Mapping Methodology

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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 systems 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 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. 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.

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  • Radio Communications

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