An Evaluation of eLoran as a Backup to GPS

Abstract

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, which has been operated by the U.S. Coast Guard for the past 40 years, 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 of non-precision approach (NPA) for the aviation community and the harbor entrance and approach (HEA) requirements for the maritime community. The accuracy requirements for Loran to be used as a backup system are 307 m for NPA and 20 m for HEA. In addition, there are integrity, availability, and continuity requirements. The current Loran system of 24 stations provides a stated absolute accuracy in navigation position of only 0.25 NM; however, enhanced Loran or eLoran has the capability of meeting the stringent requirements for NPA and HEA. In order to meet the accuracy requirements user receivers must use additional secondary factors (ASFs) in calculating the user position. ASFs are propagation time adjustments that are subtracted from the receiver's times of arrival (TOAs) to account for propagation over non-seawater paths. These ASFs vary both spatially and temporally and both types of variations need to be accounted for to meet the accuracy targets. The current approaches to meeting the needs of the aviation and maritime communities are slightly different. For maritime navigation, the spatial variations will be accounted for through the use of a grid of ASF values that is known by the receiver a priori. As one component of the eLoran system, a reference station located nearby the harbor will be used to estimate the temporal changes in the ASFs relative to the published spatial grid; these differences will be broadcast using the Loran data channel (9th pulse) to the user receiver. This general method to HEA navigation was discussed by the authors in 2003 (ION AM 2003). More recently (ION GNSS 2006) we developed a technique to process survey data into a harbor grid. For the aviation community the approach is to measure and publish a set of ASF values for each airport. These airport ASFs will be adjusted to be in the middle of the seasonal variation in order to minimize the maximum error. This approach has been discussed by the authors most recently in papers presented in 2005 (ILA 34) and 2006 (ION NTM 2006). In this paper we show results from both flight tests at various airports around the U.S. and maritime tests in the Thames River in CT. These results demonstrate the ability of eLoran to meet the accuracy requirements for both NPA and HEA using the ASF methods we have proposed.