Performance trials of an integrated Loran/GPS/IMU navigation system, part II
Date of Original Version
The 2001 Volpe National Transportation Systems Center report on GPS vulnerabilities identified Loran-C as one possible backup system for GPS. The Federal Aviation Administration (FAA) observed in its recently completed Navigation and Landing Transition Study that Loran-C, as an independent radio navigation system, is theoretically the best backup for GPS; however, this study also observed that Loran-C's potential benefits hinge upon the level of position accuracy actually realized (as measured by the 2 drms error radius). For aviation applications this is the ability to support non-precision approach (NPA) at a Required Navigation Performance (RNP) of 0.3 which equates to a 2 drms error of 309 meters and for marine applications this is the ability to support Harbor Entrance and Approach (HEA) with 8-20 m of accuracy. The recently released report of the DOT Radionavigation Task Force recommended to "complete the evaluation of enhanced Loran to validate the expectation that it will provide the performance to support aviation NPA and maritime HEA operations." To meet this need, the FAA is currently leading a team consisting of members from industry, government, and academia to provide guidance to the policy makers in their evaluation of the future of enhanced Loran (eLoran) in the United States. Through FAA sponsoring, the U.S. Coast Guard Academy (USCGA) is responsible for conducting some of the tests and evaluations to help determine whether eLoran can provide the accuracy, availability, integrity, and continuity to meet these requirements. The key to meeting HEA accuracy requirements is an accurate ASF spatial grid. This can be met by a very dense grid of ASF values; however, this increases the problems with grid distribution and storage on the receiver. Previous work (ION AM June 2004) suggested that a sparse grid can be used and accuracy targets still reached by interpolating the points in between the grid values. The difficulty is in creating a grid with accurate grid point data. Several options for uniform grids were tested (ION NTM Jan 2005) and did not yield sufficient accuracy. In this work we have created a more accurate grid using non-uniform spacing and better matching of data to grid points. An integrated Loran/GPS/IMU receiver has been developed that incorporated this new ASF grid. This receiver integrates IMU information (velocity and acceleration) and ASF data from a stored grid into the Loran position solution to improve the accuracy and consistency of the resulting position. Initial results of this receiver were reported in (ION NTM Jan 2005). Since then, extensive work has been done to characterize the IMU errors and biases in order to better incorporate the IMU data into the integrated receiver. A Kalman filter is used to integrate the information and to predict forward the position to remove the time lag caused by the Loran filtering. The GPS information (position, time) is used to measure the ASF values in real-time to track deviations from the stored ASF grid. These grid differences are used to correct the grid values in the absence of a local ASF monitor station. Performance of the receiver is presented using an ASF grid alone, an ASF grid corrected using temporal ASF variations from a local ASF monitor site, and an ASF grid corrected using the real-time calculated grid differences. Finally, how all of these efforts lead towards meeting the accuracy requirements is shown.
Proceedings of the Annual Meeting - Institute of Navigation
Johnson, Gregory, Ruslan Shalaev, Christian Oates, Richard Hartnett, Hunter Atherton, Michael Teixeira, and Peter Swaszek. "Performance trials of an integrated Loran/GPS/IMU navigation system, part II." Proceedings of the Annual Meeting - Institute of Navigation , (2005): 875-886. https://digitalcommons.uri.edu/ele_facpubs/1076