Loran: Creating a viable backup for GPS

The FAA is currently leading a team consisting of members from Industry, Government, and Academia to evaluate the future of Loran-C in the United States as an alternative or backup to the Global Positioning System (GPS) as a national navigation system. For Loran-C to be considered a viable back-up system to GPS it is must be able to meet Required Navigational Performance accuracy requirements for aircraft Non-Precision Approach of 309m and Harbor Entrance and Approach accuracy requirements of 8-20m for the maritime community. Current Loran receivers which use a weighted least-squares solution to compute a position from Time of Arrival (TOA) or Time Difference measurements cannot meet new Coast Guard and Federal Aviation Administration (FAA) accuracy requirements.; In the past 10 years advances in DSP hardware and algorithms have allowed for improvements to Loran receivers. This has led to performance improvements, especially in the accuracy of the navigation position solution. The current limitation now appears to be the difference between the assumed all-seawater propagation paths and the actual propagation path which has varying ground conductivities. This difference is called the Additional Secondary Factor (ASF). A description of various tests and analysis conducted to understand the spatial, temporal, and directional variations in the ASF values as well as a proposed method to incorporate ASFs for both maritime and aviation users is provided. A solution methodology for each user community is provided to meet both spatial and temporal ASF variations.; A proposed integrated Loran/IMU/GPS receiver is described, where an IMU is used to improve the performance of Loran in the absence of GPS. The proposed solution is to track and integrate information such as course and speed from an Inertial Measuring Unit (IMU), Additional Secondary Factor (ASF) grid data, and ASF temporal corrections into the position solution to allow the position solution to meet the accuracy requirements. This Kalman filter integrated receiver appears to work fairly well (perhaps 25m accuracy). The position accuracy is still not quite as good as needed (8-20m); however, improvement will come with a better ASF grid as the ASF is the largest source of error.