A procedure for creating optimal ASF grids for harbor entrance & approach

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 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 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 (NPA) 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 (TOA) 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 (ASF). 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 effect 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 1times;10-7. The current approach to HEA Loran navigation is to establish a spatial grid of ASF corrections for the harbor area and then supplement this with broadcast temporal corrections to the grid. 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 to the user and storage on the receiver. Previous work (ION AM 2004) suggested that a sparse grid could be used and accuracy targets still reached by interpolating the points in between the grid values. Creating an accurate grid is difficult due to the receiver averaging time leading to inaccuracies when the receiver is moving relative to the desired grid point. Several options for uniform grids were tested (ION NTM 2005) and did not yield sufficient accuracy. In recent work (ION AM 2005) we created a more accurate grid using non-uniform spacing and better matching of data to grid points. The authors have been investigating these issues and working to develop a methodology for harbor ASF grid survey procedures; i.e. how to get from measured data to a published grid. This is similar to the work being done to develop a methodology for airport surveys (most recently reported on in ION NTM 2006). Using existing harbor survey data we have formulated a proposed survey method that meets the accuracy requirements while minimizing field data collection. This includes establishing error bounds and criteria for what constitutes a valid grid point