Coverage Analysis of NDGPS Broadcast Sites

摘要

The U.S. Coast Guard is part of the U.S. Department of Transportation's team implementing the world's largest ground-based Differential Global Positioning System (DGPS) service, Nationwide DGPS (NDGPS). The NDGPS expansion effort, scheduled to have over 126 Broadcast Sites, is designed to provide double terrestrial DGPS coverage across the continental United States to meet all surface transportation navigation requirements. As a cost saving measure, the NDGPS project team is converting obsolete U.S. Air Force (USAF) Ground Wave Emergency Network (GWEN) sites into differential broadcast sites. Several of these GWEN sites have pre-existing structures nearby; such as cellular phone towers and television broadcast antennas. During the initial conversion studies, questions were raised about how these structures would affect the DGPS signal and the sites' expected coverage area. Using the MININEC computer modeling software, the U.S. Coast Guard Command and Control Engineering Center (C2CEN) Radio Frequency (RF) Engineers determined the theoretical effects on the DGPS signal strength, radiation patterns and predicted coverage area at several broadcast sites. This paper reviews C2CEN's methodology for modeling the obstructions and the predicted impact on the DGPS coverage area. It will show the method used to determine the actual coverage areas and how this is used to improve the theoretical model. We also discuss the effects on the selection of future NDGPS broadcast sites based upon C2CEN's theoretical analysis and the coverage area validation to ensure the system achieves its signal availability and coverage requirements.