Engineering the world's largest DGPS Network

The U.S. Coast Guard's is part of a team that is implementing the world's largest ground-based GPS augmentation service, Nationwide DGPS (NDGPS). The U.S. Coast Guard's original responsibility was to provide DGPS coverage to all harbors and harbor approaches of the United States. The Maritime DGPS broadcast site network built to meet this requirement provided differential corrections along the Atlantic, Pacific and Gulf coasts, the Great Lakes, Hawaii, Puerto Rico and the southern coast of Alaska. Two control sites were also built to monitor the performance and integrity of the individual DGPS broadcasts. As the number of users and uses of DGPS increased, the need to expand the DGPS coverage area has also increased. An agreement with the U.S. Army Corps of Engineers expanded the U.S. Coast Guard's DGPS coverage to the inland rivers of the U.S.. The NDGPS expansion effort, scheduled to have over 126 broadcast sites, is designed to provide double terrestrial DGPS coverage across the entire continental United States and Alaska and meet all surface transportation navigation requirements. Once the requirement of a NDGPS network was identified, the Department of Transportation (DOT) assembled a team from several DOT agencies to determine the best way to meet this need. This team decided the most cost effective and efficient way to create a nationwide system was to expand the U.S. Coast Guard's Maritime DGPS network. Based upon their experience with providing DGPS coverage, the U.S. Coast Guard was designated the lead agency responsible for the design, construction and implementation of the broadcast and control sites in the NDGPS expansion. This paper explains the basic operating concepts of the U.S. Coast Guard's DGPS Network: the equipment and functions of the broadcast site, control site and communication's network, and how the U.S. Coast Guard ensures the integrity of the system and differential corrections on a continuous basis. It then describes the challenges the U.S. Coast Guard is facing trying to provide a nationwide network and reviews the advantages and effort involved in converting obsolete U.S. Air Force Ground Wave Emergency Network sites into DGPS broadcast sites. Finally it reviews the U.S. Coast Guard's efforts to improve the accuracy, reliability and performance of the entire DGPS Network.