Differential Global Positioning System navigation: A geometrical analysis

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This thesis examines and evaluates some geometrical aspects of a method called "differential navigation", which is used as a means of real time calibration for a navigation system. Particularly, the evaluation and analysis is applied to a new satellite-based radionavigation system, known as NAVSTAR-GPS (NAVigation Satellite Timing And Ranging system-Global Positioning System). The NAVSTAR-GPS, being developed by the United States Department of Defense, is scheduled to be fully operational by the end of the decade and is capable of providing realtime continuous positions accurate to 10 meters. Field test results using some of the current prototype GPS satellites have manifested these capabilities. These early results and other studies led the United States, for national security reasons, to intentionally curtail the GPS capabilities to the general public and provide accuracies of the order of 200 meters. The need for better accuracies required by a large class of "unauthorized users" prompted us to extend the conventional performance capability of the GPS to the differential one. Differential GPS navigation provides an opportunity to thousands of unauthorized users, unable to gain the full benefit of the GPS system, to effectively make use of the system under intentionally degraded conditions and retrieve the original signals. Under certain assumptions and through a simulation computer program, this study evaluated and demonstrated the validity and feasibility of the above concept, with main emphasis on the investigation of various geometrical aspects related to the differential operation. Inferences correspond only to marine applications (two dimensional) of the GPS 18-satellite constellation considering hypothetical intentional degradations. Recommendations for the continuation of this research are also given.