Performance considerations for real-time navigation with the GPS

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The Global Positioning System, (GPS) is expected to fulfill the navigation requirements of many civilian users in the future. Whether or not it will be capable of providing sufficient navigation information to meet requirements will depend on a variety of factors. Consideration must be given to the level of integrity, reliability and accuracy which the GPS will provide. These concerns, and the related question of outages of the GPS, are examined with reference to the available literatures. First, a review of some aspects of the GPS are given as background for further discussions. Integrity, reliability and accuracy of the GPS are then each defined and evaluated considering the planned 21-satellite constellation. Next, outages of the full constellation – both those due to bad user-satellite geometry and to satellite failures – are discussed with particular emphasis on the former. Alternatives for navigation during such outages are then briefly mentioned. Bases on the aspects considered, it is then obvious that civilians users of the GPS cannot expect the system to continuously provide all the navigation information required. Additional information will be necessary during outages of the system to satisfy more stringent accuracy requirements.