GPS aviation outage prediction and reporting systems
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GPS aviation outage prediction and reporting systems

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    Use of GPS for instrument flight rule (IFR) air navigation requires that the system have integrity. Integrity is the ability to detect when a satellite is out of tolerance and should not be used in the navigation solution and then warns the pilot in a timely manner. This capability currently is provided by a Receiver Autonomous Integrity Monitoring (RAIM) or Fault Detection and Exclusion (FDE) algorithm contained within the GPS receiver. All GPS receivers currently certified for air navigation under Technical Standard Order (TSO) C129 must have RAIM for supplemental navigation and FDE to serve as a primary means system. In order for the receiver to perform RAIM, a minimum of five satellites with satisfactory geometry must be visible to the user and FDE requires a minimum of six visible satellites. Since the GPS constellation of 24 satellites was not designed to provide this type of performance, GPS RAIM/FDE is not available 100% of the time, even when all of the satellites are operational. Occasionally, satellites are taken out of service for maintenance, further degrading coverage. There is a need to interpret the satellite status information and provide GPS outage information in a format which is meaningful to the pilot during the pre-flight planning process. In other words, it is not sufficient to only report to a pilot which satellite is out of service, but rather where and when the outages will occur. The location and duration of integrity outage periods can be predicted with the aid of computer analysis, however, and reported to pilots when the flight plan is filed. This paper discusses systems that the FAA and the U.S. Air Force have implemented to disseminate GPS satellite outages, as well as airfield-specific GPS RAIM outages, to civilian and military pilots. These concepts are being adopted by other countries, such as Australia, and can easily be extended to GLONASS or a system which uses signals from both constellations. However, in the implementation of such a system for reporting GPS outages for aviation, there are issues to address such as the format of the outage data disseminated, the standardization of GPS integrity algorithms, the large range of aircraft equipment which can be certified for Required Navigation Performance (RNP) requirements, and the training necessary for pilots and air traffic controllers. This paper also considers plans to disseminate outage information for satellite and ground-based GNSS augmentation systems of the future (i.e., SBAS and GBAS) which are beginning to be discussed in the ICAO GNSS Panel.
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