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Notice to Airmen (NOTAM) System

The FAA has certified GPS as a supplemental navigation system for domestic en route, terminal, and nonprecision approach navigation, as well as for primary means oceanic use. Use of GPS for instrument flight rule (IFR) air navigation requires that the system have integrity, which is the ability to detect when a satellite is out of tolerance and should not be used in the navigation solution. Until the Wide Area Augmentation System (WAAS) is implemented, GPS integrity will be provided by algorithms within the receiver. This method is known as Receiver Autonomous Integrity Monitoring (RAIM). All GPS receivers certified under Technical Standard Order (TSO) C129 must have RAIM.

In order for a GPS receiver to perform RAIM, a minimum of five satellites with satisfactory geometry must be visible. Since the GPS constellation of 24 satellites was not designed to provide this type of performance, RAIM is not available 100% of the time, even when all 24 GPS satellites are operational. Also, satellites occasionally need to be taken out of service for maintenance, further reducing the availability of RAIM. GPS outage information needs to be provided to pilots during pre-flight planning. GPS, however, differs from traditional navigation systems due to the fact that the satellites and areas of degraded coverage are in constant motion. Therefore, if a satellite fails or is taken out of service for maintenance, it is not intuitively known which areas of the country are affected, if any. The location and duration of these outage periods can be predicted with the aid of computer analysis, however, and reported to pilots during the pre-flight planning process.

The Volpe National Transportation Systems Center has designed, developed, and implemented a system for the United States Air Force Flight Standards Agency (AFFSA) to report GPS Notices to Airmen (NOTAMs). This system, which was declared operational in May 1995, produces NOTAMs for GPS satellite outages and also provides airport specific military GPS NOTAMs for nonprecision approach (NPA) that are based on RAIM outages lasting longer than twenty minutes. These outages are disseminated through the U.S. NOTAM system as Class One M-Series NOTAMs to military aviators via the Automated Weather Network (AWN).

The Volpe Center also has developed a GPS aeronautical information system for the FAA to provide GPS outage information to civilian pilots. The requirement established by the FAA Satellite Operational Implementation Team (SOIT) is to report all RAIM outages lasting longer than five minutes during a nonprecision approach. GPS outage information is disseminated to civilian pilots via the Automated Flight Service Stations (AFSS). Although these outages are not officially NOTAMs, they are available to pilots upon request. The FAA GPS aeronautical information system was declared operational in November 1995. The Volpe Center has developed similar systems for Airservices Austrailia, DFS Deutsche Flugsicherung, and Direccion General de Aeronautica Civil de Chile.

For more information contact Karen Van Dyke at vandyke@volpe.dot.gov.