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18.1.2 System Availability and Reliability
18.1.2.1 The status of GPS satellites is broadcast as part of the data message transmitted by the GPS satellites. GPS status information is also available by means of the U.S. Coast Guard navigation information service: (703) 313.5907, Internet: http://www.navcen.uscg.gov/. Additionally, satel-lite status is available through the Notice to Airmen (NOTAM) system.
18.1.2.2 The operational status of GNSS operations depends upon the type of equipment being used. For GPS.only equipment TSO.C129a, the operational status of nonprecision approach capability for flight planning purposes is provided through a prediction program that is embedded in the receiver or provided separately.
18.1.3 Receiver Autonomous Integrity Monitoring (RAIM). When GNSS equipment is not using integrity information from WAAS or LAAS, the GPS navigation receiver using RAIM provides GPS signal integrity monitoring. RAIM is necessary since delays of up to two hours can occur before an erroneous satellite transmission can be detected and corrected by the satellite control segment. The RAIM function is also referred to as fault detection. Another capability, fault exclusion, refers to the ability of the receiver to exclude a failed satellite from the position solution and is provided by some GPS receivers and by WAAS receivers.
18.1.4 The GPS receiver verifies the integrity (usability) of the signals received from the GPS constellation through receiver autonomous integrity monitoring (RAIM) to determine if a satellite is providing corrupted information. At least 4 satellites are required to obtain a navigation solution. At least 5 satellites (or 4 satellites and a barometric altimeter for baro.aiding) are required for RAIM. This is also known as fault detection (FD). An enhanced form of integrity, known as Fault Detection and Exclusion (FDE), uses a minimum of 6 satellites (or 5 satellites with baro.aiding) to not only detect a possible faulty satellite, but to exclude it from the navigation solution in order for the navigation function to continue without interruption.

NOTE.
Baro.aiding satisfies the RAIM requirement by substitut-ing for one of the satellites. Baro.aiding is a method of augmenting the GPS integrity solution by using the barometric altimeter (a non.satellite input source).
NOTE.
The current altimeter setting must be entered into the receiver as described in the operating manual to ensure baro.aiding is available.
NOTE.
GPS derived altitude should not be relied upon to determine aircraft altitude since the vertical error can be quite large and no integrity is provided.
18.1.5 RAIM messages vary somewhat between receivers; however, generally there are two types. One type indicates that there are not enough satellites available to provide RAIM integrity monitoring and another type indicates that the RAIM integrity monitor has detected a potential error that exceeds the limit for the current phase of flight. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position.
18.1.6 Selective Availability. Selective Availability (SA) is a method by which the accuracy of GPS is intentionally degraded. This feature is designed to deny hostile use of precise GPS positioning data. SA was discontinued on May 1, 2000, but many GPS receivers are designed to assume that SA is still active. New receivers may take advantage of the discontinuance of SA based on the performance values in ICAO Annex 10, and do not need to be designed to operate outside of that performance.
18.1.7 The GPS constellation of 24 satellites is designed so that a minimum of five is always observable by a user anywhere on earth. The receiver uses data from a minimum of four satellites above the mask angle (the lowest angle above the horizon at which it can use a satellite).
18.1.8 The DOD declared initial operational capa-bility (IOC) of the U.S. GPS on December 8, 1993. The FAA has granted approval for U.S. civil operators to use properly certified GPS equipment as a primary means of navigation in oceanic airspace and certain remote areas. Properly certified GPS equipment may be used as a supplemental means of IFR navigation for domestic en route, terminal operations, and certain instrument approach proce-dures (IAPs). This approval permits the use of GPS in a manner that is consistent with current navigation requirements as well as approved air carrier operations specifications.
18.2 VFR Use of GPS
18.2.1 GPS navigation has become a great asset to VFR pilots, providing increased navigation capabili-ty and enhanced situational awareness, while reducing operating costs due to greater ease in flying direct routes. While GPS has many benefits to the VFR pilot, care must be exercised to ensure that system capabilities are not exceeded.
18.2.2 Types of receivers used for GPS navigation under VFR are varied, from a full IFR installation being used to support a VFR flight, to a VFR only installation (in either a VFR or IFR capable aircraft) to a hand.held receiver. The limitations of each type of receiver installation or use must be understood by the pilot to avoid misusing navigation information. (See TBL ENR 4.1.5.) In all cases, VFR pilots should never rely solely on one system of navigation. GPS navigation must be integrated with other forms of electronic navigation (when possible), as well as pilotage and dead reckoning. Only through the integration of these techniques can the VFR pilot ensure accuracy in navigation.
　
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