Integrity, Availability and Continuity of Service
Integrity is the ability of the system to provide timely warnings to the user when the equipment is unreliable for navigation purposes. The concept of integrity includes a failure to alarm and a false alarm.
In Europe, conventional ground-based navigation aids incorporate monitoring equipment at the ground site. Should the equipment detect an out-of-tolerance condition, the transmitter is shut down, and the user alerted by a means of a flag or loss of aural identification. GNSS integrity relates to the trust that can be placed in the correctness of the information supplied by the total system. This includes the ability of the system to notify the pilot if a satellite is radiating erroneous signals.
Individual GNSS satellites are not continuously monitored, and several hours can elapse between the onset of a failure and the detection and correction of that failure. Without some additional integrity monitoring, a clock or ephemeris error, for example, can have a significant effect on any navigation system using that satellite. Receiver Autonomous integrity monitoring (RAIM) is the most common form of integrity monitoring. Many GPS receivers do not monitor integrity and will continue to display a navigation solution based on erroneous data.
Availability may be defined as the percentage of time the services of a navigation system are accessible. It is a function of both the physical characteristics of the environment and the technical capabilities of the transmitter facilities. GNSS availability is the system’s capacity to provide the number of satellites required for position fixing within the specified coverage area. At least three satellites need to be in view to determine a two-dimensional (2D) position, while four are required to establish an accurate 3D position.
Selective Availability (SA) was a technique used by the US Department of Defense to limit the accuracy of GPS to other than approved users. It was achieved by artificially degrading the accuracy or ‘dithering’ the satellite clock, or broadcasting less accurate ephemeris parameters. With growing reliance upon GPS in civil applications, SA was
discontinued by Presidential decree in 2000.
Many early GPS receivers were “hard-wired” for SA in the expectation that civil use would always need to assume that SA was active. For receivers that cannot take advantage of SA being discontinued, average RAIM (Fault Detection) availability (is slightly less than for receivers that can take advantage of SA having been discontinued.)
Continuity of service is the ability of the total navigation system to continue to perform its function during the intended operation. Continuity is critical whenever reliance on a particular system is high, such as during an instrument approach procedure. Although the GPS constellation has been declared fully operational, the possibility exists that unserviceability will occur and reduce the number of ‘healthy’ satellites in view to less than the operational requirement.