The Global Positioning System: A Shared National Asset (1995)
Chapter: Resistance to RF Interference
that at least four satellites must be visible to a user's receiver anywhere on or near the Earth, and the satellites must be providing the required positioning accuracy for the user's application. Some GPS applications, such as static surveying, do not require continuous availability. Others, such as air navigation, can require that GPS signals be available 99.999 percent of the time. The average availability of four or more GPS satellites in view of a given receiver, at SPS accuracy levels, is currently specified as 99.85 percent.32
Continuity of Service
Continuity of service, which also is referred to as reliability, is the ability of a navigation system to provide required service over a specified period of time without interruption. The level of continuity is expressed in terms of the probability of not losing the radiated guidance signals.33 Where warranted, continuity of service is achieved by using redundant transmitters and monitors. Continuity of service and availability go together in that availability is the probability that a system will be in service when it needs to be used, and reliability is the probability that the system will continue to provide service. The global average reliability for GPS is specified as 99.97 percent.34
Resistance to RF Interference
The accuracy of a GPS receiver can be degraded in the presence of unwanted interfering signals from terrestrial or other sources. In extreme cases, the receiver is unable to provide any useful navigation or positioning capability. Unwanted and unintentional sources of interference exist, such as the third harmonic of some UHF transmitters, which many civilian users may be unaware of. Military users are also concerned with unintentional interference, but they are more concerned with deliberate efforts to prevent the use of navigation signals through jamming. While no receiver can be made entirely immune to interference (intentional or otherwise), steps can be taken in the design of the receiver to
