The Global Positioning System: A Shared National Asset (1995)
Chapter: Availability
Dilution of Precision
Dilution of precision, or DOP, is a term that describes the effect of satellite geometry on positioning, timing, and velocity accuracy. Any positioning system that relies on pseudoranging will be affected by the angular spacing between the known points that are used to measure from. The GPS constellation has been designed to give users at least four satellites in view with good geometric spacing, but terrain and man-made structures can occasionally block a receiver's view of some satellites, especially those near the horizon, making the dilution of precision less than ideal.
Improving The Capabilities Of GPS
Even before the implementation of SA in 1990, many potential GPS users envisioned a need to improve the accuracy of the system, as well as some of its other specified characteristics. Although GPS accuracy has just been discussed, other characteristics such as integrity, availability, continuity of service, and resistance to radio frequency (RF) interference require further elaboration.
Integrity
Integrity, as defined by the Federal Radionavigation Plan, is the ability of a navigation system to provide timely warnings to users if and when the system should not be used. The integrity function of a navigation system involves monitoring the system's errors and, if specified protection levels are estimated to be exceeded, giving a warning to the user that the system cannot be used for navigation. In the case of GPS, integrity is maintained by monitoring the signal emanating from each satellite and determining if the pseudorange accuracy meets specified performance criteria for a given application.
Two statistical measures of integrity are often used. One measure relates the probability that a hazardously misleading error will occur and the probability that this error will go undetected (1 minus PHE times PD, where PHE is the probability of hazardous error and PD is the probability of missed detection). The second measure of integrity is simply the time a navigation system takes to warn the user that a hazardous error exists (time-to-alarm) There is currently no specified integrity value for either the GPS SPS or the PPS.
Availability
The availability of a navigation system, which is also defined in the Federal Radionavigation Plan, is the percentage of time that the services of the system are useable. Availability is an indication of the ability of a system to provide useable service within the specified coverage area. For GPS, "useable service within the specified coverage area" means
