Each plane is inclined to the plane of the Equator at an angle whose measure is 55 . The lines that are intersections of the orbital planes with the plane of the equator also intersect Earth's axis and are distributed uniformly around the axis. At any time of any day from any location on Earth, there are likely to be at least four satellites in view. Subject to variations in arrangement of the satellites, a GPS receiver processes several signals simultaneously to determine the current location of the receiver. Quality of results also depends on signal quality.
Optimum results require favorable distribution of satellites at the time that data is collected.
Since a receiver uses several satellites, results are generally best when the satellites are in substantially different directions from the receiver and are not at extremely high or extremely low angles of elevation. From satellites at low angles of elevation, signals tend to be degraded by long paths through the atmosphere. From high angles of elevation, geometry is unfavorable for the determination of altitude of the receiver. Factors affecting signal quality also include atmospheric conditions and selective availability, the latter a deliberate degradation of signal. The degree of degradation caused by selective availability varies with need as determined for reasons of national security.