The geometric effect resulting from satellites being too close together. Satellites that appear farther apart in the sky provide a more accurate position solution than ones close together. In the latter case, the position accuracy is degraded in a similar fashion to when position lines from ground-based NAVAIDs (navigational aids) are less than 45°. Since the ephemeris of each satellite is known by the GPS (global positioning system) receiver, it is possible to calculate which combination of GPS satellites provides the best geometry at a given time. This is translated into a figure called the position dilution of precision
(PDOP). Since the satellites move across the sky relative to the user, the PDOP is always changing. A low PDOP is better. A PDOP of 4 to 6 is considered good. Position solutions calculated when the PDOP is from 6 to 10 should be used cautiously because they may have significant error. A PDOP that is above 10 indicates unacceptable error. The ICAO (International Civil Aviation Organization) states that a PDOP of less than 6 is required for en route navigation, with a PDOP of 3 or less required for non-precision approaches [i.e., not ILS (instrument landing system) types]. The geometric dilution of precision
(GDOP) is the same as the PDOP, except that it includes a factor to account for any errors in the time.