position dilution of precision


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position dilution of precision (PDOP)

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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.
References in periodicals archive ?
Wildlife GPS collars record position dilution of precision (PDOP), along with horizontal dilution of precision (HDOP), vertical dilution of precision (VDOP), time dilution of precision (TDOP), and whether or not the position is a three dimensional (3D) or two dimensional (2D) fix based on the number of observable satellites on which the position was calculated (Telonics 2009).
* Never log GPS data when Position Dilution of Precision (PDOP), a measure of satellite geometry, is greater than 7.