Selecting the best four global positioning satellites based on dilution of precision and clock performance.
详细信息
文摘
Most locations on earth have six or seven satellites in view all the time and the receiver chooses the best four satellites to make its pseudorange calculations by only considering the user Space Vehicle (SV) geometry. What is the impact if a few vehicles have larger User Range Accuracy (URA) values than nominal SVs? The URA is a statistical indicator of the ranging accuracies that are obtainable for a specific SV. The URA is the one-sigma estimate of the user range errors in the navigation data for a transmitting satellite. It includes all errors for which the Space and Control Segments are responsible for,but a major error source is clock error. The current constellation of SVs have very low User Range Errors (URE),but it may only take one or two SVs with URAs two or three times worse than the average to effectively act like the constellation has a satellite out. The URE is an estimate of "Signals in Space" errors,i.e.,ephemeris data,satellite clocks,ionospheric delay and tropospheric delay. [IS200] Usually these errors can be greatly reduced by differential and multiple frequency techniques. Users may be able to de-weight SVs with poor performing clocks so that their absence wouldn't affect the user position calculation. [Parkinson] [Pisacane].