Algorithms that use the ionosphere to control GPS errors

The ionosphere, a major source of GPS pseudorange errors, can now be used to monitor and control other pseudorange errors in GPS two-frequency systems. These errors, which may originate in the receiver, pre-amp, satellite, or elsewhere, have been a continuing correction issue for precise GPS pseudorange measurement. Ionosphere-based correction is possible through use of self-calibration of pseudorange errors (SCORE) algorithms, which apply consistency constraints on the ionospheric measurements to calibrate an installed GPS two-frequency receiver system for non-ionospheric pseudorange error. SCORE delivers the sum, for each satellite, of all system components' contributions to pseudorange error. Current SCORE processing speed and projected upgrades should support near-real-time system and ionosphere error monitoring and correction for DGPS and WAAS. SCORE algorithms have also been modified to be used with single frequency GPS data, where they can give single frequency receivers true capability to monitor absolute ionospheric pseudorange error. If accurate enough, this capability may enable single frequency systems to aid WAAS ionospheric error correction.