Modified sidereal filtering: Implications for high-rate GPS positioning

[ 1] High-rate (1-Hz) Global Positioning System (GPS) data are beginning to be used for a variety of geophysical monitoring purposes, including seismology. Improving the precision of high-rate GPS position estimates will increase the value of these 1-Hz GPS monitoring systems. One technique that has been used to improve high-rate GPS positioning takes advantage of the ground track repeat period of the satellites. This study investigates the GPS orbital repeat period and determines that it varies for each satellite and differs significantly from the generally assumed sidereal period. Orbit repeat periods are calculated and used to filter 1-Hz GPS position estimates. Using the calculated orbit repeat period significantly reduces low frequency (0.001-0.04 Hz) errors in 1-Hz GPS position estimates.