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GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L22608, doi:10.1029/2004GL021621, 2004 Modified sidereal filtering: Implications for high-rate GPS positioning Kyuhong Choi, Andria Bilich, Kristine M. Larson, and Penina Axelrad Department of Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, USA Received 27 September 2004; accepted 28 October 2004; published 30 November 2004. [1] High-rate (1-Hz) Global Positioning System (GPS) of the satellites, and the ability to model errors associated data are beginning to be used for a variety of geophysical with orbits, satellite and receiver clocks, atmospheric monitoring purposes, including seismology. Improving the delays, antenna effects, and multipath. It was noted by Bock precision of high-rate GPS position estimates will increase [1991] and Genrich and Bock [1992] that since some of the value of these 1-Hz GPS monitoring systems. One these errors are related to satellite-receiver geometry, one technique that has been used to improve high-rate GPS could take advantage of the fact that the GPS satellite period positioning takes advantage of the ground track repeat is designed to be exactly half a sidereal day to mitigate these period of the satellites. This study investigates the GPS errors. For a receiver on the Earth, this means that satellites orbital repeat period and determines that it varies for each visible in the sky today should be visible in the exact same satellite and differs significantly from the generally assumed location 23 h 56 m 4 s (the sidereal day) later. sidereal period. Orbit repeat periods are calculated and [4] The sidereal filtering concept has two steps and used to filter 1-Hz GPS position estimates. Using th