This code calculates the average orbit repeat period (or shift time) for each GPS satellite using the broadcast ephemeris. This was first described in the context of modified sidereal filtering by Choi et al. [2004]. It reads the navigation message in RINEX format from standard input and writes to standard output. The first column of output is the PRN number and the second column is the repeat time (in seconds) relative to one day.
In general, the farther you are from the ideal orbit repeat time (one day minus 245), the more this repeat time varies (+- 2 sec) within a given ground track. For satellites very far from the ideal, the ground track is not well repeated and should not be used for modified sidereal filtering.
For more discussion of the average shift time and the best repeating ground track shift time, feel free to pick up this paper. Code and a help file are available at the GPS Tool Box, specifically this site
Acknowledgements: Duncan Agnew, Penny Axelrad, Andria Bilich, Kyuhong Choi, and Yusaku Ohta.