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Hi Peter,

   Just stumbled across your message about this. I've been working on tools for a similar problem of using imaging of GPS satellites to allow astronomers to determine the errors in the timing of images :

https://www.projectpluto.com/gps_expl.htm

   The folks imaging asteroids generally have a decent idea as to where they are,  but getting the timing right on their images can be a challenge,  as described at the above URL.  Imaging navsats can be really helpful.

   I converted your positions to the format expected by the Minor Planet Center,  and fed it through under the assumption that your observations were made at the MPC code (Q62).  The cross-track errors were in the two to four arcsecond range;  along-track (the timing error),   -0.024 milliseconds to +0.101 seconds.  I could easily believe that these errors were due to the MPC's position for your observatory being off (or my assumption as to the observatory being (Q62) being wrong).  At 20000 km,  those cross-track errors would correspond to a few hundred meters.