[throwaway4891a]

A side-effect of a 4+ satellite fix is both extremely accurate and extremely precise computation of current time (in addition to location). (GPS sats broadcast time, receivers triangulate.) Some high-quality receivers (Trimble, probably others) attempt to count the number of pseudowavelengths back to the satellite, including relativistic, gravimetric and atmospheric effects. http://www.trimble.com/gps_tutorial/sub_phases.aspx

[privong]

Thanks, but I was asking why heavy use of GPS in NTP pools was a bad thing, rather than how GPS can be used to determine a reference time (I'm aware of the use of GPS as a stratum 0 clock).