[time-nuts] GPS frequency uncertainty

[Perry Sandeen]

Yo Bubba Dudes!
In a number of fascinating and highly educational for me were the explanation of accuracy by TVB and others.  I learned quite a lot, thank you.
Bob mentioned about slight orbital variations in the GPS satellites.
IIRC those slight variations meant that you could only reliably get about 1x10^12.accuracy.
Would someone please expand on what the practical frequency calibration limit might be?
Regards,
Perrier