On 2011-08-30, Chris Albertson <[email protected]> wrote: >> The time stuff I understand, even the radio wave propogation stuff, >> though obviously not as well as others here. >> I'm not sure what "cable length", "speed of light delay", or "velocity >> factor" is all about. >> > > What I was getting at was that that better GPS receivers are so accurate > with their pulse per second that you have to carefully account for the delay > in a short cable. In other words the delay due to speed of light down a > wire that is four feet long is greater then the error in the PPS. I
Well, no. That is just 4ns, and the jitter in the gps pps of most receivers is more than that. (and the ability to get that signal into a computer in order to affect the timing of that computer is much much longer than that-- of the order of 1us) > brought it up to show how insanely small the timing error is. Then > "velocity factor" is a correction the signal does not move a speed of light > in a real wire. > > None of this matters for NTP. But some people use GPS for other purposes, > like maintaining a frequency standard. or running cell phone towers. NTP is > actually one of the least demanding uses of GPS timing signals > _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
