[ntp:questions] Questions about joining pool.ntp.org

unruh unruh at wormhole.physics.ubc.ca
Tue Aug 30 15:22:27 UTC 2011

On 2011-08-30, Chris Albertson <albertson.chris at gmail.com> 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

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