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

Chris Albertson albertson.chris at gmail.com
Tue Aug 30 07:15:28 UTC 2011

> 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
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


Chris Albertson
Redondo Beach, California

More information about the questions mailing list