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

Chris Albertson albertson.chris at gmail.com
Tue Aug 30 22:18:11 UTC 2011

On Tue, Aug 30, 2011 at 8:22 AM, unruh <unruh at wormhole.physics.ubc.ca>wrote:

> 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)

Some low cost GPSes really are that good, better than 4nS.  For example this
one sells for about $60

It's overkill for an NTP server  but there are other uses of GPS that can
make use of  nanosecond level error.  Mostly the accurate PPS lets you
calibrate test equipment or a local frequency standard.

Yes you can't get this into a computer without introducing a large error but
people doing work at this level are using specialized hardware.  Maybe with
something like this

Chris Albertson
Redondo Beach, California

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