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

NPG nathan at cmpublishers.com
Mon Aug 29 22:30:45 UTC 2011

On 8/29/2011 5:03 PM, Chris Albertson wrote:
> On Mon, Aug 29, 2011 at 12:43 PM, NPG <nathan at cmpublishers.com> wrote:
>> Stupid noob question alert.
>> Does GPS provide better accuracy?
> Depending on which GPS receiver, GPS can be MANY orders of magnitude better.
>   The best GPSes have a one sigma error on the pulse per second that is on
> the order of 5 nanoseconds or less.  At this level we need carfully measure
> cable lengths and allow for the speed of light delay and "velocity factor"
> of the cable.  We are talking about literally 10,000  or even a million
> times better, literally.

LOL, I only vaguely understand what you are talking about.

> That said if the GPS is only being used to drive NTP we don't need
> nanosecond level accuracy.    

OK, I guess I don't necessarily need to understand the above.

> An NTP server set up well using GPS can keep
> time to within about 1uS or 2uS.  A GPS with a 100ns error is good enough.
> WWV on the other hand is not so good.  The problem is you don't know
> the propagation delay.  The WWV signal has to bounce off the ionosphere at
> least once to reach you and you don't know the effective height of
> the ionosphere.   Worse, the signal will take multiple paths and what you
> get is a mixture,  So you get a "smeared" time signal.  

Now that I understand perfectly.
Its all about propagation or the lack thereof.
Which can be affected by many environmental variables.
That probably explains why at least once a year one of my WWVB driven
wall clocks loses its mind and starts displaying ridiculous time.

> Radio waves travel
> 1000 feet in one microsecond and the uncertainty in your patch length in
> measured in miles not feet.   In other words WWV is ok if you care about
> milliseconds but not if you care about micro or nano seconds.
> I'm (very slowly) working on a project at home to compare WWV and GPS.  The
> purpose is to measure the ionosphere.  "Lag" in the WWV signal can tell you
> about radio propagation.

That sounds like a cool project.
Thanks for the info.


Nathan Gibbs

Systems Administrator
Christ Media

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