[ntp:questions] Questions about joining pool.ntp.org
albertson.chris at gmail.com
Mon Aug 29 21:03:38 UTC 2011
On Mon, Aug 29, 2011 at 12:43 PM, NPG <nathan at cmpublishers.com> wrote:
> > Get a Sure gps receiver for $50 do a bit
> > of wiring, and get far greater accuracy than from WWV
> 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.
That said if the GPS is only being used to drive NTP we don't need
nanosecond level accuracy. 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. 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.
The most accurate affordable GPS receiver today is this one:
It sells for about $60.
You can buy older version of the Oncore MT+ and UT+ for eBay for $30 to $20.
I paid $18 for a UT+. It has an error of about 55 nS (one sigma)
The Sure unit is OK but lacks the documentation of the Motorola designed UT
and MT receivers.
Redondo Beach, California
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