On Mon, Aug 29, 2011 at 12:43 PM, NPG <[email protected]> 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: http://www.synergy-gps.com/images/stories/pdf/m12mt_brochure.pdf 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. Chris Albertson Redondo Beach, California _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
