On 8/29/2011 5:03 PM, Chris Albertson wrote: > On Mon, Aug 29, 2011 at 12:43 PM, NPG <[email protected]> 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. -- Sincerely, Nathan Gibbs Systems Administrator Christ Media http://www.cmpublishers.com _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
