On 2010-01-05, John Hasler <jhas...@newsguy.com> wrote: > Bill Unruh writes: >> However, the only way you can really know is to get a GPS with PPS, >> hook it up to your machine and measure the difference in time between >> the Stratum 3 set clock and the GPS time. > > I think that what he wants is the "expected time" (he has that: the time > Ntpd reports) and a 95% confidence interval for it. That is, he wants > to be able to say "The time is within +- 4.2usec of 2300UTC with 95% > confidence".
Unfortunately he cannot get that. the random part he can get (jitter) but the systematic part he cannot. The dispersion is a very very conservative estimate of the systematic part, but it is in almost all situations far far larger than the true 95% confidence interval. As I said the only way to estimate the systematics is by having a local gps pps clock to compare it with, but that is obviously overkill, since then you would just use the gps to give the time. for example, I have a machine with a gps pps and with another source being a stratum 1 server 2000km away. The round trip time is 45ms, which would give a "dispersion" component of 22.5ms. But actually the offset of that source from the local gps time is only about +-.1ms.Thus the estimate -- 1/2 the roundtrip -- of the systematic error is out by about a factor of 200. Thus the estimate -- 1/2 the roundtrip -- of the systematic error is out by about a factor of 200. As I said, it is possible that all outgoing ntp requests go via a 1Gb ethernet, and all return packets go via a 300baud modem. In that case the estimate of 1/2 the round trip would be a good estimate of the systematic error. Unfortunately there is absolutely nothing ntp can do on its own to figure that out. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions