On 2013-05-26, Miguel Gon?alves <[email protected]> wrote: > Hi Paul! > > On Fri, May 24, 2013 at 9:28 PM, Paul G <[email protected]> wrote: >> I have four GPS receivers into four different computers. I've assumed that I >> should be able to get small offsets between them but to do so I need >> to set time1. If that's the correct approach how do I pick the "right" >> system to be the benchmark. So far I've just been assuming that >> the purpose built M. Tharp server should have the lowest latency but I'm not >> sure -- in any case, at present, I can't adjust it. >> >> Each machine in the truncated table below (except 210) has a time1 >> value derived in an ad hoc fashion. >> >> l r refid st delay offset jitter >> ================================================== >> 1 o22.0 .PPS. 0 0.000 -0.001 0.001 >> 2 +244 .PPS. 1 0.068 0.000 0.006 >> 2 *210 .GPS. 1 0.514 -0.008 0.136 >> 2 +192 .PPS. 1 0.455 0.008 0.032 >> >> >> I feel as if this topic should be clearly explained somewhere but I've been >> unable to find it. > > My recent experience with 4 stratum 1 servers (one of them an > appliance from Meinberg - great box BTW) is that you will never have > every server in sync with each other due to the latency and delays of > the network (even LAN, same segment as is in this case). I don't depend > on external NTP servers.
Lan will have delays of the order of .15ms (worse for gigabit) with accuracy of the order of 10-20us. Direct PPS will be of order 2-5us, but only if you have only one. If you have more than one, then they get in the way of each other ( the interrupts all arrive at the same time, and each has to wait for th other to finish which is about 10us. ) > > My Meinberg NTP server: > > $ ntpq -p 10.0.2.1 > remote refid st t when poll reach delay offset jitter >============================================================================== > LOCAL(0) .LOCL. 10 l 8 16 377 0.000 0.000 0.001 > +GENERIC(0) .GPS. 0 l 13 16 377 0.000 0.000 0.001 > oPPS(0) .PPS. 0 l 7 16 377 0.000 -0.001 0.001 > -10.0.2.2 .PPS. 1 u 37 64 377 0.199 0.007 0.031 > -10.0.2.3 .PPS. 1 u 6 64 377 0.171 -0.004 0.008 > +10.0.2.4 .PPS. 1 u 51 64 377 0.157 -0.002 0.009 > > My Garmin 18 LVC on the roof: > > $ ntpq -p 10.0.2.2 > remote refid st t when poll reach delay offset jitter >============================================================================== > oGPS_NMEA(0) .PPS. 0 l 5 16 377 0.000 0.000 0.004 > +10.0.2.1 .PPS. 1 u 2 16 377 0.157 0.005 0.013 > +10.0.2.3 .PPS. 1 u 1 16 377 0.144 -0.001 0.004 > +10.0.2.4 .PPS. 1 u 16 16 377 0.140 -0.002 0.004 > > My Sure GPS board on the window sill: > > $ ntpq -p 10.0.2.3 > remote refid st t when poll reach delay offset jitter >============================================================================== > oGPS_NMEA(0) .PPS. 0 l 13 16 377 0.000 0.001 0.004 > +10.0.2.1 .PPS. 1 u 10 16 377 0.166 0.003 0.008 > +10.0.2.2 .PPS. 1 u 5 16 377 0.138 0.004 0.004 > +10.0.2.4 .PPS. 1 u 8 16 377 0.140 0.001 0.004 > > My 2nd Sure GPS board on the same window sill: > > $ ntpq -p 10.0.2.4 > remote refid st t when poll reach delay offset jitter >============================================================================== > oGPS_NMEA(0) .PPS. 0 l 7 16 377 0.000 0.000 0.004 > +10.0.2.1 .PPS. 1 u 4 16 377 0.201 0.000 0.011 > +10.0.2.2 .PPS. 1 u 3 16 377 0.136 0.002 0.004 > +10.0.2.3 .PPS. 1 u 2 16 377 0.140 0.001 0.004 > > For me < 10 us offsets is OK and acceptable between stratum 1 machines. > > I believe this is what is expected. > > Regards, > Miguel _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
