jthul...@gmail.com <jthul...@gmail.com> wrote: > Hi, > the PPS signal comes from one GPS clock. > > At the cold start the drift could be several seconds, so we plan to perform a > ntpdate before launching the ntp client. > > The sync accuracy we want to achieve for this particular system is below 3ms > offset. > > After the ntpdate we assume that the offset can be several 100 ms, and we > would like to optimise the time to get close to the 3 ms offset. Therefore we > wonder if the use of PPS signal on the client could speed up the convergence > time in this case.
It depends on the particular GPS clock. The time obtained from a GPS clock via its serial interface is usually not within 3ms. For some receivers it is better than for others, but an error of 10ms or more is usually to be expected, for some it can be like 400ms. ntpd will first believe that this time is correct, attempt to sync to it, and after a while it starts believing the PPS and will lock to that. So at first you will see it steer the wrong direction, and it will remember this fact and have difficulty locking because it has calculated the wrong frequency offset during that interval. I think ntpd should not adjust variables like the frequency offset during the initial locking, but just use the stored ntp.drift and stay with that until it is back to a reliable reference. But what do I know? I'm only a longtime nptd user and not dr. Mills. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
