valizade...@gmail.com wrote: > i have a ntp server based on Raspberry Pi3 with PPS (from u-blox6 ). i have > also added a hardware clock to this system. > > > SOME times there is a large offset(3-18 sec) on the SHM (ntp driver 28). > i did some experiments and figured out it is related to "leap seconds". > when i COLD-RESET the ublox(via USB through u-center) this offset appears on > the ntp and both PPS and SHM become false-tickers.
GPS internal system time is off UTC by an integral number of seconds corresponding to the number of leap seconds that have been observed since 1980. As Jakob has already pointed out, the satellites send this information once every ~12 minutes. The GPS receiver needs this information to convert the raw GPS time received from the satellites to UTC, so it usually store this information in its non-volatile memory to have it available right after power-up. However, if you force the GPS receiver into cold boot mode then this information gets lost, so the receiver outputs GPS time instead of UTC. I'm not sure if the output contains some status that indicates that the time sent by the receiver is not UTC. Anyway, gpsd seems to accept the raw GPS time as UTC time, feeds it to ntpd, and ntpd sets the Linux system time off by a number of seconds. Up to 12 minutes later, after the GPS receiver has seen the UTC correction parameters from the satellites, the GPS receiver starts to convert GPS time to true UTC, and the time output jumps by a number of seconds. ntpd observes the sudden time step by a number of seconds, but accepts it only after it has persisted by a certain interval, by default ~15 minutes. So your system time should be stepped to the correct time, but only after up to 12 + 15 minutes after power-up. I think the best solution would be to provide your GPS receiver with a backup battery, if the device supports this, to make sure the UTC correction parameters don't get lost while the system is powered off. Martin -- Martin Burnicki Meinberg Funkuhren Bad Pyrmont Germany _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
