Manuel Bouyer <[email protected]> writes: >> If the concern is to keep time sync when the Internet is down, 1 ms of >> fuzz is ok. If you are trying to build something to distribute time to >> other people, and especially to be a public stratum 1, then it's not ok. > > Sure; my feeling (but I may be wrong) is that is this case the 1s NMEA > messages > may be good enough for NTP to sync, and the PPS may not bring much. > > I have a setup where I only use the NMEA message with gpsd and ntpd, no PPS. > ntpd has to problem to sync but I don't know how acurate it is (this host > is not connected to internet). For this use case 1s acuracy is good enough :)
If within 1s is good enough, NMEA only is ok. If you have a system that is well-connected and you set up NMEA you will find out that there are delays and that these vary by type of receiver. You use 'time1' in ntp.conf to adjust this. I have 3 devices that need values of 0.116 0.0445 0.072 to be close. That is very coarse compared to 1 ms for PPS. But indeed, if you have a machine that is not connected at all, it's really hard to tell it is a second off. So basically you might think NMEA only: 200 ms USB PPS: 1 ms gpio PPS: 100 us and to do better (or to know you are doing better) you need to really pay attention.
