Re: NetBSD as an NTP stratum 1 server

[Greg Troxel <gdt%lexort.com@localhost>]

Manuel Bouyer <bouyer%antioche.eu.org@localhost> 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.