Mailinglist Archive: opensuse (1420 mails)

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Re: [opensuse] openSUSE + gpsd + PPS + NTP
  • From: Per Jessen <per@xxxxxxxxxxxx>
  • Date: Tue, 14 Jan 2014 21:09:44 +0100
  • Message-id: <lb45ik$1ap$1@saturn.local.net>
John Andersen wrote:

On 1/14/2014 12:51 AM, Roger Oberholtzer wrote:
Hi

Is anyone using a GPS (gpsd) with the PPS signal as a time source for
NTP?

My main interest is how such systems are acting at system boot. We
have a mobile measurement system that syncs time using home brewed
software. We would like instead to use the standard tools.

Our main question is how the system acts if, at boot, there is no GPS
data. Is the time left alone? When the time does arrive, how quickly
will the system clock reflect the time? For example, it may turn out
that the PC clock and the GPS time differ by some hours. Does NTP
always do a gradual time shift? Can that be disabled? These systems
are booted a couple times a day.

Any other things to consider?

TIA for any experiences.


Your system should run using it's internal clock until there
is a NTP source, be that via GPS or over the net. (Your local
computer clock is set up as a stratum 10 time source, which means
it will be used as a last resort).

NTP will often NOT compensate for several hours of difference.

I think the limit is one hour.

This is by design, but, there may be a switch to over-ride it.

on ntpdate it used to "-g", I think.

Your GPS source should provide you UTC time.
(Well, US GPS sats provide GMT plus an offset to arrive at UTC, but
Glonas and Galileo only provide UTC).

Er, GMT is UTC+0, i.e. the same thing.

However, I've seen many references on the net about GPS being
a poor source, probably due to the delay in getting time
as you mentioned. It might be wise to not start NTP until
you have believable time from the GPS.

AFAIK, the main GPS advantage of DCF77 (et al) is getting the PPS
signal.


--
Per Jessen, Zürich (2.5°C)
http://www.hostsuisse.com/ - dedicated server rental in Switzerland.

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