Mailinglist Archive: opensuse (1420 mails)

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Re: [opensuse] openSUSE + gpsd + PPS + NTP
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.

This is by design, but, there may be a switch to over-ride it.
When it does get a connection to NTP, even after weeks of being
disconnected or turned off, it will sync immediately, and adjust
time all at once, provide the clocks are reasonably close.
When I say immediately, I mean as soon as NTP elects a clock
from the list you provide to it.

I don't recall any gradual time shift, but I haven't read the man
page in a while.

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).

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.



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