| Re: [chrony-users] Setting up Chrony with PPS |
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William G. Unruh | Canadian Institute for| Tel: +1(604)822-3273
Physics&Astronomy | Advanced Research | Fax: +1(604)822-5324
UBC, Vancouver,BC | Program in Cosmology | unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 | and Gravity | www.theory.physics.ubc.ca/
On Mon, 11 Apr 2016, Deven Hickingbotham wrote:
On 4/10/2016 11:30 PM, Miroslav Lichvar wrote:
Hm, did the configuration of the GPS change? Maybe it's giving fewer
NMEA sentences than before? It looks like the NMEA samples are now
~400 ms off from the expected offset and PPS can't lock to them.
You could try changing the SHM offset to 0.1 and maybe increase the
delay to 0.6 or so, but I'm not sure if that will be enough if the
NMEA offset is changing so much over time.
Yes, but I didn't expect that it would affect chrony. The update rate
was increased from 1 per second to 10 time per second (since this didn't
affect the PPS signal, I didn't expect it to impact chrony).
??? chrony is using the nmea (ie the reported time from the GPS) to determine
what the seconds are for the PPS. If it does not trust the NMEA time, it
cannot figure out what the "second" time is for the PPS. (PPS ONLY determines
the "microseconds" part of the time, not the seconds part)
Changing the update rate back to 1 per second corrected the issue, but I
need an update rate of at least 5 per second (preferably 10).
Why? I think you are confused about something, but I am not sure what.
chrony disciplines the system clock so that it delivers the correct time to
within a few microseconds no matter when you query it. This is completely
indepndent of how often the GPS updates anything. In fact the GPS time is
completely ignored once chrony has settled down. It is far far far more
accurate than the GPS is. ( about 10000 times more accurate).
Would it be correct to assume that different update rates require
different chrony offsets and delays? And if so, what values should I try?
I think you need to tell us why you thing you want different update rates
first before this question can be answered.
If the gps delivered one update per day, (but had pps) the accuracy of your
system clock would not change one iota.
So please tell us what it is you are trying to do that makes you think you
want more frequency updates.
Thanks,
Deven
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