On Thu, Sep 11, 2014 at 07:37:10PM +0100, David Taylor wrote:
It has been pointed out to me that this page:
http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver28.html
says: The gpsd man page suggests setting minpoll and maxpoll to 4. That was
an attempt to reduce jitter. The SHM driver was fixed (ntp-4.2.5p138) to
collect data each second rather than once per polling interval so that
suggestion is no longer reasonable
So what should minpoll and maxpoll be set to for the GPSD shared memory
driver? Or should they be omitted? I'm confused
Hm, that paragraph doesn't make much sense to me either as the default
refclock poll is 6.
When collecting samples each second and processing them in the median
filter, the output has a lower jitter, so it's better to use a shorter
poll if the goal is to get the best accuracy, not longer.
On Linux when the GPS PPS signal has 1us jitter, poll 3 or 4 usually
works best for me.
FWIW, in clknetsim simulations with 1us jitter and 1ppb/s wander I get
these results:
pollRMS time error (s) RMS freq error (1)
3 6.0e-07 1.7e-08
4 1.5e-06 9.5e-09
5 4.3e-06 1.0e-08
6 1.1e-05 1.3e-08
With 10us jitter:
pollRMS time error (s) RMS freq error (1)
3 2.4e-06 1.6e-07
4 2.2e-06 6.7e-08
5 4.5e-06 2.7e-08
6 1.3e-05 1.7e-08
On other systems (using the standard PLL time constant shift) the best
poll would be even shorter.
--
Miroslav Lichvar
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