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Re: [gpsd-users] Garmin 18X-5Hz

From: Miroslav Lichvar
Subject: Re: [gpsd-users] Garmin 18X-5Hz
Date: Fri, 12 Aug 2016 10:52:02 +0200
User-agent: Mutt/1.6.2 (2016-07-01)

On Thu, Aug 11, 2016 at 12:19:46PM -0700, Gary E. Miller wrote:
> OTOH, ntpd will not accept time corrections faster than every 8 seconds
> without a patch.  I find that every every 16 or 32 seconds gives better
> results.  Even longer may be even better, depending on local clock 
> stability.

Yes, the stability of the clock is the most important factor here. If
the machine is just sitting idle all day, longer update intervals may
work better, but when the temperature starts changing rapidly due to
CPU or other components with variable power consumption, even 8 second
update interval may be too long to keep the kernel PLL locked. In such
case giving ntpd more samples by increasing the PPS rate would be

Another issue with ntpd is that it reads from SHM only once per second
(and this can't be easily fixed), so in order to take advantage of a
higher rate PPS it needs to use some mechanism based on sockets or
pipes (maybe the GPSD JSON driver?).

Miroslav Lichvar

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