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

From: Robin Schwab
Subject: Re: [gpsd-users] Garmin 18X-5Hz
Date: Mon, 15 Aug 2016 13:22:06 +0200
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

Hi Gary

Am 14.08.2016 um 23:14 schrieb Gary E. Miller:
No, I mean hard fail.  Like loss of signal, no PPS output, so ntpd goes
free running.  Or worse yet, just a wild hare output., like 14 Aug about
16:00 UTC here:

Tht one happens every few weeks, driving me crazy to find and fix.

I think you have already seen how inaccurate a GPS can be with poor signal i.e. when you come out of a tunnel. There is a huge difference in precision between any fix and a good fix.

NMEA Dilution Of Precision (DOP) values are available so I see no point in not using them. Ublox modules send even a specific Time DOP signal (PUBX,00,…). When you have a bad fix in a stream of good fixes the bad fix should get discarded. Maybe just a bird sitting on your antenna :-)

Would be interesting to see if that solves your problem.

chronyd was designed to handle loss of refclocks.  The authors
considered intermittent uptime and intermittent fixes.  It has
techniques to adjust for them and converges fast.

ntpd, by design, is for constant on operation with multiple inputs.  It
converges very badly and slowly on startup, it uses voting to select
averages of the best times.

If you are only going to have one input you should be using chronyd.

I'll consider using chrony. I do see the point of NTPd providing a robust networks of clocks that are maybe 1s accurate for login purposes.

But if you have a GPS clock that is <1us accurate (it has to be because otherwise the position would be wrong) I do not see the advantage of correcting that time with a network peer clock that is >100 ms away. You can use those network clocks for statistics and cross-checking but basically you have to trust your own GPS clock. Of course ntpd accounts for the latency over the network but since the latency is big and varies the correction can't be that good.


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