[ntp:questions] Help: fudge time2 value for NMEA driver

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Thu Nov 3 07:03:18 UTC 2016

On 2016-11-02 17:51, Paul wrote:
> On Sun, Oct 30, 2016 at 1:29 PM, ogre up <orgeup at gmail.com> wrote:
>> Hello everyone, I've setup a NMEA+PPS ntp server, but both ref clock have
>> strange offset value reported by ntpq -p.
> Your billboard is fine.  If you want less jitter in your NMEA sentences you
> need to buy better hardware.  Not necessarily more expensive but better.
> However it doesn't matter as long as you stay within a few hundred
> milliseconds of the correct second since the timing is done with PPS and
> the GPS output is only used to number the seconds.  Unfortunately I have no
> experience with your PPS problem.
> I prefer using ATOM (PPS) + a GPS specific driver.  In the past I've been
> unhappy using the PPS option in the NMEA driver so I don't bother.
> Output from a low jitter NMEA compatible GPSDO:
>      remote           refid      st t when poll reach   delay   offset
> jitter
> ==============================================================================
> o127.127.22.0    .GPPS.           0 l    2    8  377    0.000    0.000
> 0.002
> *    .FURY.           0 l    1    8  377    0.000   -0.003
> 0.011

The uBlox LEA-6T is probably as good as the older M12+ used in the JLT Fury,
and has been used for DIY GPS-DOs.
With proper GPS antenna, receiver, and system configuration, the results
achievable should be comparable, limited by system and NTP performance,
compared to the standalone Fury GPS-DO [TO]CXO which will not be affected
by temperature.

Follow the instructions for timing setup on a uBlox LEA-6T to get the best
results; see sections 10 Time Mode Configuration and 11 Timepulse in:

You may want to do the setup in uBlox mode at higher speed, especially if
you load ALP data from Ublox, before switching to 9600 bps and NMEA output.

Once the setup is done properly, it would be worth comparing the performance
using the NMEA driver with kernel PPS against the separate PPS/ATOM driver.

Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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