Re: Slow convergence of NTP with GPS/PPS - NTP

This is a discussion on Re: Slow convergence of NTP with GPS/PPS - NTP ; An Update from what we have decided to do. As NTP is designed to achieve accuracy slowly and conservatively. We have decided not to go down the NTP route. We have decided to use the PPS signal from the GPS ...

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Thread: Re: Slow convergence of NTP with GPS/PPS

  1. Re: Slow convergence of NTP with GPS/PPS

    An Update from what we have decided to do.

    As NTP is designed to achieve accuracy slowly and conservatively. We
    have decided not to go down the NTP route.

    We have decided to use the PPS signal from the GPS and force the system
    time into being correct.

    It is a great shame the NTP doesn't achieve a high accuracy quickly, as
    the method we have adopted has a low level of accuracy compared to NTP

    I am posting this mainly to help Nicola, with a solution.

    Many Thanks for all your help

    Best Regards

    David

  2. Re: Slow convergence of NTP with GPS/PPS

    davida@pipstechnology.co.uk (David Ashmore) writes:

    >An Update from what we have decided to do.


    >As NTP is designed to achieve accuracy slowly and conservatively. We
    >have decided not to go down the NTP route.


    >We have decided to use the PPS signal from the GPS and force the system
    >time into being correct.


    I have no idea what this means. doyou mean that you reset the clock each
    time it receives a PPS pulse? Ie, stepping the clock each time? That can be
    dangerous. a) backward steps are possible confusing the filesystem.
    b) GPS PPPS lines can get affected by noise. My gps PPS line every once in
    a while will put out up to 10 interrupts in a second-- presumably some
    weird noise on the system. Your system would be resetting the clock like
    mad, creating a totally inaccurate time during that second. And if you
    assumed that the time advanced one second on each interrupt you would be
    out 10 seconds.

    Ie, you have to sanity check the PPS to make sure that what you think is
    correct, really IS correct.


    >It is a great shame the NTP doesn't achieve a high accuracy quickly, as
    >the method we have adopted has a low level of accuracy compared to NTP


    >I am posting this mainly to help Nicola, with a solution.


    >Many Thanks for all your help


    >Best Regards


    >David


  3. Re: Slow convergence of NTP with GPS/PPS


    >Ie, you have to sanity check the PPS to make sure that what you think is
    >correct, really IS correct.


    It's even worse than that. When the PPS ticks, you know you
    are on a second boundary, but you don't know which second.
    You have to get the second from someplace else, perhaps by
    parsing the NMEA messages or whatever your GPS device uses.

    --
    These are my opinions, not necessarily my employer's. I hate spam.


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