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Re: [gpsd-users] About date and time validity

From: Gary E. Miller
Subject: Re: [gpsd-users] About date and time validity
Date: Wed, 12 Dec 2018 12:55:35 -0800

Yo Massimiliano!

On Wed, 12 Dec 2018 10:28:48 +0100
Massimiliano Fago <address@hidden> wrote:

> Unfortunately, my system can not always stay on. It is turned on the
> morning of the race, maybe a few hours before the start of the
> competition for this I wanted to know the "state of the time", to
> understand if between the various points of passage the time is equal
> at least to ms.

A few hours warm up should be more than enough for millisecond accuracy.
The crystal drift correction will be off a bit, partly due to lack
of recent disiplining and partly due to thermal effects.  But should be
well under a milliseecond.  Keep an eye on the jitter to quantify
your errors.

Running ntpviz will allow you to watch ntpd tame the system clock as it
warms up.

> I use this model of GPS: VK2828U7G5LF GPS. Is it a good choice?

An interesting part for around $10.  It has a TCXO which helps accuracy,
but the clock on your CPU is probably not as good as a TXCO.

It looks to be based on a u-blox UBX-G7020-KT.  Pretty good, not quite
as precise as a generation 8 chip.  It is rated to cold-start in only 30
seconds.  CEP(?) of 2 meters.  Much more accurate in time than whatever
CPU clock you have.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin

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