[ntp:questions] ntpd - gpsd communication

Steven Sommars stevesommarsntp at gmail.com
Mon Jun 8 23:48:02 UTC 2020


Gary,

Sounds sort of plausible.  However if the GPS doesn't have the almanac, how
can it get a fix?    If this is a known issue, shouldn't the GPS delay
sending validity="A" or otherwise signal that the UTC offset may be
incorrect?

This should be reproducible, correct?  What kind of reset will cause the
GPS to lose the almanac information?  Many Rpi boards have batteries or
supercaps for power backup.  khronos.mikieboy.net is a Rpi+Uputronics board

Steve


On Mon, Jun 8, 2020 at 5:25 PM Gary E. Miller <gem at rellim.com> wrote:

> Yo Steven!
>
> On Mon, 8 Jun 2020 17:06:55 -0500
> Steven Sommars <stevesommarsntp at gmail.com> wrote:
>
> > The off by N-second errors are often seen soon after NTP server
> > initialization.
>
> This is common to all GPS based NTP servers.  It is distinct from
> long lasting off-by one errors that one version of gpsd could fall
> victimm to.
>
> When a GPS starts up, it often uses the value for the current leap
> second stored in the firmware.  By "leap second" I mean the correction
> from GPS tim to UTC time which is always in integer seconds.
>
> This stored leap second is often off by one or two seconds, or more,
> for a while.
>
> After receiving the Almanac, which can take up to 25 minutes on some
> older GPS, the current leap second offset is known.  But now ntpd is
> set in its ways, and will take a while to slew to the correct time.
>
> RGDS
> GARY
> ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
>         gem at rellim.com  Tel:+1 541 382 8588
>
>             Veritas liberabit vos. -- Quid est veritas?
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