[ntp:questions] ntpd - gpsd communication

Gary E. Miller gem at rellim.com
Tue Jun 9 01:19:32 UTC 2020


Yo Steven!

On Mon, 8 Jun 2020 18:48:02 -0500
Steven Sommars <stevesommarsntp at gmail.com> wrote:

> Sounds sort of plausible.  However if the GPS doesn't have the
> almanac, how can it get a fix?

A GPS never uses the Almanac to compute a fix.  A GPS uses the
Ephemeris to compute a fix.  Each satellite sends its own
Ephemeris every 30 seconds.  The Almanac is a reduced precision
ephemeris only used to decide what sats should be in use.

If you have a receiver that can receive many satellites at once then
the Almanc is not needed at atll.  But there are other things int
subframe messages of use.

> If this is a known issue, shouldn't
> the GPS delay sending validity="A" or otherwise signal that the UTC
> offset may be incorrect?

Most do not.

> This should be reproducible, correct?

Yes, depending on the receiver model, very repeatable.

> What kind of reset will cause
> the GPS to lose the almanac information?

Depends on the receiver type, and if/where is stores the current GPS-UTC
offset.  Some GPS have no power down storage, some have battery backed
ram (BBR) and some have FLASH.


> Many Rpi boards have
> batteries or supercaps for power backup.

Yup, but if the receiver is powered down long enough, the stored
GPS-UTC offset will become out of data.

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?
    "If you can't measure it, you can't improve it." - Lord Kelvin
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