Re: [gpsd-dev] Off by a second

[Gary E. Miller]

Yo Hal!

On Wed, 01 Jul 2015 14:16:27 -0700
Hal Murray <address@hidden> wrote:

> 
> address@hidden said:
> > A power cycle will not help.  The GPS needs to download a new
> > almanac with the new leap second data.  AFAIK it is not in the
> > ephemeris. 
> 
> It's working now.

What I expected.

> Your almanac suggestion may be the key.  I didn't add the battery to
> this board so I could look for startup quirks from a really cold
> start.

My guess is that all your cold starts will be off one second until your
next firmware update.

> There is an interesting tangle with cold starts.  Relative to the
> marketing speak of startup times, it takes a long long time to
> download an almanac. Can a unit provide time before that?

Well, yes, and no.  Each sat transmits its GPS time in each of its 5
300 bit subframes.  At 50 bps that means the GPS can get its time
to 'rough' accuracy in 6 to 12 seconds.  This would depend on the GPS
having a lock on a SAT, which is a given on a 30 channel receiver, and
when in the subframe it starts listening.

For full accuracy the GPS would need subframes 1,  2 and 3, which are each
transmitted every 30 seconds.  So an ideal time to gather the ephemeris
cold is 18 to 48 seconds, depenindg on when in the cycle listening starts.

Except for the offset of GPS time and UTC time.  This is only in page
18 of subframe 4.  Which is only transmitted once every 12.5 minutes.  
Ideally that page would be received in 6 seconds to 25 minutes.


>  My guess
> is they compiled in a default GPS-UTC offset.  This is a reasonably
> new chip.  It might be newer than 2012 so a then current default
> would be correct until a few days ago.

Yup.

> I've got lots of log files.  It will take me a while to dig out the
> right data.  More later.

Cool, let us know.

I expect a few cold start tests will be illuminating.

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

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