Re: [gpsd-dev] RFC: GPSd on Android as a system service

[Gary E. Miller]

Yo Greg!

On Fri, 03 May 2019 07:12:02 -0400
Greg Troxel <address@hidden> wrote:

> "Gary E. Miller" <address@hidden> writes:
> 
> > You have an interesting heuristic.  I looked at the u-blox
> > regressions with ubxtool.  Once you have elevation and azimuth, you
> > have the Ephemeris.
> >
> > Makes sense because you computer elevation and azimuth from your
> > position and the Ephmeris data.  
> 
> I don't follow this, because the almanac exists to let you compute
> elevation and azimuth roughly, so you can know what's above the
> horizon and guess at the doppler shift.

Yes.  In the old days when a GPS had only 4 or 8 receive channels, the
Almanac data was needed to get close enough to lock on.  Getting the
Almanac required guessing which of 32 channels might be in sight until
you locked on one to get the Almanac.

With a modern 70+ channel GPS, the GPS just tries to listen on all
signals at once.  6 seconds later it has all the Ephemeris it needs
to start.

> I have a new device ("VFAN", with ublox M8030), and I was seeing
> satellites listed with az/el, el near 0, 0 C/N, and apparently not
> tracked.   I suspect those have almanac and not ephemeris.

Why guess?  ubxtool tells you.  Let us not make decisions on guesses.

# ubxtool
[...]
UBX-NAV-SAT:
  iTOW 501050000 ms, version 1 numSvs 41
   gnssd 0 svid   1 cno 42 elev  22 azim  83 prRes    -10 flags 0x191f
    qualityInd 7 svUsed health 1 orbitSource 1 ephAvail almAvail 
   gnssd 0 svid   7 cno 45 elev  17 azim 123 prRes     12 flags 0x91f
    qualityInd 7 svUsed health 1 orbitSource 1 ephAvail 
   gnssd 0 svid   8 cno 24 elev   3 azim  39 prRes      0 flags 0x1914
    qualityInd 4 health 1 orbitSource 1 ephAvail almAvail 
   gnssd 0 svid  11 cno 46 elev  31 azim  66 prRes     89 flags 0x191f
    qualityInd 7 svUsed health 1 orbitSource 1 ephAvail almAvail 
   gnssd 0 svid  13 cno 49 elev  52 azim 265 prRes     85 flags 0x191f
    qualityInd 7 svUsed health 1 orbitSource 1 ephAvail almAvail 
   gnssd 0 svid  15 cno 38 elev  29 azim 300 prRes     15 flags 0x91f
    qualityInd 7 svUsed health 1 orbitSource 1 ephAvail 
   gnssd 0 svid  17 cno 51 elev  53 azim 182 prRes    -29 flags 0x191f
    qualityInd 7 svUsed health 1 orbitSource 1 ephAvail almAvail 
   gnssd 0 svid  18 cno 43 elev  18 azim  56 prRes     95 flags 0x191f
    qualityInd 7 svUsed health 1 orbitSource 1 ephAvail almAvail 
[...]


> > Or just set them on all the time as gpsd does not put sats with no
> > elevation and azimuth in the JSON.  
> 
> I think it's useful to know the locations the satellites that aren't
> being tracked.

And, in fact, gpsd does pass that data through, if the GPS provides
it.  What gpsd does not do is pass on the ephAvail and almAvail
since they provide no useful info to the user and only a few GPS
provide it.

> But maybe I'm missing something.

We all are.  :-)

RGDS
GARY
---------------------------------------------------------------------------
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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