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

From: Greg Troxel
Subject: Re: [gpsd-dev] RFC: GPSd on Android as a system service
Date: Fri, 03 May 2019 07:12:02 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (berkeley-unix)

"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.

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.

> In almost all cases the GPS either has both Almanac and Ephemeris at the
> same.  A few cases of startup have the Almanac a few cycles after the
> Ephemeris, not worth trying to figure out that narrow window.  Just set
> both if you have elevation and azimuth.

Agreed that almanac is not worth worrying about, as usually it is kept
and around later.

> 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.

But maybe I'm missing something.

> ---------------------------------------------------------------------------
> 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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