Re: [gpsd-dev] gpsd - AT command control on modems

[Gary E. Miller]

Yo Michael!

On Tue, 16 Jul 2019 08:53:49 +0100
"Michael J. Tubby B.Sc. MIET" <address@hidden> wrote:

> While it is true that several GPS receivers work without needing the 
> system Almanac, you have to be careful as strange things can happen 
> since the Almanac contains the SV Operational Health for each of the 
> SV/RPNs.

Yes, there are a few tidbits in the almanac not present elsewhere.
Sat health is not one of them as each sat broadcasts that in its
ephermeis.

> Remember that SVN23/PRN23 had a failure of its onboard atomic clock
> back in Feb 2016 and that any GPS receiver using it in its navigation 
> solution was off by around 13.7uS ... this threw out GPS receivers
> using it in the position solution by a mile or two.  We had hundreds
> of emergency services vehicles in the south-west of England in the
> wrong location for several hours until SVN23 was marked bad.

Yes, because the receiver failed to check the health status in the
ephmeris.

> While starting up without the system almanac is common place these
> days it is optimistic since it assumes that every SVN that you
> acquire lock to is behaving and there is a chance that one or more is
> not - there's also increased chance of trusting a spoofed signal... I
> know, I tried ... its amazing what you can do with a HackRF-One,
> Rubidium frequency standard borrowed from a friend and Linux GPSSIM
> software.

Before you can use a sat for a fix you need to get lock, then download
the ephemeris.  If the receiver forgets to check the health bit in
the ephermeis then you bought a crappy receiver.

Spoofing is a much more complex issue.  To handle that you need a
really new receiver that knows how to check for that.  If you look at
the u-blox AED doc they are very clear to say their spoofing detection
is not to be trusted.

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

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

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