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

From: Gary E. Miller
Subject: Re: [gpsd-dev] gpsd - AT command control on modems
Date: Tue, 16 Jul 2019 11:17:14 -0700

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

> 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

> 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.

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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