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

[Paul Fertser]

Hey Gary,

On Sun, Jul 14, 2019 at 11:24:08AM -0700, Gary E. Miller wrote:
> That is not the policy of this list.  Or any other list I am on.
> Reply All has long been discouraged here.  Last time we stopped
> it was because of the echo storms it caused.

I am using "group reply" on this list and nobody complained so far :)
So we must be talking about different things here; I'm talking about
the feature of a MUA.

> > > No modern GPS needs assistance data.  That just speeds up the
> > > start, which for a new GPS should still be under 2 minutes.  
> > 
> > I would speculate assistance data can help a lot when the reception
> > conditions are too poor (EMI, physical obstacles) for reliable
> > reception of ephemeris from the sats but good enough to be getting
> > time signal from them. Am I wrong here?
> 
> Here we prefer data and experience to speculation.
> 
> A modern GPS will just listen to all GPS at once on startup.  If there
> is a strong enough signal then the ephemeris for that sat is downloaded
> in 90 seconds.

If we are talking about legacy GPS it should be 30s IIRC.

> If the signal is not strong enough, then the ephemeris is useless.

I thought the signal requirements for the C/A code to be used for
ranging are a bit lower than they're for demodulating LNAV from
it. And with L2C this should be even more evident with the CL
code. This is just a speculation on my part, indeed.

I think I've heard about receivers being able "to keep the fix" in
conditions where it was impossible to acquire it from a fresh
state. However, I'm not able to provide any real data indeed.

> So only GPS with only a few receive channels, mostly older ones, benefit
> from assistance data.

Yes, the number of correlators. Also they were benefiting from
remembering almanac and the last known date and position so they would
try decoding signals from sats that were supposed to be visible in
that particular moment and place. Good ol' times eh ;)

> This is why the more modern GPS no longer even support assistance data.

I remember ublox (ANTARIS 4) being able to get a fix within few
seconds when current ephemeris were known. It might have been
meaningful from power saving point of view. Not sure what exactly
changed since then (there already were enough correlators to
concurrently decode signal from all the birds flying).

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