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Re: xgps and cgps are flickering

From: Gary E. Miller
Subject: Re: xgps and cgps are flickering
Date: Thu, 28 Jul 2022 13:01:59 -0700


On Thu, 28 Jul 2022 21:14:18 +0200 wrote:

> environment
> hw: raspberry pi4, Ublox ZED-F9P Expansion from uputronics
> sw: Debian bullseye, gpsd: 3.24.1~dev (revision 
> release-3.24-211-gfda79f2b8)

Looks good.

> When I use my 6 year old gps-hat I learned to have one update per
> second using xgpd or cgps

Yeah, those older receivers could not fit a lot of data in 9600 bps.

> With the new ZED-F9P there are 2 updates per second but not even
> this, it shows different information.
> One view shows about 76 seen satellites and 36 used the other view
> shows about 44 satellites and 16 used

Yup.  NMEA is showing its age.  You are likely getting NMEA data, and
u-blox binary, in every epoch.  They can't agree because the NMEA does
not understand newwer multi-frequency receivers.

To make it worse, older NMEA limits GPGSV to just 3 messages...

> It's obviously it has something to do with the GPGSV sentence. There
> are two blocks within one second. The first one has 11 messages the
> other has 19 messages. Of course this varies from time to time.

Yup.  That is NMEA trying to be back compatible, yet handle, sort of,
newer stuff.  One set of GPGSV is L2, and the other L1.  So the concept of
a "seen" satellite is insufficient to describe the situation.

Best to just turn off the NMEA and go u-blox only.

> The interesting part is that some satellites are in both blocks, for 
> example 02 or 05. Sat 05 is even twice in the same block only SNR is 
> different.

Yup.  Look at the last GPGSV field, just before the checksum.  It is the
new "GNSS Signal ID".

NMEA tells you how many lines will be in which GPGSV set, but not
how many GSV sets there will be.  So gpsd has to output them as it
gets them.  Garbage in, garbage out.

I'd like to describe it from the standard, but the standard is closed and
unavailable to us.  A little more info here:

And here:

> Is there a way to switch off this permanently switching between two 
> views ?  Maybe a hidden option or key combination ?

You could try to get NMEA to fix their "standard".  Or switch to
u-blox binary.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703  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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