Yo Chris! On Sun, 12 Oct 2025 18:39:18 +0100 Chris Blackburn <[email protected]> wrote:
> I'm using a U-blox X20P GPS chipset connected to gpsd to provide
> location information to a local network.
Cool. Those are hard to get. What version gpsd? The gpsd in git head
has a lot of updates for the X20P that are not in any gpsd release yet.
Of special note, u-blox broke automatic configuration starting at
Gen10. So gpsd can no longer automatically configure your device for
best performance. Unless you have spent a lot of time with ubxtools
configuring your device, then it is configured sub-optimally.
The defaults are marginal. They are selected to minimize support
complaints.
> I
> notice that the estimated error is much higher than I expected.
All error estimates from all GNSS receivers are junk, and that includes
from u-blox. They never specify what they are measuring. There is a
big difference between CEP(99) over 24 hours, and CEP(50) and 6 minutes.
Error estimates from GNSS receivers are only useful as a short term
figure of merit.
The only way to know is to create a scatter plot using gpsprof using
12 or 24 hours of data. See: "man gpsprof", etc.
> It's
> also much higher than the value I'm getting out of the official
> U-center software form u-blox.
U-blox has a large number of ways reporting errors. They all differ
in precision and accuracy. If you find out where the numbers in
u-center are coming from, then you will see the same numbers with
gpsd.
> This is what I see from cgps
About what I see using RTCM3 on an F9P.
> However, the output below shows that the actual error being reported
> by the GPS is 14mm horizontal and 12mm vertical.
"Actual"? Did you read the doc? Is that CEP(1) or CEP(99)? Only
useful as a figure of merit.
> $ ubxtool -p NAV-PVT localhost:2947:/dev/ttyACM0o
I recommend that you use the "-v 2" option for a bit more verbosity.
Your first mistake. Do not use NAV-PVT! NAV-PVT does not support
the full precision of the X20P. Use NAV-HPOSLLH, and the other NAV-HP*
messages. You have a lot of doc to read.
gpsd handles this configuration for you on Gen8, But for Gen9 and Gen10
your are on your own.
> Basically, what I want to ask is: Is this expected behavior or is this
> something that's broken?
Yes. Embrace the power of "and".
> What I mean is that, if there's a good reason that the wrong number is
> being reported.
Define "wrong"? The doc specifies nothing, so about any number is not
"wrong".
> Or, is it that this is just something that's
> missing and the fields from the UBX message are just not used by the
> gpsd code,
Getting way ahead of yourself here. You have said nothing about how
your X20P is configired. Defaults? NMEA, Which version NMEA? UBX?
Which UBX messages.
If you have not spent a lot of time on your message configuration, and
it does not sound like you have, then PEBCAK.
> I'm happy to have a
> rummage around in the code and see if I can get it to use the correct
> figures.
Please do. The X20P is very new. You have the first one we have heard
of. Pretty much every thing needs to be double checked. Starting with
your configuration.
RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
[email protected] 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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