Re: Help needed with USB GPS device

[Gary E. Miller]

Yo Gavin!

Replies to the list please.  Private email is for paying
clients.

On Fri, 1 Mar 2024 09:31:02 +0000
Gavin Davenport <gavdav@gavdav.net> wrote:

> >> It helps if you tell us what model GPS you have.    
> I'm not sure I can accurately answer this.

The gpsdebuginfo might show us.

> >>So before you look at chrony, you need to have a good lat/lon from
> >>gpsd.  
> According to cgps I do have a good lat/long. 

A good start.  So I assume also good time.

> I had not seen any links to gpsdebuginfo.

No links needed, gpsdebuginfo is part of gpsd, so you already had it. But
the linked one is the latest one.

> + gpsd -V
> gpsd: 3.25 (revision 3.25)

Good, the latest release.  Almost time for a new release.

> # Magic Hat enabled.

Are you on a Raspberry PI?  If not, this should be off.

> + ps ax
>     PID TTY      STAT   TIME COMMAND
>  60205 ?        S      0:04 /usr/sbin/chronyd -F 2 106673 ?
> 106673 ?        S<sl 125:49 /usr/sbin/gpsd -n /dev/ttyUSB0 /dev/pps0

And there is your chronyd and gpsd.  And the first problem.  Unless
you are on a Raspberry Pi, or similar, you shold not have a separate
/dev/pps0.

If PPS is available from the ttyUSB0 it is read from mttyUSB0.

> {"class":"DEVICES","devices":[{"class":"DEVICE","path":"/dev/ttyUSB0",
>  "driver":"SiRF","subtype":"GSW3.5.0_3.5.00.00-SDK-3EP2.01

And there is the answer to your GPS type.  It is s SiRF-III.  A tad
old, not so good indoors.  My condolences.  SiRF was bought out, and
disappearing from the market.

> + ntpshmmon -n 6 -t 10
> ntpshmmon: version 3.25

You are getting basic time, no PPS, from the device:

> NTP0 1709284857.228735488 1709284856.591468173 1709284856.570000000 0  -1

Not good enough to be a good NTP server.

> + ls -l /dev/pps0 /dev/pps1 /dev/pps2 /dev/pps3 /dev/pps4

A weird number of /dev/ppsX devices.

> + lsusb
> Bus 001 Device 010: ID 067b:2303 Prolific Technology, Inc. PL2303
> Serial Port / Mobile Phone Data Cable

Not a real USB GPS, using a Serial/USB chip.  That can be a pain.

> + cat /etc/sysconfig/gpsd
> # Options for gpsd, including serial devices
> OPTIONS=""
> # Set to 'true' to add USB devices automatically via udev
> USBAUTO="false"
> DEVICES="/dev/ttyUSB0 /dev/pps0"
                        ^^^^^^^^^^  Wrong, remove it.

You will never get PPS out of that device.

> I didn't know USB GPS devices tended not to have PPS.

And now you do.

> This is as much to do with understanding a bit more about GPS and how
> it gets into ntp/chrony, so yes, a few 100msecs accuracy is fine. I
> can look at alternatives if I need anything better.

If no PPS is good, then you just need to figure out if gpsd is talking to
chronyd.  Another email from another list user had things to check on that
part.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  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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