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Re: Glitching GPS feed


From: Mike Tubby G8TIC
Subject: Re: Glitching GPS feed
Date: Wed, 18 Oct 2023 18:58:48 +0100
User-agent: Mozilla Thunderbird

Nick,

Firstly, I rarely use NMEA GPS - I prefer uBlox binary mode, but my observations:

* I am used to seeing $GPGGA and/or $GPRMC fixes from GPS

* you have $GNRMC and $GNGGA messages which are the newer GNSS agnostic messages

* there does appear to be two levels of connection - shown in your data: gpsd://10.253.154.161:2947#tcp://10.253.132.170:7000

* you PDOP is 1.04 and you're seeing enough satellites for a fix

* GPSD appears to be parsing and decoding your location:

            "lat":57.314202167,"lon":-111.764077667,"alt":223.400

    which puts you in Fort McKay, Canada, here:

https://www.google.com/maps/@57.186758,-111.6367851,3a,75y,90t/data=!3m8!1e2!3m6!1sAF1QipNCGXldgZQf-2D2rP2XGO_pnMIYE6ngcnkat85w!2e10!3e12!6shttps:%2F%2Flh5.googleusercontent.com%2Fp%2FAF1QipNCGXldgZQf-2D2rP2XGO_pnMIYE6ngcnkat85w%3Dw114-h86-k-no!7i4608!8i3456?entry=ttu


* The $GNTXT messages appear to be an indication that it's transmit buffer is overflowing - it may be configured to generate messages faster than it's serial port can deliver them to the host and is dropping frames, or may be being delivered over busiy I2C or SPI bus - depending on the architecture of the device with the GNSS.


There is plenty of discussion of this issue on the interweb if you search for "$GNTXT,01,01,00,txbuf alloc*61" and some suggestions that it is common place on some of the Chinese knock-off uBlox replicas.


Mike



On 18/10/2023 09:55, Nick Taylor wrote:
Hi all

After some advice...

We have a customer who is providing us with a feed via their gpsd daemon - we don't have exact details of how they are doing this behind the scenes but it sounds like there are a couple of devices in between us and the actual GPS receiver.

We can pickup feed using gpsd:// source, however when using this feed cgps seems to glitch, keeps resetting and never seems to get a fix.

I'm attaching dumps of the received feed (one is gpspipe -r and the other gpspipe -R). These show feed seems to contain multiple GNTXT messages which may or may not be causing the issue.

Any suggestions would be much appreciated.

Thanks and regards

Nick




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