How gpsd can interfere with your GNSS receiver and how to fix it

Pi and u-blox

Raspberry Pi 3 Model B and u-blox C94-M8P

In order to work with the u-blox C94-M8P on the Raspberry Pi, I installed a GPS interface called gpsd on the Pi. Documentation on gpsd can be found here. And this is the tutorial I used to install it on the Pi. However, there is one issue with gpsd that neither tutorial mentioned.

It turns out, when gpsd attempts to reconfigure certain Bluetooth- or USB-connected GPS receivers, it can interfere in strange ways with their functioning. I found that certain gpsd commands interfered with the C94-M8P’s ability to reach RTK mode. The receiver would initially reach RTK mode, then quickly lose it and achieve only a 2D/3D fix while gpsd was running.

After much digging, I found a solution. You can configure gpsd to run with the -b option, which is a read-only mode that prevents it from writing to the GPS receiver. This can be achieved when you install it, by passing -b to the Options to gpsd prompt, or, if you have already installed gpsd, you simply need to change the default parameters, as detailed below:

  • Start up your Pi and open the default file for gpsd with:

sudo nano /etc/default/gpsd

  • Set the parameters to look like this:

gpsd default settings

  • That’s it. Exit and reboot.

From the gpsd man page:

-b

Broken-device-safety mode, otherwise known as read-only mode. A few bluetooth and USB receivers lock up or become totally inaccessible when probed or reconfigured; see the hardware compatibility list on the GPSD project website for details. This switch prevents gpsd from writing to a receiver. This means that gpsd cannot configure the receiver for optimal performance, but it also means that gpsd cannot break the receiver. A better solution would be for Bluetooth to not be so fragile. A platform independent method to identify serial-over-Bluetooth devices would also be nice.

The read-only mode was a simple solution to a problem that plagued our group for over a week. If you are finding your Pi-connected USB or Bluetooth GPS mysteriously goes in and out of RTK mode and you have exhausted other causes, such as poor satellite reception at the base receiver or incorrect RTCM messages at the roving receiver, try checking your gpsd default parameters and adding this option.

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