GPS-IMU v4 Weak GPS signal (and picks up GPS satellites only)

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    I have a pair of GPS-IMU v4 units, with (as far as I know) identical setups on a pair of raspberry pi 4s using external antennas.

    One works fine, the other has a very weak signal (and apparently only picks up GPS satellites – no other GNSS constellation).

    I was wondering if I had somehow managed to get this receiver into a bad state? I have tried using the cold-reboot command from the FAQ but this doesn’t seem to make any difference.

    The GPS appears to be outputting binary data, so minicom and gpsmon are not able to look at the NMEA sentences. cgps and gpspipe are able to parse the GPS binary output though (but doesn’t show more than a few satellites).

    I am pretty sure I have the antenna connected and selected correctly (and a good view of the sky, as confirmed by the neighbouring, operating system), is there anything else I can try?

    Thanks a lot

    Mark Williams


    If you want to see NMEA sentences in gpsmon, kill gpsd using sudo killall gpsd Then run gpsmon specifying the serial interface gpsmon /dev/serial0

    You should be able to see the HDOP value, this is a good indication of accuracy;

    For the second GPS, if HDOP is between 0.5-1.2.. seeing other GNSS will not improve the accuracy greatly.
    If the HDOP is really high.. then check these two items;
    -switch is set to Ext for external antenna. (I assume you already have this set)
    -If using external antenna which uses SMA-uFl adapter.. make sure all screws ands nuts, especially on the back of the SMA connector, are tight



    You could also try a more aggressive reset which resets the entire module
    echo -e -n "\xb5\x62\x06\x04\x04\x00\xff\xff\x01\x00\x0e\x61" > /dev/serial0

    You can also use U-Center to see what exactly is happening.

    Mark --


    Thanks Mark, this is really useful. The tips about GNSS constellation are handy – I wasn’t concerned as such (GPS alone should be fine), but perhaps I just get better GPS signals in my location.

    Just for some further followup (incase anyone else finds this), after leaving the offending unit on overnight, it produced a 3D fix. However, later in the day, both units then lost signal completely. As they are now mostly synchronised in their ability (or not) to get a fix, I think the remaining issues are due to my testing location (and so they will hopefully work in the field).



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