gpsd/gpsmon on R-Pi 3 re-activates disabled NMEA messages?

Question

I have a Raspberry Pi 3 with Adafruit's Ultimate GPS Pi Hat. I was able to successfully install gpsd and chrony. I am now getting NMEA and PPS thumbs up But I have a weird issue.

The Ultimate GPS module is spitting out more NMEA messages that I want: I want only GPGGA and GPRMC....but it is also replying with GPGSA, GPGSV, and GPZDA.

So Adafruit has some Python code/libraries that allows you to interface with the GPS module and 'talk' to it. (https://github.com/adafruit/Adafruit_GPS). Using this code I was able to send the following packet using the PMTK command packet (https://cdn-shop.adafruit.com/datasheets/PMTK_A11.pdf)

$PMTK314,0,1,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0*28

I then opened up Putty and using a 9600 serial connection to /dev/ttyAMA0 I see indeed that only GPGGA and GPRMC are being processed out:

BUT...as soon as I reopen gpsmon/gpsd, all the NMEA message streams are reactivated'...what the heck ??

If I don't open gpsd/gpsmon...indeed only GPGGA and GPRMC are active.

Has anyone come across this issue before ?

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Answer 1

gpsd re-enables most sentence types when it detects that it is talking to the GPS chip that the Adafruit Ultimate GPS Hat uses (MTK3339).

The relevant code in gpsd reacts when it sees a sentence from the GPS Hat starting $PMTK705,, which identifies this GPS hardware. gpsd then sends a few configuration strings, including one for PMTK314:

// Set NMEA sentences.
(void)nmea_send(session, "$PMTK314,0,1,0,1,1,5,1,1,0,0,0,0,0,0,0,0,0,1,0");

There's no way in gpsd to disable this behaviour except a sledgehammer workaround, which is to invoke gpsd with the -b option to prevent it from writing to the device at all. On Raspbian, editing /etc/default/gpsd will do:

# Other options you want to pass to gpsd
GPSD_OPTIONS="-b -n"

Needless to say, disabling gpsd from writing to the device might have undesired consequences.