Sierra Wireless GPS Setup

Sierra Wireless mPCIE modems often combine 3G/4G wireless connectivity along with GPS functionality

In this example we'll be using a MC7304 modem, for other models please check with the vendor to confirm GPS funtionality and GPS antenna type needed.

We'll start by installing the udev rule (20-modem-7304.rules) in /etc/udev/rules.d from the modem rule pack

This sets up shortcut references to allow us to consistently identify which of the ttyUSBx ports do which functions

root@raspberrypi:~# ls /dev/modem* -l
lrwxrwxrwx 1 root root 7 Jan  4 16:45 /dev/modemAT -> ttyUSB3
lrwxrwxrwx 1 root root 7 Jan  4 16:45 /dev/modemGPS -> ttyUSB2
lrwxrwxrwx 1 root root 7 Jan  4 16:45 /dev/modemS0 -> ttyUSB1
lrwxrwxrwx 1 root root 7 Jan  4 16:45 /dev/modemS2 -> ttyUSB2
lrwxrwxrwx 1 root root 7 Jan  4 16:45 /dev/modemS3 -> ttyUSB3

As the MC7304 modem has active GPS antenna compatibility we can use most of the gps aerials on the market or combo antennas.

After connecting up a combo antenna let's do a quick check to make sure the modem is configured correctly:


Having the SIM inserted and registered on the network gives the modem the correct time and can help with GPS fix time

There are a few commands we need to understand to correctly configure the GPS setup on the modem

AT+WANTGNSSPWR=1                  Enable Active GPS antenna support (MC73xx)
AT+WANT=1                         Enable Active GPS antenna support (MC74xx)
AT!GPSTRACK=1,255,1000,1000,1     Starts GPS service + NMEA Feed on /dev/modemGPS
AT!GPSSTATUS?                     Get GPS fix status
AT!GPSLOC?                        Run on AT command port to give a one-shot reading
AT!GPSEND=0                       Stops GPS Service + NMEA Feed on /dev/modemGPS



Let's drop into a minicom session and start up the GPS system. an un-assisted  "First Fix" can take anywhere from a few seconds to a few minutes, depending on the number of satellites visible.





To enable the GPS NMEA feed on /dev/modemGPS you need as a one off to run the below :

echo \$GPS_START >/dev/modemGPS
We can can then check this by doing :

minicom -D /dev/modemGPS


MC73x4 AT Command Reference

MC73x4 Product Technical Guide

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