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ModemManager-powered location indicator for gnome-shell

TL;DR: Show location information retrieved directly from ModemManager in a gnome-shell indicator.

Location Indicator in gnome-shell

I’ve been wanting to try to write a gnome-shell extension from some time, and I ended up finding enough free time during the past weeks. So I ended up writing a gnome-shell extension which would show information about your current location as provided by your modem. Note that this information is not coming from GeoClue; the new ModemManager1 interface is not integrated yet there.

ModemManager exposes these types of information in the Location interface:

  • 3GPP location, which is given as a set of 4 parameters, and specifies which is the current network cell which is giving mobile coverage to the modem:
    • MCC/MNC: Identifies the ‘mobile country code’ and the ‘mobile network code’; i.e. country and operator.
    • LAC and Cell ID: Identifies the ‘location area code’ and ‘cell id’ pair.

    Once all these items are known, a query to an open database of cell IDs can give us the exact location of the tower giving us coverage, and therefore a good approximation of where we are located.

  • GPS location, given if the modem has a built-in GPS. Currently Option/HSO modems are supported, as well as QMI-powered modems with the ‘PDS’ service implemented.
  • CDMA Base Station location, given as Latitude/Longitude coordinates, and available in QMI-powered modems prepared for CDMA networks. As with the 3GPP one, this location information specifies where the station giving coverage to your modem is located, which is a good approximation of where you are located.

Beware! You’ll need ModemManager >= 0.7 (i.e. git master) in order for this extension to work.

You can download and install the extension in the following link:
https://extensions.gnome.org/extension/569/location-indicator

The sources are kept in a git repository in gitorious.org:
https://gitorious.org/aleksander/gnome-shell-location-indicator

If you find the extension itself mostly useless, you may still want to take a look at the source code to see yet another example of how to talk to DBus interfaces from within gnome-shell, including the use of the new standard ObjectManager interface with a Gio.DBusObjectManagerClient.

Enabling GPS location in ModemManager

Location information in ModemManager

Since MM 0.5 at least, ModemManager comes with a Location interface which is defined to expose not only 3GPP-specific location information (operator MMC/MNC + Location area code + Cell ID), but also GPS-specific location information (cached NMEA traces and raw location information). In MM 0.7, this Location interface will actually work for GPS location, at least for Option modems with GPS capabilities.

The HSO plugin

The High-Speed Option plugin is probably one of the most complex plugins being handled in ModemManager. In the new ModemManager 0.7 codebase, HSO modems (MMBroadbandModemHso) are implemented as a subclass of the standard Option modems (MMBroadbandModemOption), as they share lots of common features (allowed modes management, access technology and signal quality reporting...); but they also implement lots of connection related HSO-specific features (e.g. success of a connection establishment gets notified via unsolicited messages), which are most of them handled in a new HSO-specific bearer. Some of these advanced Option modems come with additional surprises: GPS-specific ports (AT port for control, raw serial port for NMEA traces).

The HSO modems provide detailed information about the type of each port being exposed in the USB interface; and so during the probing phase we are able to easily grab the GPS-control AT port and the additional serial port for NMEA traces (handled by a new MMGpsSerialPort port type). Once the HSO modem knows it has these two ports, it will allow to enable or disable any of the two GPS-specific location sources (NMEA or raw), or both at the same time. The NMEA location source will enable the modem to expose the last cached NMEA traces; while the raw location source will tell the modem to just expose generic location information such as time, latitude, longitude and altitude. In the case of the HSO modem (and possibly in every other case), both location sources are actually implemented based on the same NMEA traces received (raw information is parsed from $GTGGA traces). In addition to the core GPS location handling and the HSO-specific implementation, libmm-glib and mmcli where also improved to easily play with the new features.

Trying the GPS location retrieval with your Option modem

If you want to give a quick try to these capabilities, grab ModemManager git master, compile, install and plug in your modem. You first need to check whether the modem has GPS-specific location capabilities. Note that we'll assume the modem is exposed as index 0; if you have more than one modem, just use --list-modems to check the proper modem index:

$ mmcli -m 0 --location-status
/org/freedesktop/ModemManager1/Modem/0
  ----------------------------
  Location | capabilities: '3gpp-lac-ci, gps-raw, gps-nmea'
           |      enabled: 'none'
           |      signals: 'no'

The output says that the modem supports 3GPP Location area code/Cell ID, GPS raw and GPS-NMEA location sources. None is enabled yet, as we didn't enable the modem, which we can do issuing:

$ sudo mmcli -m 0 --enable
successfully enabled the modem

$ mmcli -m 0 --location-status
/org/freedesktop/ModemManager1/Modem/0
  ----------------------------
  Location | capabilities: '3gpp-lac-ci, gps-raw, gps-nmea'
           |      enabled: '3gpp-lac-ci'
           |      signals: 'no'

The 3GPP location source will always be enabled by default if it is supported by the modem, while the GPS-specific ones will always be disabled by default. The reasoning behind this is that the 3GPP LAC/CI comes for free in C(G)REG notifications when C(G)REG=2 is supported, while the GPS location is usually an additional module in the modem, with high power consumption. Using mmcli, we can now enable both GPS raw and NMEA location sources:

$ sudo mmcli -m 0 \
             --location-enable-gps-raw \
             --location-enable-gps-nmea
successfully setup location gathering

If we do check again the status, we'll see the GPS-specific locations are enabled:

$ mmcli -m 0 --location-status
/org/freedesktop/ModemManager1/Modem/0
  ----------------------------
  Location | capabilities: '3gpp-lac-ci, gps-raw, gps-nmea'
           |      enabled: '3gpp-lac-ci, gps-raw, gps-nmea'
           |      signals: 'no'

Once they are enabled, ModemManager will start receiving NMEA traces in the GPS raw port, and will start to cache and process them. mmcli allows to show location-source-specific information with --location-get-3gpp, --location-get-gps-nmea and --location-get-gps-raw; but it also allows to query for all at the same time:

$ sudo mmcli -m 0 --location-get
/org/freedesktop/ModemManager1/Modem/0
  -------------------------
  3GPP location   | Mobile country code: '214'
                  | Mobile network code: '3'
                  |  Location area code: '21071'
                  |             Cell ID: '7033737'
  -------------------------
  GPS NMEA traces | $GPGGA,,,,,,0,,,,,,,,*66
                  | $GPRMC,,V,,,,,,,,,,N*53
                  | $GPGSA,A,1,,,,,,,,,,,,,,,*1E
                  | $GPGSV,4,1,16,24,,,,29,,,,05,,,,18,,,*7A
                  | $GPGSV,4,2,16,22,,,,14,,,,11,,,,17,,,*7B
                  | $GPGSV,4,3,16,03,,,,12,,,,30,,,,13,,,*78
                  | $GPGSV,4,4,16,23,,,,15,,,,27,,,,07,,,*79
                  | $GPVTG,,T,,M,,N,,K,N*2C
  -------------------------
  Raw GPS         | Not available

Note that ModemManager will expose only the last received NMEA traces of each kind, except for those ($GPGSV here) that are actually building a sequence.

Once the GPS module fixes a position, we'll process the $GPGGA NMEA trace to get valid GPS location information:

$ sudo mmcli -m 0 --location-get-gps-raw
/org/freedesktop/ModemManager1/Modem/0
  -------------------------
  Raw GPS         |  UTC time: '155142.2'
                  | Longitude: '-3.513941'
                  |  Latitude: '40.502603'
                  |  Altitude: '18.000000'

What about other modems?

Well, if the modem exposes GPS-specific ports in the same USB interface, it shouldn't be difficult to add support for them, specially now that all the core/libmm-glib/mmcli work is already available.

A special case is when the modem exposes a single AT port via USB and the GPS port via RS232 (e.g. Sierra Wireless FXT009 with GPS extension). This case is a bit more tricky as there is no easy way for ModemManager to tell that the RS232 port is to be managed by the same modem object created after grabbing the AT port. In this case, the user may need to explicitly set some magic udev tags to let ModemManager handle the needed logic, but that also may get tricky if the user has more than one such modem plugged in (not sure for what, but anyway).

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