A server to collect data from GPS Locators



This work is inspired by this project, but it is more of a complete re-implementation than a derived work. There also exists an industrial strength open source server that supports multiple types of trackers.

When powered up, the module makes TCP connection to the configured (via SMS) server, identifies itself (with IMEI), and continues to send periodic messages with location and other status updates. Some of these messages require a response from the server. In particular, when zx303 has no GPS coverage, it sends information about nearby GSM+ cell towers and WiFi access points, to which the server is expected to respond with a message containing approximate location derived from this data. To do that, the server may need to consult with some external service.

Because we would usually want location information reach consumer instantly upon arrival, and also to be stored, it makes sense to design the system in “microservices” way, using a message bus in “publish-subscribe” model. And then, since we already have a message-passing infrastructure anyway, it makes sense to decouple the server process that maintains TCP connections with the the tracker terminals from the processes that analyses messages and prepares responses.

This leads us to this implementation, that has consists of five daemons that talk to each other over Zeromq:

There is also a command-line tool to send messages to the terminal.

Configuring the Terminal

Send SMS to the telephone number of the SIM card plugged in the terminal, with the text

“123456” is the default password on that kind of trackers, that you can change. If “123456” does not work, try “523681”.

Server address may be FQDN or a literal IP address. Port is a number; by default, this application listens on the port 4303. A different port can be configured in the config file.

It is recommended to always keep the service running while the terminal is powered up: it is possible that the terminal is programmed to reset itself to the default configuration if it cannot connect to the server for prolonged time.

Websocket messages

Websockets server communicates with the web page using json encoded text messages. The only supported message from the web page to the server is subscription message. Recognised elements are:

Each subscription request nullifies preexisting list of IMEIs associated with the web client, and replaces it with the list supplied in the message.

Example of a subscription request:


Server sends to the client a backlog of last locations of the terminals, that it fetches from the database maintained by the storage service, one location per websocket message. It then continues to send further messages when they are received from the module, in real time, including gps location, responses with approximated location, and status with the precentage of battery charge.

Example of a location message:

{"type": "location",
 "imei": "8354369077195199",
 "timestamp": "2022-05-09 21:52:34.643277+00:00",
 "longitude": 17.465816,
 "latitude": 47.52013,
 "accuracy": "gps"} // or "approximate"

Example of a status message

{"type": "status",
 "imei": "8354369077195199",
 "timestamp": "2022-05-09 21:52:34.643277+00:00",
 "battery": 46}

Rectifier service

When the terminal has no gps reception, it uses secondary sources of location hints: list of nearby cell towers, and list of MAC addresses of nearby WiFi access point, with signal strength. It expects a response from the server with approximated location. In order to get such approximation, the server system needs a source of information about cell towers and/or WiFi access points in the area. We support two ways to get approximated location: querying Google geolocation service, and using locally installed database filled with data downloaded from opencellid crowdsourced source. For both options, you will need an access token. Google service is “online”, you are making a request for each approximation (and thus reveal location of your users to Google). Opencellid service is “offline”: you download the file with locations of all cell towers in the country (or worldwide) once, or refresh it at relatively long intervals, such as a week or a month, and then all queries are fulfilled locally. Note that opencellid data does not contain WiFi access points, so the approximation will less accurate.

Rectifier service can be configured to use either of the options by assigning backend = opencellid or backend = googlemaps in the configuration file (/etc/loctrkd.conf by default). Then, the path to the file with the auth token needs to be specified in the [opencellid] section or [googlemaps] section of the configuration file respectively.

Note that in both cases, the value in the configuration file needs to point to the file that contains the token, rather than contain the token itself. The file needs to be readable for the user under which services are executed. That is the user loctrkd if this software was installed as the Debian package.

This part of setup cannot be automated, because each user needs to obtain their own access token for one of the above services.

Termconfig Service

To configure terminal settings, such as SOS numbers, update intervals etc., “termconfig” service consults the configuration file. It should contain the section [termconfig], and optionally sections named after the IMEIs of individual terminals. [termconfig] values are used when the individual section is not present.

For a bigger multi-client setup the user will want to re-implement this service to use some kind of a database, with the data configurable by the owners of the terminals.

Homepage and source

Home page is http://www.average.org/loctrkd/ Get the source from the origin git://git.average.org/loctrkd.git or from Github mirror.