Receiving and decoding ASTRID messages
The Belgian ASTRID network is a pager system for emergency services, like ambulances and firefighters, to get notified when their assistance is required somewhere. This network uses the POCSAG protocol and is unencrypted, so anyone in Belgium or close to the borders can receive all messages and decode them.
There was once even a website called cpabreda.nl that recorded all messages and grouped them for each part of the country. Unfortunatly they had to shut down their website. But luckily for us, it is very easy to do it ourselves.
How to receive and decode ASTRID messages
You don’t need a lot to get started. All you need is
- a RTL-SDR
- a suitable antenna (I use an antenna like the ones found on handheld radios like the Baofeng UV-5R)
- a computer
On that computer you will need to have installed
rtl-fm takes care of the receiving, while multimon-ng does the decoding.
These two programs get combined into a single command:
rtl_fm -f 169625000 -s 22050 | multimon-ng -t raw -a POCSAG2400 -f alpha --timestamp -e /dev/stdin
Let’s dissect this command.
The first part is rtl_fm -f 169625000 -s 22050. This launches rtl-fm and sets the frequency to be received to 169625000 Hz, or 169.625 MHz, the frequency on which the ASTRID network operates. It also sets the bandwidth to 22050 Hz.
All the received data is then piped into multimon-ng. To make this decode the messages properly we need to set the input type to raw (-t raw), as this is what rtl-fm outputs.
Next we add a demodulator. The ASTRID network uses the POCSAG 2400 protocol, so we choose this (-a POCSAG2400). multimon-ng tries to auto-detect the type of data that is being receives, but it does not work really well, so we need to stop it from guessing. To do this we set the decoding to alphanumeric (-f alpha). Next we add a timestamp (--timestamp) and hide the empty messages (-e).
The last thing multimon-ng needs is an input file. We use a pipe to supply the data, so in our case it is /dev/stdin.
Conclusion
It does not take much to be able to receive and decode ASTRID messages. I also think this counts as OSINT. Technically it is not “open source”, but more like “source available”, as you are not really supposed to be able to do this.
If you are located in or near Belgium, I can highly recommend trying this out if you have some free time.