[This post has been marinating as a draft for a while, I wrote it up as I got the idea some time back in 2013.]
In Sweden we have an encrypted (though encryption is not always on, for performance issues) digital radio network (based on and essentially the same as TETRA used by throughout Europe and US) for authorities, like police, customs, fire brigade, ambulance, for inter-communication between them in case of emergencies and major events.
Between 2006 and 2010 the Swedish network RAKEL (RAdioKommunikation för Effektiv Ledning, roughly translated to something like “radio communication system for effective governance”) was rolled out and effectively quieting the analog networks S70 and S80, though they are still in place as fallback. Many swedes had radio scanners (radios that swept through lots of channels in rapid succession, scanning for broadcast) and listened in on police/fire brigade/ambulance activity, some people also helped the police by being alert to broadcasts in and about their neighborhoods. With the digital radio system, they effectively shut out everyone not directly tied into that network.
I’m one of those swedes with a scanner, this has irritated me a little that we’re no longer “trusted” to listen in on the police (they work for us, the public, the taxpayers).
So, I’ve been playing with a thought for a while.
As electronics of different flavors has become readily available in many areas, no one has come up with a publicly available scanner for RAKEL/TETRA yet. The reasons is several, digital radio with encryption is one, authorized devices is another and there is probably more.
My thoughts has focused around the fact that they are radio-based, every radio broadcast and thereby it’s also locate-able by triangulation, two or more receivers can pinpoint a general location of a broadcasting device, add more and you get even more accuracy.
As radio waves are unidirectional (unless focused like microwave radio-links) they broadcast in all directions, which is possible to pick up with “simple” receivers. With a sectorized (passive) or a spinning (active) antenna one could passively listen for specific frequencies between 380Mhz – 430Mhz (where Swedish RAKEL resides) and register in what general direction (with a sectorized antenna) or specific direction (with a spinning antenna). If these “directions” are pooled together with other peoples readings, one could pinpoint individual devices pretty accurately.
Another use for such a set up could be to scan for flying drones (popular in the US) as they communicate in both directions too, commands for flying and/or telemetric/video-feedback.
To set up such equipment for listening, would require a few simple things;
GPS-device / Compass
Active/Passive antenna for a specific frequency-range
Computer / Software for processing/analyzing signals
It would only require to find a certain signal on a specific frequency to point out that it is something “there”, you would not need to know what the broadcast was about, just the presence of the signal is enough.
Taking it a step further, would be to analyze the signals, as it is digital, it is “packets” within a protocol, which could be broken down into headers and payloads for further analysis. Packets contain information about which device broadcasting, which makes it possible to point out a single device – along with triangulated position and second-by-second updates, it would be possible to passively following a single device/car/person around on a map.
This could be done on a massive scale to crowd-source data like this, with historical data, spanning several days, weeks or months, patterns would emerge.
Little brother could be watching Big brother.