Carriers Everywhere

GNU Radio / RTL SDR

Recently, they placed a pair of wireless traffic lights right in front of our university. Today, I had the time to have a brief look. Very convenient to put them close to eduroam and coffee :-)

Turns out, they use the same frequency band as the traffic lights that I studied earlier (around 170MHz), but use a different modulation and frame format.

Recording the signal with GQRX and loading it in Inspectrum, revealed a kind of FM in FM modulation, similar to AFSK. The inner FM signal is 1200 baud, altering between 1200Hz and 1800Hz.

Extracting the preamble is straightforward thanks to Inspectrum. Once the preamble is known, GNU Radio will help us to demodulate the data. This time, I though I do a quick recording, showing how I use vim to reverse the frame format.

The work flow, described in a previous post, allows me to easily switch between offline signal processing and a live receiver. So creating a web GUI with live updates was basically no effort.

The light was pretty bad this morning, so I couldn’t record a nice video. Here is the one from the last year in case you are interested; it’s the very same setup.

Update

A (shaky) video of the new traffic light.