Radio enthusiasts have a long history of eavesdropping on non-broadcast stations–police, fire, and public transportation frequencies, for example. These days, though, a lot of interesting communications are digital. When [bastibl] wanted to read data displayed on bus stop signs, he turned to software defined radio. He used gr-fosphor to monitor the radio spectrum as buses drove by and discovered a strong signal near 151 MHz (see photo below).
That, however, was just the start. Using a variety of tools, he figured out the modulation scheme, how the data framing worked, and even the error correction scheme. Armed with all the information, he built a GNU Radio receiver to pick up the data. A little number crunching and programming and [bastibl] was able to recover data about individual buses including their position and schedule.
A little programming, and you wind up with live bus maps. Granted, if you don’t live in Paderborn Germany, this might not be directly useful to you. But it was a detective story worthy of a radio-version of CSI.