Capture the Signal, or simply CTS, is is a challenge-based contest, pretty much like a CTF. The CTS focuses exclusively on the reverse engineering of radio signals. This activity is also known as “blind signal analysis” as the specifications of the signal are not known to the contestant (you). So, we're not talking about Bluetooth, Wi-Fi, or some other standard protocols.
You can use your RF-hacking-fu and tools like GNURadio, GQRX, URH, Python, C, black magic, to examine a series of increasingly complex radio signals to extract key information leading them to the next signal.