A pure-Python decoder library for Mode S and ADS-B messages: feed it hex frames (or stream them from dump1090 over TCP/Beast) and it returns the decoded fields — ICAO address, callsign, airborne/surface position (CPR), velocity, altitude — plus DF4/5/11/20/21 surveillance and Comm-B BDS registers. The reference way to interpret message contents and to run plausibility / anti-spoof checks on a decoded feed in your own code.