gr-air-modes implements a software-defined radio receiver for Mode S transponder signals, including ADS-B reports from equipped aircraft.
Mode S is the transponder protocol used in modern commercial aircraft. A Mode S-equipped aircraft replies to radar interrogation by either ground radar (secondary surveillance) or other aircraft ("Traffic Collision Avoidance System", or TCAS). The protocol is an extended version of the Mode A/C protocol used in transponders since the 1940s. Mode S reports include a unique airframe identifier (referred to as the "ICAO number") and altitude (to facilitate separation control). This receiver listens to the 1090MHz downlink channel; interrogation requests at 1030MHz are not received or decoded by this program.
Automatic Dependent Surveillance-Broadcast (ADS-B) is a communication protocol using the Extended Squitter capability of the Mode S transport layer. There are other implementations (VDL Mode 2 and UAT, for instance) but Mode S remains the primary ADS-B transport for commercial use. The protocol is:
ADS-B-equipped aircraft broadcast ("squitter") their position, velocity, flight number, and other interesting information to any receiver within range of the aircraft. Position reports are typically generated once per second and flight indentification every five seconds.