This invention relates to interrogator-transponder systems, including identification coding on the transmissions.
At the present day, both the Federal Aviation Agency (FAA) and the Department of Defense (DOD) use the air traffic radar beacon system (ATCRBS) as the primary means of controlling U.S. air traffic. The ATCRBS network consists of nearly 700 ground stations, 184,000 aircraft transponders, and 108,000 altitude encoders for Mode C altitude reporting. It is estimated that DOD owns 108,000 transponders. The U.S. ATCRBS hardware is compatible with the Secondary Surveillance Radar (SSR) international air traffic control system governed by the International Civil Aviation Organization (ICAO).
Interrogator-transponder beacon systems are also used for Identification Friend or Foe (IFF) systems.
A transponder is a device that produces an active output signal in response to an incoming interrogation signal. Transponders adapted to be carried on aircraft or other vehicles respond to signals received from other station transmitters and provide reply signals which are received by the receiver at the originating station. It is desired to code the reply signals with specified pulse formats so that the signal received at the originating station identifies the particular aircraft or vehicle from which the reply is received, or to convey other information. In some applications a simple two-pulse reply is provided, with the spacing between the two pulses identifying the different transponders. See for example U.S. Pat. No. 4,129,867. Other modulation techniques are also used. Other U.S. patents of interest include my U.S. Pat. No. 4,161,729 for a Beacon Add-On Subsystem for Collision Avoidance System, hereby incorporated by reference, and U.S. Pat. No. 4,090,196 for an Inverse Gain Modulator.