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A typical altitude encoder, the ACK Technologies A-30. Note the 15-way D-type connector to send the Gillham code to the transponder and the port on the top of the case that connects to the aircraft's static pressure system. An altitude encoder takes the form of a small metal box containing a pressure sensor and signal conditioning electronics.
Mode C uses a spacing of 21 μs, and requests the aircraft's pressure altitude, provided by the altitude encoder. Mode 2 uses a spacing of 5 μs and requests the aircraft to transmit its Military identification code. The latter is only assigned to Military aircraft and so only a small percentage of aircraft actually reply to a mode 2 interrogation.
Without the pressure altitude reporting, the air traffic controller has no display of accurate altitude information, and must rely on the altitude reported by the pilot via radio. [ 6 ] [ 7 ] Similarly, the traffic collision avoidance system (TCAS) installed on some aircraft needs the altitude information supplied by transponder signals.
Provides 4-digit octal unit code (set on ground for fighters, can be changed in flight by transport aircraft) [1] 3: A: Provides a 4-digit octal identification code for the aircraft, set in the cockpit but assigned by the air traffic controller. Mode 3/A is often combined with Mode C to provide altitude information as well. [2] C
Similar standards are maintained by other aviation authorities. For example European Technical Standard Orders (ETSO) by EASA for the European Union, [3] with limited reciprocal equivalence on a per-country basis. [4] These often have the same numbers as FAA TSOs. For example, the FAA TSO for aviation headsets is C139.
ARINC 429, [1] the "Mark 33 Digital Information Transfer System (DITS)," is the ARINC technical standard for the predominant avionics data bus used on most higher-end commercial and transport aircraft. [2]
A pitot–static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot–static system generally consists of a pitot tube , a static port, and the pitot–static instruments. [ 1 ]
ICAO Doc 4444 requires that radar systems should provide for the display of safety-related alerts including the presentation of minimum safe altitude warning. [2] The radar equipment predicts an aircraft’s position in 2 minutes based on present path of flight, and the controller issues a safety alert if the projected path encounters terrain or an obstruction.