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Non-discrete mode A code reserved use in mode S radar/ADS-B environment where the aircraft identification will be used to correlate the flight plan instead of the mode A code. [1] US: Used exclusively by ADS-B aircraft to inhibit mode 3A transmission. [3] US: Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2.
A discrete transponder code (often called a squawk code) is assigned by air traffic controllers to identify an aircraft uniquely in a flight information region (FIR). This allows easy identification of aircraft on radar. [6] [7] Codes are made of four octal digits; the dials on a transponder read from zero to seven, inclusive. Four octal digits ...
The civilian aircraft frequency for voice distress alerting is 121.5 MHz. Military aircraft use 243 MHz (which is a harmonic of 121.5 MHz, and therefore civilian beacons transmit on this frequency as well). Aircraft can also signal an emergency by setting one of several special transponder codes, such as 7700.
The encoded number changes day-to-day. When the number is received and decoded in the aircraft transponder, a further cryptographic encoding is applied. If the result of that operation matches the value dialled into the IFF in the aircraft, the transponder replies with a Mode 3 response as before. If the values do not match, it does not respond.
Depending on the type of interrogation, the transponder sends back a transponder code (or "squawk code", Mode A) or altitude information (Mode C) to help air traffic controllers to identify the aircraft and to maintain separation between planes. Another mode called Mode S (Mode Select) is designed to help avoiding over-interrogation of the ...
Upon interrogation, Mode S transponders transmit information about the aircraft to the SSR system, to TCAS receivers on board aircraft and to the ADS-B SSR system. This information includes the call sign of the aircraft and/or the aircraft's permanent ICAO 24-bit address (which is represented for human interface purposes as six hexadecimal ...
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The aircraft's transponder began to emit squawk 7500 — the international code for an aircraft hijacking — while flying north over Sudan. [5] When the pilot exited the cockpit to use the restroom, the co-pilot locked the cockpit door and continued to fly the aircraft. [6]