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Mode-S transponder was introduced to to eliminate reduced codes available to designate aircraft (4096 squawk codes) in a congested airspace. With the introduction of mode-S transponders, unique ICAO 24-bit address (17million codes) was permanently assigned to an aircraft. But still squawk codes are assigned to mode-S transponders, why ?

Example of ICAO 24-bit address: (taken from flightradar24.com), 89611E

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2 Answers 2

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Mode S was not introduced because of a shortage of Mode A (squawk) codes in congested airspace, but to improve the radar performance.

In congested airspace, a Mode A/C radar suffers from two technical phenomena: garbling and FRUIT.

Garbling occurs when the reply of two transponders overlap, causing the radar to have trouble in distinguishing the replies. This happens when aircraft are nearby each other.

FRUIT (Foreign Replies Unsynchronised in Time) occurs when a radar interprets a transponder's reply to an interrogation by another radar as a reply to its own interrogation.

Mode S (S = Selective) addressed both of these problems by introducing addressed interrogation, allowing radars to selectively solicit for replies of aircraft. For this the ICAO 24-bit address is used, uniquely assigned to each aircraft. Radars have a similar, albeit non-unique address, the IC/II (interrogator code / interrogator identifier).


So why can't the 24-bit address replace the 12 bit squawk code?

  1. not everywhere on earth Mode S radars are used. There are still Mode A/C only radars around. For a long time, it was very common to have Mode A/C only. For the same reason, not every aircraft is equipped with a Mode-S transponder.

  2. the squawk codes have a number of functions, some cannot be replaced by a static 24-bit address.

  • correlating the radar track to a flight plan. This could be done based on 24-bit address
  • signifying the nature of the flight (e.g. VFR 7000/1200 codes, 2000 code, dedicated codes for Helicopter Emergency Service etc) - this cannot be done based on the 24-bit address
  • indicating emergency status (7500, 7600, 7700), this cannot be done based on the 24-bit address either.

In order to address the shortage of squawk codes, one of the approaches taken is to use the Flight ID (call sign) transmitted by the Mode-S transponder in lieu of the Squawk code for associating the radar track to a flight plan. In this case, the squawk code 1000 is used to signal to the flight data processing system that association shall be done base on Flight ID.

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  • $\begingroup$ "indicating emergency status" Mode-S has built-in facilities for doing this, see Type Code 28/BDS 6,1, which encodes more emergency status, including lifeguard/medical and minimum fuel. Mode-S transponders current automatically output this message depending on Mode A setting. $\endgroup$
    – user71659
    Dec 19, 2022 at 20:20
  • $\begingroup$ @user71659 TC 28 / BDS 6,1 is part of 1090ES (ADS-B), not of the Mode S radar protocol. But it is a fair point, and shows that you know an awful lot about the nitty gritty details of ATC surveiilance. $\endgroup$
    – DeltaLima
    Dec 19, 2022 at 22:26
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In some countries, if the equipment codes in the flight plan indicate mode S capability, the computer will assign a squawk code of 1000, which is a special code that causes the transponder to not transmit mode A at all. This works because in those countries, ATC’s software can use the mode S aircraft ID (aka “hex code”) to correlate a target with a flight plan.

However, not all countries have upgraded their software to use the mode S code this way, so they still need the mode A code for that purpose.

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  • $\begingroup$ Excellent explanation. But this led me to few more questions. 1. If the equipment on board an aircraft is mode-S transponder and still ATC asks aircraft to squawk certain 4-digit code, how will the transponder behave now ? Will it transmit mode-S data (24-bit address along with FL, Speed, Callsign, Lat & Long) and mode-A code or upon entering 4-digit code, the transponder downgrades its capability and transmits only mode-A/C information (Squawk code + altitude) ? $\endgroup$ Dec 18, 2022 at 5:04
  • $\begingroup$ @StephenS do you have any examples of ATC systems doing correlation on 24-bit address when Mode =1000? Normally the correlation is done based on FlightId/Callsign for Mode A = 1000 tracks. $\endgroup$
    – DeltaLima
    Dec 19, 2022 at 11:41
  • $\begingroup$ @KrishnaKanth Mode S transponders also send mode A/C information, except mode A is suppressed if the squawk code entered is 1000. $\endgroup$
    – StephenS
    Dec 19, 2022 at 16:08

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