Mode S Transponder

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The ATC transponder is the airborne component of air traffic control. It is usually referred to as secondary surveillance radar (SSR). Thanks to SSR, air traffic controllers can identify aircraft and determine their altitude. The SSR complements the primary surveillance radar (PSR).

 

Types of transponders

There are different types of transponders:

  • Mode A Transponder: used to identify an aircraft by the ground radar through transmission of an identification number that is stored in the transponder.
  • Mode C Transponder: an additional box connected to the transponder enables the ground radar to accurately determine the aircraft's altitude, converted to 1013 mb.
  • Mode S Transponder: additional information can be transmitted to the ground (registration, aircraft type, etc.). The Mode S code also allows information from the aircraft to be transmitted via TCAS to other aircraft in the same flight zone, and to suggest avoidance maneuvres if there is a risk of collision.
  • Mode S Elementary Surveillance (ELS):  based on the State of registration assigning a unique code (24 bits) to each aircraft, which enables the aircraft to be identified by ground radars and its altitude to be determined with an accuracy of approximately 25 ft. EASA has issued an information letter (Pdf, 29 Kb) on this subject.
  • Mode S Enhanced Surveillance (EHS): in addition to the information transmitted by the ELS, the EHS uses supplementary data (e.g., ground speed, magnetic heading, etc.) provided by the aircraft to facilitate air traffic control information and traffic management. EASA has issued an information letter (Pdf, 29 Kb) on this subject.
  • Mode S Transponder + TIC: future standard that will enable traffic and weather information to be transmitted to aircraft.

The power of the transponder is mainly used to enable faster identification by ground radars, with the most powerful transponders being detected first.

 

 

Regulatory aspects

ICAO Annex 10, Volume III, Part 1, Chapter 9 provides for the allocation of aircraft addresses that can be used by navigation systems worldwide. Several rules should be observed:

  • a same address can only be assigned to one aircraft.
  • when an aircraft changes its State of registration, it loses the assigned Mode S code, which becomes obsolete as it is linked to the registration itself. Therefore, the aircraft owner/operator must submit a new application.

These two rules are an essential element of air navigation safety.

Since 1 January 2003, aircraft with MTOW > 15.000 kg or carrying more than 30 passengers must be equipped with an ACAS II system and therefore have a Mode S code.

Since 1 January 2005, aircraft with MTOW > 5.700 kg or carrying more than 19 passengers must be equipped with an ACAS II system and therefore have a Mode S code.

All aircraft authorized to fly in RVSM airspace must be equipped with a transponder and the corresponding address.

See the respective AIP Belgium and Luxembourg and AICs for the specific requirements regarding aircraft equipment depending on the areas flown over in the Brussels FIR/UIR.

 

 

Transponder coding

ICAO manages the coding system, which assigns the code for each State of registration. This code is composed of 24 bits. The aircraft owner/operator must submit the request for Mode S code to the DAC on plain paper or by e-mail.

The DAC issues a Code Mode S to each newly registered aircraft that required one. When the aircraft operates in RVSM airspace, the DAC informs the operator and Eurocontrol via the Form F2. Eurocontrol then informs all other offices concerned.

The Mode S code can be used as an identification code in emergency locator transmitters (ELT).

 

 

Composition of the Mode S code

The Mode S code is defined by the DAC. Each address block identifies the country of registration, the operator, the aircraft registration mark and the equipment standard of the TCAS system installed. The complete code is composed of a binary code or its equivalent in hexadecimal code, known as the " Mode S code ".

Each code is composed of 24 bits.

  • Code for Luxembourg (0100 11 010 000 0-)
  • Operator code (4 bits)
  • Aircraft code (6 bits)

The hyphen represents a bit value equal to 0 or 1.