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The two main design functions of SSR Mode S are…

  • A
    the elimination of ground to air communications and the introduction of automatic separation between aircraft using TCAS II.
  • B
    collision avoidance using TCAS II and improved HF communication capability.
  • C
    continuous automatic position reporting using GPS satellites and collision avoidance using TCAS II.
  • D
    air to ground and ground to air data link communications and improved ATC aircraft surveillance capability.

MODE S

Modes A and C are limited to 4096 available codes. With an increasing volume of air traffic, these systems become prone to interference. Mode S was invented to overcome these limitations. "S" stands for Selective Addressing, which means that a specific aircraft my be interrogated by the controller. The main features of Mode S include:

  • Availability of codes. The aircraft address code will be made up of a 24 bit code. Considering a 24-bit binary code, it provides over 16 million (224 = 16 777 216) possible hard-coded individual addresses to be allocated. Mode S differs from Modes A and C due to the additional data message up to 112 bits that is transmitted via interrogation.

  • Data Link. Mode S can provide ground-to-air, air-to- ground and air-to-air data exchange using communications protocols. Thus reducing the overall workload and capacity demands of ATC frequencies.

  • Reduction of Voice Communications. Messages to and from an aircraft will be exchanged via the aircraft’s CDU resulting in a reduction in voice communications.

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