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With mode-S each aircraft can be uniquely interrogated and identified by using the..
  • A
    twelve-pulse train, giving 212 possible codes.
  • B
    long (30.25 msec) P3 pulse.
  • C
    24-bits address code, giving more than 16 million possible codes.
  • D
    Special Position Identification (SPI) pulse.


For identification, the aircraft is assigned with a four digit octal code (SQUAWK CODE). Each character of the designator may contain 8 values: numbers 0 - 7 with four digit codes. With each of the four characters having 8 possible values, the number of possible permutations for codes equals:

  • 8 x 8 x 8 x 8 = 4 096


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. One of the main features of Mode S is the 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.

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