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The SSR interrogation format (sometimes called uplink format) in Mark X standard is very simple, consisting of two pulses (P1 and P3) of 0.8 µs width which are separated by a certain time - this determines the mode of interrogation. The table shows the time spacing of the different military and civil modes and indicates their use.

Mode Distance between
P1 - P3
mode description
mili­tary civil
1   3 µs (±0.2) µs Military Identification
Military mode 1 is used to support 32 military identification codes (although 4096 mode 1 codes could also be used). Normally, the 32 codes could be used to indicate role / mission / type. However, this mode itself is not in common use in a normal peacetime environment.
2   5 µs (±0.2) µs Military Identification
Military mode 2 provides 4096 ID codes for military use (as for mode A). Normally used to identify an individual aircraft airframe.
3 A 8 µs (±0.2) µs Civil / Military Identification
Provides 4096 ID codes for civil / military use. The commonly used mode
  B 17 µs (±0.2) µs not used
  C 21 µs (±0.2) µs Civil, Pressure Altitude Extraction
mode C is used to extract the pressure altitude mode C value (or true altitude if below the transition altitude).
  D 25 µs (±0.2) µs not (never) used

Table 1: uplink-formats in Mark X Standard

Military mode 1 is usually used to indicate role, mission or type of aircraft (hence several aircraft may give the same mode 1 reply value). Mode 2 is usually used to indicate an individual aircraft airframe (which is a number set in the aircraft, usually before it takes off).

Military mode 3 and civil mode A are the same interrogation mode (hence often referred to as 3/A). It requests an identity used for air traffic control purposes. Since this identity is only 12 bits (constrained by the down link reply format - see later), there are only 4096 possible values. Values of mode 3/A codes to use in various regions are allocated by air traffic control authorities. The identity code value is set (as 4 octal digits) by the pilot, as directed by air traffic control instructions. The value may sometimes be changed during flight.

The other essential information required by air traffic control is obtained from the mode C interrogation, requesting the aircraft flight level. This is derived from the aircraft pressure altimeter or the radar altimeter.

Civil mode B and D, although originally defined, have never been used for civil ATC purposes. Hence, the present civil SSR system is usually referred to as SSR mode A/C. Not all aircraft transponders are able to reply to all modes of interrogation. Military aircraft transponders will reply to modes 1,2,3/A and many also have mode C capability. Civil transponders will not recognise Modes 1 and 2, but must recognise mode 3/A. Most will also have mode C capability.

The ground interrogator will change the interrogation modes made in a regular way – this is usually referred to as the interlace pattern. Usually civil SSR interrogators alternate mode A and C each interrogation– i.e. an AC interlace. Military interrogators may include mode 1 or 2 e.g. a 1AC2AC interlace. (Some military interrogators may interchange mode 1 and 2 each scan.)

Some military IFF systems (IFF Mk XII) also include mode 4. However, mode 4 uses very different formats. In particular, the uplink format consists of a sequence of many pulses that contain encryption data so that only aircraft carrying the correct decipher key can be recognised.

The P2 pulse, shown darkgreen colored in the pictures of the mode-table, is used for side lobe suppression purposes, as will be described later.