page_title: IFF Mark X
section_title: Response format
hierarchical_section_title: IFF Mark X / Description / Response format
caption_attribution_description: English: This is a widely used diagram associated with Secondary Surveillance Radar
context_page_description: IFF Mark X was the NATO standard military identification friend or foe transponder system from the early 1950s until it was slowly replaced by the IFF Mark XII in the 1970s. It was also adopted by ICAO, with some modifications, as the civilian air traffic control secondary radar transponder. The X in the name does not mean "tenth", but "eXperimental". Later IFF models acted as if it was the tenth in the series and used subsequent numbers.
  For most of World War II the standard IFF system used by the allied air forces was the IFF Mark III. Mark III responded on the same frequency as the trigger signal, returning a selected pulse pattern. Originally, the Mark X was simply a version of Mark III operating at a higher frequency, which has several practical advantages. Three return patterns, or Modes, were available. As it was being introduced, the new Selective Identification Feature, or SIF, allowed the response signal to be modified with bit encoding, providing the ability for each aircraft to produce a unique response using octal digits. This was initially handled through a separate box that connected to the original Mark X.
context_section_description: In the original pre-SIF system, the response to a properly received interrogation pulse is normally either one pulse in Mode 1 and 2, or two pulses in Mode 2. Turning on Emergency Mode produced four pulses in all modes.
  For SIF-equipped systems, the response to a properly received interrogation pulse is a chain of 0.45 µs (±0.1 µs) long pulses 1.45 µs apart framed by start and stop pulses. The pulses are labeled F1 and F2 for start and stop, and A1, B1, C1, A2, B2... for the response pulses. The responses are interleaved together, C1, A1, C2,...A4 followed by B1, D1... for a total of 12 possible pulses in four octal digits, A, B, C and D. A single extra "X" pulse in the center is left unused, so the total packet with framing is 20.2 µs long.
  The pulses were timed out originally through the use of delay lines. The initial specification had been for 3 µs between pulses in a given digit, or 1.5 µs between the interleaved pulses of A/C or B/D. When the first sample delay lines arrived they were defective and delayed only 2.9 µs, resulting in the 1.45 µs timing between pulses.
  Military Mode 1 transmits two digits, A and B, leaving the rest of the possible pulses empty. Only a subset of possible combinations are possible, with the complete 0 to 7 for the first digit but only 0 to 3 for the second, allowing 32 codes between 00 and 73. This is sometimes known as the "mission signal" and set by the air controllers prior to flight. In Mode 2 and 3, all 4096 possible 4-digit codes from 0000 to 7777 are allowed.
  For Mode 3/A, the value of each of the digits, 0 to 7, is set through front-panel switches. For civilian use this is a code provided by the ATC controllers. Most general aviation aircraft in North America are told to "squawk 1200", meaning that they should set their transponder to 1200, whereas in the rest of the world, 7000 is used for this same purpose. Three special-purpose codes are also used, 7500 means the aircraft is being hijacked, 7600 means their voice radio set is not working, and 7700 is a general emergency.
  For mode C, the altitude is encoded using Gillham code, using 11 bits. The lowest possible code is 000000000001, representing an altitude of -1200 feet. Every 100 feet of additional altitude above -1200 adds 1 to the total, so, for instance, 000000110100 is 1200 feet.
  SIF also adds a single additional optional pulse, Special Purpose Identification or SPI, which is sent 4.35 µs after the F2. SPI is triggered manually by the transponder operator to identify a single aircraft by pressing a button. The SPI continues to be broadcast for 18 seconds. Under ICAO rules, SPI should only be added to Mode 3/A.
caption: Mode A and C reply format.
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