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Kremnij-2   Кремний-2
Identification and recognition system - this page is a stub

Kremnij-2, also written as Kremniy-2, was a Soviet military identification and recognition system, or IFF, 1 introduced in the late 1950s as the successor to the Kremnij-1 system. The system was used by the countries of the Soviet Union (USSR), and also by the Warsaw Pact (WP) nations, albeit with a limited set of codes. Non-WP countries had to settle with the older Kremnij-1 system [1].

The interrogating ground station sends three pulses with a specific length and distance in UHF band III (668 MHz). A friendly airplane answers by impressing a specific frequency onto these pulses, defined by a plug-in code filter unit.

At the ground station, the returned modulated signal is decoded and, when correct, shown as a curve behind the object on the primary radar image. Of the 12 available plug-in units, only filters 1 thru 8 were used in piece time, whilst 9 thru 12 were reserved for the event of war. Filter 4 was only used for system and alignment [2].
Plug-in units for the Soviet Kremnij-2 identification system

The plug-in filter units were swapped in a secret pre-arranged scheme every one to six hours. The system was able to send a distress signal by responding twice to the interrogation system, resulting in a wider curve on the primary radar display. Being an unencrypted system, Kremnij-2 was vulnerable to interception and spoofing. It is known that American systems like QRC-248 and APX-80/81 (Combat Tree), were able to trigger responses from the Kremnij-2 IFF transponders.

  1. IFF = Identification Friend or Foe.

Sealed storage case with plug-ins
Seal on the case lids
Plug-in units for the Soviet Kremnij-2 identification system
Plug-in detail
The diagram below shows the three modes in which the Kremnij-2 system can be used. In Mode 1, the transponder is triggered by a signal from the secondary radar system, which operates on band III and coincides with the pulse from the primary radar system operating on band I or II. The transponder replies by returning two amplitude modulated (AM) signals of a specific length, and a distance of 4.5µs in band III, on the same frequency as the base station's interrogation signal.

In Mode 2, the transponder is triggered by a pulse from the primary radar system in band I or II, and replies with the same two pulses in band III. Mode 3 is the so-called autonomous mode, in which the transponder is only triggered by a pulse in band III and replies on the same frequency.

The diagram above shows the interrogation pulse in more detail. A correct interrogation consists of 3 consecutive pulses, to which the transponder replies with a similar set of (modulated) pulses. As a safety measure, the ground stations intermittently sends sequences of 4 pulses, to which the transponder should not respond. It should only reply to a correctly timed three-pulse sequence.

  • SRO
    IFF transponder
  • SRZ
    IFF interrogator
  • SRZO
    IFF interrogator/transponder
  1. Primary radar 8600 - 9700 MHz
  2. Primary radar 2520 - 3380 MHz (phased out in the early 1980s)
  3. Interrogation and response 668 MHz
  1. 1.67 MHz
  2. 2.33 MHz
  3. 3.00 MHz
  4. 3.67 MHz ← used for test and alignment
  5. 4.33 MHz
  6. 5.00 MHz
  7. 5.67 MHz
  8. 6.33 MHz
  9. 7.00 MHz ← wartime only
  10. 7.67 MHz ← wartime only
  11. 8.33 MHz ← wartime only
  12. 9.00 MHz ← wartime only
  1. Secret Projects, Soviet/Russian IFF
    Website forum. 17 September 2006.

  2. H.J. Kaiser, Kenningssysstem Kremnij-2
    Website Raketen- und Waffentechnischer Dienst im Kdo. MB III (German).
    Retrieved May 2017.

  3. Wikipedia, Sekundärradar
    Retrieved May 2017 (German).
Further information
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© Crypto Museum. Created: Friday 19 May 2017. Last changed: Wednesday, 26 August 2020 - 15:24 CET.
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