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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].
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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].
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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.
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IFF = Identification Friend or Foe.
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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.
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SRO IFF transponder SRZ IFF interrogator SRZO IFF interrogator/transponder
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- Primary radar 8600 - 9700 MHz
- Primary radar 2520 - 3380 MHz (phased out in the early 1980s)
- Interrogation and response 668 MHz
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- 1.67 MHz
- 2.33 MHz
- 3.00 MHz
- 3.67 MHz ← used for test and alignment
- 4.33 MHz
- 5.00 MHz
- 5.67 MHz
- 6.33 MHz
- 7.00 MHz ← wartime only
- 7.67 MHz ← wartime only
- 8.33 MHz ← wartime only
- 9.00 MHz ← wartime only
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© Crypto Museum. Created: Friday 19 May 2017. Last changed: Wednesday, 26 August 2020 - 14:24 CET.
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