Friend-foe identifier

In telecommunications, the identifier friend-foe, or IFF for its acronym in English (Identification Friend or Foe), is a cryptographic identification system. Within the military field, it serves to distinguish enemy aircraft or vehicles from those that are not. (Specifically it serves to identify friends, the unidentified are presumed enemies)

Its operation is based on establishing a huge list of challenges that only friendly teams can solve. The system that wants to know if another system is friend or foe interrogates the other system proposing a challenge. Depending on whether the answer is correct or not, it will be identified as a friend or foe. Observe that even if the enemy listens and copies the answer, it will not work, since the next time the system interrogates, the challenge used will be changed.

History

The world's first operational IFF was the FuG 25a Erstling (deutsch), developed by Germany in 1940. The system used radar frequencies of 125 MHz (Radar Freya) and 550-580 MHz (Wurzburg Radar).

The Allies captured some German planes during the war and from these, they developed their first series of electronic equipment that was built for identification and received the name of "Trasponders" (Equipment that receives on one frequency and transmits on another).

The first one developed by the English industry was operational from 1941, receiving the name of Mark I. In general, it was a piece of equipment of limited capacity without operational use.

Subsequent developments, this time carried out in the USA, gave rise to the MARK II. It operates in a similar way to the MARK I, with the difference that it has the capacity to work in two or more bands. The time taken to track each radar band was approximately 3 microseconds. The biggest drawback was that several ground-based radar sets were needed to get the different frequencies at the same time. Another problem with this equipment was that the identification echo could not be separated from the normal radar echo.

The MARK III was considered the first complete IFF kit. Its reliance on radar was removed and another component called the interrogator was added.

The interrogator was used to make the transponder work in the same way as the radar. The transponder was always mechanically clocked once every 3 microseconds approximately over the 30 MHz bandwidth, working in a range between 157 and 187 MHz. The difference with the MARK II was that the transponder could now select 6 response combinations. different, in successive scans.

Later, and in a short period, the MARK IV, V, VI, VIII and IX emerged.

Although these systems had been in service since World War II, they were not completely satisfactory for heavy, high-speed aircraft traffic as occurs today. This led to the development of the MARK X IFF in 1948. It employed an interrogation (transmit) frequency of 1030 MHz and a receive frequency of 1090 MHz. The interrogation codes were designated MODE 1, 2, and 3. Each Interrogation consisted of a pair of code pulses 3, 5, and 10 separated from each other. 8 microseconds respectively.

A later modification of the MARK X consisted of introducing a selective identification feature (SIF/Selective Identification Feature) and the system remained as MARK XI IFF/SIF.

Mode of operation

Two systems are needed: an interrogator (a secondary radar, for example) and a transponder. There are different modes of interrogation. The interrogation in most of these modes consists of two pulses separated by a precise time interval.

Mode 1

It's a military mode with a two-digit code. The first digit can go from 0 to 7 and the second from 0 to 3, so there are 32 possible codes. It is used in military air traffic control, to determine the type of aircraft and the type of mission. The interrogation consists of two pulses 3 microseconds apart.

Mode 2

This is a military-type mode with a four-digit code. All the numbers can have values from 0 to 7, with up to 4096 different codes. It is used to identify each particular aircraft, transmitting the "tail number". The interrogation in mode 2 is carried out through the transmission of pulses every 5 microseconds.

Mode 3/A

This is a standard civil/military air traffic control mode. It is used internationally, in conjunction with mode C. Its code has the same format as mode 2. That is, four digits with values from 0 to 7. This code is assigned by the aircraft's departure airport. Several codes are reserved to indicate specific situations:

• 1200: Aircraft with visual air control
• 7500: Abduction of aircraft
• 7600: Radio communication failure
• 7700: Other emergencies
• 7720-7727: Search and rescue operations
• 7777: Ground tests

Interrogations in this mode are made through pulses separated by 8 microseconds.

C-Mode

This exclusively civil mode, which is used to transmit the barometric height of the aircraft. The altitude value is transmitted in feet, with a given resolution. Interrogations consist of pulses 21 microseconds apart.

S or selective mode

Advanced civil mode that allows selective data links to be established with each aircraft. With this mode, aircraft are identified by an address and a flight number. This mode allows you to perform "squitters" (answers without having received a question mark). The equipment that allows to send interrogations in mode S is used in air traffic control.

Mode 4

Encrypted military mode. The interrogations are more complex than the simple transmission of equidistant pulses. The transmitted frame consists of a first part of synchronization pulses, followed by an encrypted code.

Mode 5

This is the military equivalent of S mode.

Other unused modes

Mode B, deprecated, consisted of a 17 microsecond interrogation. Mode D has not been assigned, a 21 microsecond poll has been defined.