The IFF acronym is derived from the words Identification, Friend or Foe (also nicknamed "Reply or Die"). Modern day IFF systems are basically Question/Answer systems. An interrogator system sends out a coded radio signal that asks any number of queries, including: Who are you? The Interrogator system is frequently associated with a primary radar installation, but it may also be installed aboard a ship or another airplane. The interrogation code or challenge, as it is called, is received by an electronic system known as a Transponder that is aboard the target aircraft. If the transponder receives the proper electronic code from an interrogator, it automatically transmits the requested identification back to the interrogating radar.

Working Principles

Modern IFF is a two channel system, with one frequency (1.03 GHz) used for the interrogating signals and another (1.09 GHz) for the reply. The system is further broken down into four SIF (Selective Identification Feature) modes of operation (two for both military and civilian aircraft and two strictly for military use) and an additional cryptographic mode wich actually performs the Friend/Foe classification when operating in a battle environment.

Each mode of operation elicits a specific type of information from the aircraft that is being challenged:

Mode 1: This mode has 64 reply codes (00-73 expressed in octal). It is used in military air traffic control to determine what type of aircraft is answering or what type of mission it is on.

Mode 2: This mode is only for military use, requests the "tail number" that identifies a particular aircraft. There are 4096 possible reply codes in this mode (0000-7777 expressed in octal).

Mode 3/A: This mode is the standard air traffic control mode. It is used internationally in conjunction with the automatic altitude reporting mode (Mode C) to provide positive control of all aircraft flying under instrument flight rules ( IFR). Such aircraft are assigned unique mode 3/A codes by the airport departure controller. General aviation aircraft flying under visual flight rules ( VFR) are not under constant positive control, and such aircraft use a common Mode 3/A code of 1200. In either case, the assigned code number is manually entered into the transponder control unit by the pilot or a crew member.

Altitude information is provided to the transponder by the aircraft's air data computer in increments of 100 feet. When interrogated in Mode C, the transponder automatically replies with the aircraft altitude. Ground interrogators normally interlace modes by alternately sending Mode 3/A and Mode C challenges thus receiving continuous identity and altitude data from the controlled aircraft.

Mode 4: This mode is a cryptographic mode which requires both the Interrogator and the Transponder to work in conjunction with an external Crypgraphic Unit in charge of the "secure" encoding/decoding of interrogations and replies. This mode uses a very long challenge word which contains a preamble that tells the transponder it is about to receive a secure message. The challenge itself is encrypted at the interrogator and the transponder routes the challenge to a separate device that uses the inverse algorithms to decode the challenge. In effect, each challenge is telling the transponder to respond in a certain way. If the transponder cannot decipher the challenge, it will not be able to respond the proper way and thus will not be identified as a FRIEND.

To prevent unauthorized use of either the interrogation equipment or the transponders should they fall into hostile hands, several key codes must be entered into each device on a daily basis with each key code being valid only for a period of 12 hours. To eliminate the chance of a random guess by a hostile target corresponding with the proper response, each identification consists of a rapid series of challenges each requiring a different response that must be all correct before the target is confirmed as a FRIEND.

The following figure shows the aspect of interrogations for each mode of interrogation.

As it can be seen, the difference between the modes is the spacing between pulses P1 and P3. The transmission of both interrogations and replies is a matter of a simple pulsed amplitude modulation. The response consists of two framing pulses and a set of 12 pulses encoding the ABCD code of the aircraft. Codes 7500 / 7600 / 7700 are reserved to broadcast an emergency from the platform.

The most popular Transponder antenna is certainly the blade antenna (see photo below). Usually two of this units are installed on the fuselage, commonly named as Top Antenna and Bottom Antenna (located on the dorsal and ventral fuselage) operating in a so called diversity operation, meaning that the reply is trasmitted over the antenna receiving the strongest interrogation signal. Bottom antenna usually replies to ground stations and Top antenna replies when air-air interrogations are received.

High-speed military aircraft may require somewhat different antennas. To avoid aerodynamic drag, all antennas are made flat with the surface of the fuselage or wings. An annular-slot military IFF antenna is shown in the photo below.

F-15E Implementation

The F-15E utilises a so called "diversity" IFF transponder. This means that two aerials are located on the fuselage, one on the upper half and one on the lower half of it. The two antennae provide nearly spherical coverage for interrogator reception. Diversity is a very important feature for combat aircraft which have external stores or other objects which may block the antenna from view by the interrogator or the fuselage of the aircraft itself may obscure antennas during maneuvers. This doubled antenna coverage ensures that all round IFF coverage is maintained for all orientations of the aircraft at all times. The following figure illustrates the placement of IFF transponder antennae on the F-15E. Note that these antennae are not exclusively for IFF purposes: these are UHF 1 / IFF antennae.

The Northrop Grumman AN/APX-101(V) Mark XII Diversity Transponder is the standard IFF Transponder on USAF F-16, F-15, A-10, KC-10A, E3A and F5E/F jets. The APX-101 contains two separate receiver and video processing channels, one for the upper antenna and one for the lower antenna. Normally the transmitter on the lower antenna is selected for transmission, since the lower antenna is usually seen by ground interrogators. The video processing circuitry measures the strength of the received messages, processes the stronger signal and directs the transmitter to reply using the antenna which received the stronger signal.

As for interrogator side, a BAE Systems (formerly Hazeltine) AN/APX-76 IFF interrogator informs the F-15E pilot if an aircraft seen visually or on radar is friendly or not. A Litton reply evaluator for the IFF system operates in conjunction with the AN/APX-76.

Ten T-shaped dipoles are placed all over the surface of the AN/APG-70 planar array. These dipoles working together are actually conforming the IFF main beam (sigma channel) as well as the omni directional beam (diff channel). Embedded into the radar antenna dish the IFF antennae can take advantage of the scanning movement of the planar. The AN/APX-76 Interrogator is able to generate 2-8 kW pulse power at a frequency of 1.03 GHz.

Working with the AN/APG-70 Radar

The following figure illustrates the placement of the interrogator dipole antennae on the F-15E.

The IFF Interrogator usually supports two modes of operation. When the radar operates in TWS (Track-While-Scan) mode the Interrogator can be continuously interrogating to detect the codes of all platforms carrying an operative Transponder, since the IFF also has the capacity of locating targets in distance and azimuth, the echo detected by the radar and the plot detected by the IFF can be integrated in a single target and presented to the pilot as a cooperative platform. Due to this mode of operation a picture of all collaborative platforms in the airspace can be obtained. However this mode is seldom used in a battle environment since the interrogating platform constitutes a source of continuous high power radio emission that can be easily detected by an RWR (Radar Warning) system which immediately classifies it as a military platform.

The other mode of operation is when the radar is operating in STT (Single Target Track) mode in a battle environment and a particular target is locked by the radar. Then a last chance of identification is performed by sending a burst on Mode 4 interrogation ("squawk ID" request) to the platform locked up in order to obtain a friendly response. If no feedback response (an audio signal informs the pilot about target friendship) is obtained and no voice-radio contact is obtained, then usually the platform is classified as hostile and treated accordingly.

The Transponder is a fully asynchronous system. The pilot manually enters the codes of all modes of operation (Modes 1, 2, 3/A or 4) before taking off and activates the Transponder. When the patform is interrogated by a ground station (Air Traffic Control Tower using only modes 3/A and C) the Transponder automatically sends the reply without the pilot being aware of this. If the mission requires EMCON (Emission Control, wich means, no electromagnetic signals should be transmitted) the Transponder is automatically switched to standby mode.

Note that the pilot has ways of knowing when his platform is interrogated in Mode 4 (cryptographic). If a Mode 4 interrogation is received and a FRIEND response is transmitted, then the REPLY light illuminates and/or an audio tone sounds, depending on the pilot's preferences (see below). On the other hand, if a Mode 4 interrogation is received and the Transponder is not able to send a FRIEND reply, this generates an IFF Mode 4 caution on the MPD/ MPCD and the Master Caution light illuminates, thus letting the pilot know that he is treated as enemy by the interrogating platform.

Pilot Controls

The codes for modes 1 and 3/A can be set in the cockpit. Mode 2 is set using the control box in door 3R. Mode 4 is keyed in door 3R by maintenance personnel using the KIK. Mode 2 cannot be changed in flight. Mode 4 can be changed between 4A and 4B during flight and the codes can be zeroed.

The Interrogator control panel contains the controls providing A/A target identiciation ( AAI). Controls for the IFF Transponder are located on the remote intercom panel in the front cockpit and the UFC. The following figure illustrates the placement of the IFF Transponder controls:

The functions of these controls are the following:

Mode 4 Selector Switch: This is a lever-lock switch with the following positions:

Mode 4 Reply Switch: This switch has the following positions:

Master Switch: This switch has the following positions:

IFF Antenna Selector Switch: The Antenna Selector Switch is located on the pilot's left console next to the ICCP. This switch has the following positions:

Mode 4 Crypto Switch: This is a lever-lock switch with the following positions:

Upon ejection from the cockpit the IFF emergency mode automatically becomes active if Mode 1, 2, 3/A or C is enabled.

Up-Front Controller Menu Functions

The IFF submenu can be selected from the general UFC menu by pressing the pushbutton next to the 'IFF' readout. IFF functions and settings can be reached from this submenu. The IFF submenu readouts on the UFC are illustrated on the following figure:

Use of the submenu is fairly simple. The different modes of IFF operation and the C 'submode' can be activated/deactivated by pushing their appropriate pushbutton (see 2, 3, 4 and 6 on the figure above). An asterisk next to the mode readout indicates that the mode is currently active. If the mode is inactive, no asterisk is displayed next to its readout. In case of Mode 1 and Mode 3, the code can be entered in this submenu as well (by entering the code first on the numeric buttons and then by pushing the Mode 1 or Mode 3 pushbutton).

The IFF system can be programmed to operate at specific modes and codes as a function of time. This is referred to as phasing (denoted as PH on the UFC display). Phasing permits the selection of up to 13 mission segments for automatic change of IFF operation based on TOD (Time of Day) information and is expected to be based on authentication procedures for a given theatre of operation. Phasing will be programmed by the DTM (Data Transfer Module) by default but can be reprogrammed by the pilot.

Programming mode can be activated/deactivated by pushing the programming pushbutton (see 7 on the figure above). An asterisk is displayed next to the programming readout when the programming mode is active. Data for only one phase are displayed at a time, the phase number is displayed on readout 1 (see figure above). Pushing the phase pushbutton cycles through the phases of the program, incrementing the phase number by 1 on each push of the button. When one phase is displayed, phase data can me changed by using the other pushbuttons. When programming mode is exited by pressing the programming pushbutton (see 7 on the figure above) all changes are stored in memory for the rest of the mission.

Sources

• BAE Systems
• Northrop Grumman
• F-15E Flight Manual TO 1F-15E-1 (courtesy of eFlightManuals.com)

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