Identification Systems

The aircraft is equipped with a set of interrogator systems AN/APX-76, -80A and -81A, as well as with a transponder to react to interrogations from other aircraft.

The interrogator can be controlled by the WSO with a panel on the left sub-panel. The transponder is set up by the pilot on the right console.

Transponder System

The transponder automatically responds to challenges from surface or airborne radar sets and serves supplementary purposes such as providing momentary identification of position upon request and transmitting a specially coded response to indicate an emergency.

The system operates by receiving coded interrogation signals and transmitting coded response signals to the source of the challenge, with a proper reply indicating the target is friendly.

The system features four modes. Mode 1, Mode 2, and Mode 3/A—are provided for security identification, personal identification, and traffic identification, respectively.

Mode 4 is controlled through the interrogator panel by the WSO. Codes for Modes 1 and 3/A can be set in the cockpit, while the code for Mode 2 must be set on the ground, ranging from 0000 to 7777.

💡 Due to engine limitations, the settings on the panel have no effect for DCS. However, they are exposed to external tools, such as SRS.

Self Test operation

To self test Modes 2 and 3/A, place the master switch (1) to NORM and hold the switch for the desired test mode to the upper position. If the test light on the IFF control panel illuminates, this indicates the mode is operating properly.

Mode 1 and Mode C do not have self testing capabilities.

Normal Operation

To operate the IFF system, start by rotating the master switch (1) to STBY. After an approximate 80-second warmup delay, the system receives full power, but interrogations are blocked.

Set the Mode 1, Mode 2, Mode 3/A, Mode 4, and Mode C switches (6) as directed, along with the Mode 1 and Mode 3/A code selector switches (10) and Mode 4 function switch (8). Set the master switch (1) to NORM to make the system ready for operation on the selected modes. If the master switch (1) is rotated from OFF directly to an operating mode, it also has to go through the warmup period first before it is fully operational.

Interrogation of Position

For Interrogation of Position (I/P) switch operation, place the I/P switch (9) in the IDENT position or place it in the MIC position and press the UHF microphone. The IFF system responds with special I/P signals.

If the IFF warning light and MASTER CAUTION light come on momentarily, check the Mode 4 selector switch (8) ON and the master switch (1) NORMAL. Repeated illumination of the MASTER CAUTION light may be stopped only by placing the master switch (1) OFF, resulting in the loss of all IFF capability, or by placing the Mode 4 function switch (8) to ZERO. Before or during flight, if the master switch (1) is placed OFF, the IFF and MASTER CAUTION lights will not illuminate upon interrogation.

Normal IFF operation will be available, after an 80-second warm-up, when the master switch (1) is again placed to NORMAL. If the Mode 4 function switch (8) is placed to ZERO, the IFF light will come on steady, and the MASTER CAUTION may then be reset. Mode 4 will not be available during the remainder of the flight.

Emergency Operation

Upon ejection from either cockpit, the IFF emergency operation automatically becomes active.

If the master switch (1) is in the OFF position before ejection, the system will begin operation after an approximate 80-second delay.

In an emergency, rotate the master switch (1) to EMER. The replies for Modes 1 and 2 are special emergency signals of the codes selected on the applicable dials, while Mode 3/A replies are special emergency signals of code 7700.

Interrogator Systems

The Phantom combines three systems, AN/APX-76, -80A and -81A, for interrogating and challenging other aircraft to detect whether they are friend or foe.

The AN/APX-76 system enables regular interrogation with friendly transponder systems.

Further, the US reverse-engineered some Soviet transponder systems actively used between 1960 and 1980 enough to be able to develop the spoofing system AN/APX-81A Combat-Tree. Combat-Tree sends compatible interrogation requests to Soviet systems which they would identify as friendly systems, hence sending back a valid response. This allows the Phantom to not only identify friendly systems, but also some likely-hostile aircraft.

💡 Soviets quickly realized the problem and patched their IFF transponder systems, while also encrypting the communication to prevent another breach.

Operation

Controls are combined on a panel, accessible to the WSO on the left sub-panel area.

The interrogation mode is set on the first roller-display and can be set to OFF or Mode 1, 2, 3, 4/A or 4/B.

💡 In DCS, only Mode 4 (either A or B) is effective and can be used for interrogation.

The other four digits are used to set the IFF code to interrogate for Modes 1 to 3.

Once setup, interrogation can be initiated by pressing the Challenge Button on the Antenna Hand Control Stick.

The radar screen will now display lines above and below a radar return if it was able to detect that a contact is friendly. A single line is shown above the return if the transponder is set to a compatible mode, but the code differs. This is usually the case for civilian aircraft.

🚧 The Combat-Tree system will be made available later during Early-Access.

Indications

Challenge Lights

The control panel features two lamps that indicate active interrogation by either interrogation system:

• TEST/CHAL lamp in lower left corner - active APX-81A Combat-Tree Interrogation
• CHAL lamp in upper right corner - active APX-76 Interrogation

Activity Lights

Right next to either AoA Indexer in the WSO cockpit is a light that indicates activity detected by the APX-81A Combat Tree system.

Illuminated each time the system detects IFF responses by hostile aircraft.

💡 Due to engine limitations, the activity lights are not simulated in-game.