Identification Systems

The transponder and interrogator can be controlled by the RIO with a panel on the right console.

AN/APX-72 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 are provided for security identification, personal identification, and traffic identification, respectively.

Codes for Modes 1 and 3 can be set in the cockpit, while the code for Mode 2 must be set on the ground, ranging from 0000 to 7777. Mode 1 is limited to two digits 00 to 77.

The codes for Mode 4 are automatically inserted by Ground Crew before flight, the Crew Chief can be asked to insert them on demand as well.

💡 The Tomcat features a full IFF simulation. In DCS this system also works with any other cooperating aircraft, such as the F-4E, M-2000C, F1, JF-17, F-15E, Harrier and more. Other aircraft, or AI-controlled aircraft fall back to coalition based interrogation, assuming the Tomcats transponder to always be enabled and set to the correct M4 code for the current coalition, ignoring the real state of the transponder.

Self Test operation

To self test Modes 2 and 3, 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, Mode 4, and Mode C switches (6) as directed, along with the Mode 1 and Mode 3 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.

Warning Light

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.

M4 Zero/Hold

The codes for Mode 4 are automatically erased on shutdown, when the IFF system loses power. This can also be commanded by moving the Mode 4 function switch (8) into the ZERO position and holding it there for approximately 5 seconds. Once zeroed, the IFF warning light will illuminate and Mode 4 will not be available during the remainder of the flight.

Codes can be inserted again by the Ground Crew. The HOLD position of the Mode 4 function switch is inoperative on the DSCG variant of the F-4E and was added later for the DMAS only. Holding the switch in that position for 15 seconds will retain the mode 4 codes during the next shutdown, preventing them from automatically getting zeroed.

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 replies are special emergency signals of code 7700.

Interrogator System AN/APX-76

The AN/APX-76 IFF (Identification Friend or Foe) interrogator is integrated into the AN/AWG-9 operation. Then interrogator antenna itself is located on the AN/AWG-9 antenna gimbal platform.

An IFF system works by sending out an interrogation pulse and then listening for returns from cooperating transponders. In addition to the unencrypted civilian mode the AN/APX-76 is capable of interrogating in the encrypted military mode 4. This ensures that targets replying to mode 4 interrogations are indeed friendly.

The AN/APX-76 can be used both in search radar modes and in STT radar modes. To enable interrogation the IFF switch is depressed on the Detail Data Display Panel which then activates the interrogator for as long as the button is held up to 10 seconds max.

When enabled IFF received IFF returns are then overlaid on the normal AN/AWG-9 radar returns on the DDD. A friendly target will be indicated with two bars, one above and one below the normal radar return. As the AN/APX-76 is a secondary mode radar (transponder radar) apart from the AWG-9 the IFF can sometimes also detect targets not detected by the AWG-9. In this case the IFF return will not have a radar echo inside it.

In the search mode this is overlaid over each target replying and in STT over the STT target. Additionally, if the STT target is hooked on the TID the DDD will switch from normal range display to a ±10 NM display to enable display of multiple returns in case of closely grouped targets.

• 1. M4 ALARM OVERRIDE switch

  • Switch disabling the mode 4 tone alarm in the RIO headset.

• 2. TEST-CHAL CC switch

  • Switch spring-loaded to center controlling IFF challenge and test.
    • TEST: Momentary actuation, tests the AN/APX-76 by interrogating own transponder. If the same codes are set, two solid lines appear on DDD at 3 and 4 miles.
    • CHAL CC: Momentary actuation starts a 10-second interrogation cycle, only showing returns with the correct mode and code on DDD.

• 3. CODE selector thumbwheels

  • Thumbwheels controlling mode and code used for interrogation. The first wheel sets the mode, and the last four set the code.

• 4. CHAL light

  • Light indicating active interrogation in progress.

• 5. FAULT light

  • Light indicating a fault in the AN/APX-76.

To challenge friendly or civilian aircraft using the AN/APX-76, the RIO sets the interrogation mode on the first roller-display. It can be set to either OFF, or Mode 1, 2, 3, 4/A or 4/B (3).

The other four digits are used to set the IFF code to interrogate for Modes 1 to 3 (3). The code for Mode 4/A and 4/B is set by the ground personal.

Once setup, interrogation can be initiated by either pressing the IFF button on the DDD or moving the Test/Challenge Switch (2) to the CHAL CODE position.

💡 The Test/Challenge switch will only send a challenge via the AN/APX-76 system, while the Challenge Button includes an interrogation by Combat-Tree, if activated.

The radar screen presents the results of the interrogation with lines around the contacts position:

• line above; the aircraft has a matching transponder mode (likely friendly, or at least neutral)
• line below; the aircraft has a matching transponder code (likely friendly)

Each time the AN/APX-76 is sending an interrogation, the challenge light (4) illuminates.

IFF Displays

Display formats for IFF operation are shown below. During IFF interrogation, the radar continues its normal operations. With the radar in a pulse mode (search, track, or acquisitions), the IFF returns are mixed with the radar video and appropriate pulse radar display formats are displayed on the DDD. The IFF range scale is defined by the radar range scale. If the RIO commands an IFF interrogation during any pulse Doppler single target track or pulse single target track mode (PDSTT or PSTT) or when a target is designated (hooked) on the TID, an expanded IFF range display is generated on the DDD. The nominal target range is displayed at the vertical center of the DDD. The range scale is indicated on the range scale indicator at ±10 and it cannot be changed. The azimuth of the current radar mode is retained in the expanded IFF format. By unhooking the single-target track, normal range vs. azimuth is displayed along with the IFF display.

With the radar operating in the pulse Doppler mode, the DDD presentation is switched to a B-scan format. In this format, the pulse Doppler antenna scan pattern is retained but IFF range sweeps are displayed. A computer-generated symbol representing actual target range is displayed along with the IFF video on the DDD. The IFF scale is indicated on the range indicator. It may be changed by pressing the range-select pushbutton. In the pulse-Doppler, STT mode, the attack symbols are retained and the IFF sweep is displayed at the azimuth of the AWG-9 tracking antenna.

When a target is detected, the RIO must tell the computer whether the target is friend or foe. The APX-76 cannot perform this function. It can only determine if IFF is being utilized by the target.

DDD TWS positive IFF response

(1) Transponder Mode detected.

(2) Transponder Code detected.

(3) DDD Radar Range Gate.

(4) AGC Trace.

DDD STT positive IFF response

(1) STT target azimuth indicator.

(2) STT track with IFF bars. Two bars are shown positive transponder mode and code detected.