AVIATION ELECTRICITY & ELECTRONICS--RADAR

CHAPTER 2 IDENTIFICATION FRIEND OR FOE (IFF) SYSTEMS INTRODUCTION With the destructive power of modern weapon systems and the speed of modern weapon delivery systems, it is not always practical to wait until a detected radar target has been identified by visual means before preparing for battle. Therefore, a means of identifying friendly targets from enemy targets at long range is necessary. It was this need that brought about the advent of the IFF system. The IFF system provides automatic identification friend or foe (IFF) of an air or surface vehicle to all IFF interrogator-equipped aircraft, ships, and ground forces within the operating range of the system. IFF systems also provide other information, such as type of craft, squadron, side number, mission, and aircraft altitude. GENERAL THEORY OF OPERATION LEARNING OBJECTIVE: Recognize gen- eral operating principles, components, charac- teristics, and maintenance concepts of typical airborne IFF systems. IFF completes the identification process in three basic steps: (1) challenge, (2) decode, and (3) reply. A simplified block diagram of a typical IFF system is shown in figure 2-1. The system is comprised of an interrogator (challenging unit) and a transponder (reply unit). The interrogator interfaces physically with the radar system in which its operations (transmissions and receptions) are synchronized. When radar contacts of interest are interrogated, the transponder unit of the interrogated aircraft replies to the challenges and provides the proper coded response, which, in turn, indicates the challenged contact is friendly. Responses not incorporating the proper identification codes are interpreted as coming from hostile units. All naval aircraft contain a transponder set; however, an interrogator set is installed only in platforms that have a need to interrogate and identify aircraft from a distance for tactical purposes. 2-1 Note: Interrogation plat form also contains a transponder system. Antenna Array Computer Decoder Radar Display Control Panel Receiver Transmitter INTERROGATOR Antenna Array Computer DecoderReceiver Transmitter TRANSPONDER Coded Challenges Coded Replies AE/ATf2-01 Figure 2-1.—IFF system block diagram.

INTERROGATION (CHALLENGE) The interrogator is a pulse-type transmitter that is triggered by the coder synchronizer. The coder synchronizer is synchronized to the radar system so that reception of the IFF response and radar echo signals cannot occur simultaneously. The output (challenge signal) of the interrogator is different for different modes of IFF operation. As long as the aircraft transponder is operating in the correct mode, it will receive these interrogation signals and will transmit back to the interrogator the proper coded reply pulses. The IFF reply pulses are sent via the coder synchronizer to a display indicator, which is part of the challenging units radar system. The coded replies from a friendly craft normally appear as dashed lines just beyond the target blip, as shown in figure 2-2. Interrogation Modes and Codes There are presently five modes of IFF operation used by naval aircraft; an additional mode is available for loop testing. These modes are designated as Modes 1, 2, 3/A, C, Mode 4 (A or B) and Test. Modes 1, 2, 3/A, and C, and Test use two interrogation pulses, 0.8 ± 0.1 µs wide with pulse spacing as shown in table 2-1. Table 2-1.—IFF Pulse Spacing MODE SPACING 1 2.9 to 3.1 µs 2 4.8 to 5.2 µs 3/A 7.8 to 8.2 µs C 20.8 to 21.2 µs TEST 6.3 to 6.7 µs Each mode of operation obtains a specific type of information from the craft that is being challenged. Modes 1 and 2 are exclusively used by the military as tactical modes for mission and identification purposes. Mode 3/A is the standard air traffic control mode used by military and civilian air traffic control stations. It is used in conjunction with the automatic altitude-reporting mode (Mode C), which reports the aircraft’s altitude as provided by the onboard air data computer. Mode 4 is a military encrypted mode, which is classified and requires the use of a transponder computer. Only interrogators and transponders using the same encrypted codes can communicate with each other. Additionally, you can use system testing to check for proper system operation when completing a loop test. Interrogation Pulse Characteristics The interrogation pulse characteristics for the various modes of IFF operation are shown in figure 2-3. These pulses are transmitted at a frequency of 1030 MHz, and are recognized by the transponder through pulsewidth and spacing. Modes 1, 2, 3/A, C, and Test use two interrogation and one side-lobe suppression (SLS) pulse, each respectively 0.8 µs wide. The side-lobe suppression pulse occurs at 2 µs after the leading edge of the first interrogation pulse in each case. The SLS pulse allows the transponder to accept the main lobe and to reject minor lobe signals from interrogating stations. This ensures proper operation of the system. Mode 4 interrogation pulses consist of four pulses plus a SLS pulse. Each pulse is between 0.4 and 0.6 µs wide, and occurs in multiples of 2.0 µs intervals from the leading edge of the first pulse. The four pulses may be followed by as many as 32 additional pulses spaced as close as 2 µs apart. The fifth pulse, which is the SLS pulse for Mode 4, is spaced 8 µs from the leading edge of the first interrogation pulse used. 2-2 Mode 1 Reply Mode 2 Reply Radar EchoUnfriendly or Unequipped Craf t Mode 3 / A Reply I / P Reply Mode 1, 2 or 3 / AEmergency Reply Mode 1, 2 or 3 / A AE?ATf02-02 Figure 2-2.—Typical radar display showing several IFF responses.

TRANSPONDER (REPLY) The IFF transponder is a receiver/transmitter combination, which automatically replies to coded challenges when Mode decoding is satisfactory. The receiver section identifies the interrogation Mode by decoding the spacing between P 1 and P3 (for Modes 1, 2, 3/A, C, and Test) and comparing P 2 (SLS) with the main lobe pulses P1 and P3. If the SLS control pulse is at least 9 dB less than P1 or P3, the interrogator pulse train is evaluated and decoded as being in a main lobe, which automatically keys the transmitter section to send a prearranged reply signal at a frequency of 1090 MHz. If not, it is evaluated and decoded as being in a side lobe, and the transponder system disables any reply response to such an interrogation. Modes 1, 2, 3/A, and Test IFF replies use selective-identification-function (SIF) encoding. SIF/IFF codes are octal numbers that consist of three binary bit positions. A standard SIF reply (fig. 2-4) has 2-3 * 01 2 3 4 5 6 7 8 91 0 1112 13 14 15 1617 1819 20 21 22 23 24 25 26 Time (In ) s Mode 1 P3 P3 P3 P3 P3 Mode 2 Test Mode Mode 3/A Mode C Mode 4 - Side lobe suppression pulse (SLS). (1) - Not present when testing with simulated input. (2) - SLS pulse may be followed by as many as 32 pulses. Spaced at 2 s intervals. P1 P2 P P P P P P 1 1 1 1 1 3 P P P P P P 2 2 2 2 2 4 * * * * * * (1) (2) AE/ATf02-03 Figure 2-3.—IFF interrogator pulse characteristics. 01 2 3 4 5 6 7 8 91 0 1112 13 14 15 1617 1819 20 21 22 23 24 25 26 Time (In ) s Mode 1 Mode 2, 3/A and Test Mode C Mode 4 Note: F and F pulses are reply framing pulses. * Reply occurs between 200 and 270 us after a mode 4 trigger F F F C C F F F A A A C C A A A C C B B B D B B B B B D D D D 1 1 1 1 1 2 2 2 A A A P4 4 * 1 1 1 2 2 2 2 2 4 4 4 4 4 1 1 1 1 1 2 2 2 2 4 4 2 2 4 4 1.75 us 1.75 us AE/ATf02004 Figure 2-4.—IFF transponder reply pulse characteristics.

provisions for four octal characters (A, B, C, and D) and is bounded by two framing (bracket) pulses (F1 and F2). The seventh bit position, between bit position A and B, is designated X. Bit position X is not part of any octal character and is logical 0 in any standard reply. Mode 1 uses only octal characters A (A 4-A2-A1) and two least significant bit positions of character B (B 2-B1) to represent a code range from 00 to 73 (rendering 32 possible combinations). Modes 2 and 3/A use all four octal characters (A, B, C, D) to represent a code range from 0000 to 7777 (4,096 possible combinations are capable). Mode 1 is set by two pushbutton switches or thumbwheels (depending on the model of IFF system being used), Mode 3/A is set by four switches, while Mode 2 (four switches) is preset prior to flight by maintenance personnel. Mode C reply uses the same standard format as a SIF reply, except that bit position D 1 is not used, which allows for a total of 2,048 possible codes. Mode 4 replies are generated by the transponder computer and sent to the transponder receiver/transmitter for RF transmission as three-pulse, time-coded reply. A valid Mode 4 exists if the Mode 4 reply rate and transmission are satisfactory. Mode 4 reply pulse coding is accomplished by crypto equipment and is classified information. This information can be found in technical manuals for the KIT-1/TSEC equipment. The operation of the interrogated aircraft's transponder (mode of operation and specific codes, etc.) are controlled by a panel (depending on IFF system) similar to the one shown in figure 2-5. This panel contains switches, code selectors, lights, and operational modes. MASTER Control Switch The MASTER control switch is a four position rotary switch used to select the operating condition. In the STBY position, power is applied to the IFF coder, but interrogations are inhibited. In the NORM position, the transmitter-receiver is enabled for normal sensitivity. In the EMER position, the IFF coder transmits emergency replies to interrogations in Modes 1, 2, or 3/A. The Mode 3/A emergency reply includes code 7700. With EMER selected, Mode 4 is enabled regardless of the position of the Mode 4 switch, and Mode C continues to function normally if selected. To select the EMER position, pull the control outward and rotate to the EMER position. IDENT/OUT/MIC Switch The IDENT/OUT/MIC switch is a three-position toggle switch that is spring-loaded to return to the OUT position, which controls the transmission of the Identification of Position (IP) pulse. It is used in conjunction with Modes 1, 2, and 3/A to identify a specific friendly aircraft among other friendly aircraft that reply with the correct code. When the switch is 2-4 DIMPR E S S TO TEST DIMPRE S S TO TEST DIMPRE S S TO TEST A B HOLD ZERO EMERNORM STBYOFF TEST AUDIO REPLY OUT OUT O N O N O N L I G H T MODE 3/AMODE 1 OUT RAD TEST STATUS ALT KIT ANT IDENT O U T A N T M-1 M-2 M-VA M-C TEST OUT CODE N O OO GO MIC D I V TOP MASTER BOT TEST/MONTEST I F F MODE 2 MODE 2 4 1 747 1 MODE 4 AE/ATf02-04 Figure 2-5.—Typical IFF transponder set control box.

momentarily set to the IDENT position, the IFF coder adds an identification of position response to Modes 1, 2, and 3/A for 15 to 30 seconds. In the MIC position, the identification of position function is activated for 15 to 30 seconds whenever the microphone switch is actuated. In the OUT position, the IDENT reply is disabled. Modes 1, 2, and 3/A Code Selectors Four eight-position push-button switches are provided for selection of Modes 2 and 3/A, while Mode 1 has one eight-position and one four-position push-button switch for selection of a particular code to be transmitted. The Mode 2 reply code setting is set by the maintenance person prior to flight by loosening the Mode-2 screws on the front panel of the transponder receiver/transmitter and sliding the plate up. The proper code is then set by using the four Mode 2 push-button switches. The plate is then returned to its normal position and the screws are retightened. Mode-Selector Test Switches and Lights Four three-position toggle switches, labeled "M-1," "M-2," "M-3/A," and "M-C", are used for selecting the proper mode of operation. Placing anyone of the switches to ON selects that mode for transmission. By placing the switch to OUT, it de-selects the mode of operation; and by placing the switch to TEST, it initiates the built-in test (BIT) operation for that mode. As a result, a Mode 1, 2, 3/A, or C test enable signal is applied to the bit control circuits in the transponder-transmitter, which enables generation of a test signal of proper parameters. If the parameters are found satisfactory, the TEST GO indicator light will illuminate on the transponder receiver/transmitter control panel. An error causes the TEST/MON NO-GO indicator light to illuminate, indicating a malfunction has occurred. RAD TEST/OUT Switch The RAD TEST/OUT switch is a two-position toggle switch that is spring-loaded to the OUT or (down) position. When RAD TEST is selected, the system responds to TEST mode interrogations from a test set during ground maintenance testing. Mode 4 Operation The Mode 4 controls and indicator light are grouped across the center of the control panel. The MASTER rotary switch controls transponder operation in Mode 4 as well as in the other modes of operation. Mode 4 will operate normally, when selected, in either the NORM or EMER position. The Mode 4 function will be inoperative in either the STBY or OFF position. With the transponder functioning, Mode 4 operation is selected by placing the 3-position Mode 4 TEST-ON-OUT toggle switch to ON. Placing the switch to OUT disables Mode 4 operation, and placing it to TEST enables the BIT operation. The Mode 4 CODE control selects either of the two (A or B) Mode 4 codes that are stored in the classified transponder computer. The rotary switch has two additional positions, HOLD and ZERO. At a designated daily time and/or prior to the day's mission, maintenance personnel electronically load the Mode 4 codes into the transponder computer. The present code period is placed in position A, and the code for the succeeding code period is placed in position B. Both code settings will automatically zeroize when power is turned off or lost after the landing gear interlock switch supplies a ground signal to the computer. Activating the HOLD function will retain the code settings. This is normally done while the aircraft is landing, and before power is removed from the transponder. Place the Mode 4 CODE control to HOLD, and release. Allow transponder power to remain on for at least 15 seconds, and then turn it off. The code setting will be retained when aircraft power is turned off. With aircraft power on and the MASTER rotary switch in any position except OFF, both code settings can be zeroized at any time by placing the CODE switch to the ZERO position. Both code settings will also be zeroized if the HOLD function has not been properly actuated before the MASTER switch is turned to OFF. (Inadvertent selection of OFF is prevented by switch design, which requires that the rotary knob be pulled out before it can be turned to OFF.) When the CODE switch is placed in the A position, the aircraft transponder will respond to Mode 4 interrogations from an interrogator using the same code setting as that set into the aircraft's code A position. In the B position, interrogations from an interrogator using the same code setting as that set into the aircraft's code B position will be answered. The changeover time from code A to code B use is operationally directed. The Mode 4 AUDIO/LIGHT/OUT switch selects the aircraft indication for Mode 4 replies. In the LIGHT position, the Mode 4 REPLY lamp will light when a correct Mode 4 reply is transmitted. In the AUDIO position, an IFF transponder caution tone will be heard 2-5

in the pilot’s headset if an unidentified Mode 4 interrogation is received. In the OUT position, both light and audio indications are inoperative. Transponder Special Reply Functions The transponder may transmit replies in four special modes: the emergency mode; the identification of position (I/P) mode; the X-pulse mode, which is used in Modes 1, 2, and 3/A; and the altitude special position identification (SPI) used with Mode C replies. Mode 4 is not affected by the special reply functions. Refer to figure 2-6 for the special replies generated for these special features. EMERGENCY REPLY.—Setting the MASTER selector switch to EMER controls the emergency mode of operation of the transponder. This condition is initiated when the operator has an onboard emergency or has ejected from the aircraft. The emergency function affects the operation of Modes 1, 2, and 3/A. In Modes 1 and 2, the reply pulse train containing the code in use is transmitted once for each interrogation pulse received, followed by three sets of framing pulses and no information pulses. For each interrogation pulse received in Mode 3/A, one reply pulse train containing the code 7700 (regardless of the Mode 3 control dial settings) is transmitted, followed by three sets of framing pulses and no information pulses. The framing pulses are spaced in the same manner as Modes 1 and 2. During the emergency function the IFF transponder system replies to interrogations in Modes 1, 2, and 3/A automatically, regardless of which mode is enabled on the receiver/transmitter. By referring back to figure 2-2, the interrogating source radar screen should show four display bars or markers behind the target instead of just one row as in normal operation. IDENTIFICATION OF POSITION (I/P).—The I/P function is controlled by the IDENT/OUT/MIC switch and affects the operation of Modes 1, 2, and 3/A. Upon radio request from an interrogating source, the I/P operation is selected by placing the transponders IDENT-OUT-MIC switch momentarily to IDENT, which forces the reply into an I/P format. In Mode 1, the reply pulse train containing the code in use is transmitted twice, instead of once, for each interrogation pulse received. In Modes 2 and 3/A, the reply pulse train containing the code in use is transmitted once, followed by a special position indicator (SPI) pulse for each interrogation pulse received. The result of the extra pulse train in Mode 1 and the SPI pulse in Modes 2 and 3 can be seen in figure 2-6. At the interrogating source radar screen, an IFF reply display with an I/P reply is differentiated from standard IFF reply displays by two display bars or markers appearing behind the target, as shown in figure 2-2. This mode is useful when the desired aircraft is in the vicinity of other aircraft using the same coding. X-PULSE.—The X-pulse appears following the initial framing pulse in the normally unused position of Modes 1, 2, and 3/A. It can only be obtained by modifying the controls. When modified, all replies in Modes 1, 2, and 3/A will include this pulse, along with the normally selected information reply pulses, between the two framing pulses. To date, the use of this pulse has been used to identify “special” status, such as an unmanned or experimental aircraft. 2-6 0 1 2 3 4 5 10 30 35 40 4515 50 55 60 6520 70 75 80 8525 90 95 Time (In ) s Mode 1&2 F3 F3 F3 F4 F4 F4 F5 F5 F6 F6 F7 F7 F8 F8AB4 4 X SP1 SP1 Mode 3/A Mode 1 Normal Reply Normal Reply Normal Reply Normal Reply Normal Reply ReplyNormal Mode 2 & 3/A X Pulse Mode 1, 2& 3/A SP1 Mode C F1 F F F F F 1 1 1 1 1 F2 F2 F2 F2 F2 D4 F2 AB1 1AB2 2 Identification of Position Emergency AE/ATf02-06 Figure 2-6.—Special reply pulse characteristics.

SPECIAL POSITION IDENTIFICATION (SPI) PULSE.—The SPI pulse is generated whenever aD 4 pulse is used in a Mode C reply. This pulse is in the same position as the initial framing pulse of a second reply train, F 3. The SPI mode is used with Mode C replies to assist the operator at the challenging station to segregate aircraft above or below an altitude of approximately 31,000 feet. IFF MAINTENANCE CONCEPTS IFF systems are designed for rapid recognition of hardware failures, ease in replacement of the unit, accuracy of fault isolation, and ease of board and module replacement and repair. Built-In-Test (BIT) circuits are used to verify system readiness and perform fault isolation to the failed Weapons Replaceable Assembly (WRA). Corrective maintenance consists of trouble- shooting, removal, and replacement of WRAs. The BIT features are used to perform fault isolation to failed WRA in most cases. External confidence checks are performed using the AN/APM-424(V)2 (Star Wars Gun) transponder test set, shown in figure 2-7. This test set performs a radiated test on the aircraft transponders system to verify proper operation of Modes 1, 2, 3/A, C, and 4. When a failure in the transponder system occurs, the test set displays a coded digit to assist the operator in fault isolation of the system. Q2-1. What does the acronym IFF stands for? Q2-2. What are the three basic steps in the iden- tification process of a typical IFF system? Q2-3. True or False. All naval aircraft contain an interrogator system. Q2-4. What is the pulse spacing of Mode 1? Q2-5. Of the six modes of IFF operation, which one is used by military and civilian aircraft control stations? Q2-6. What mode IFF is the military encrypted mode? Q2-7. What is the transmission frequency of the interrogator? Q2-8. In a typical IFF/SIF system, what total number of codes can be selected in Mode 2? Q2-9. The Identification of Position (I/P) pulse is used for what purpose? Q2-10. What mode of IFF is set prior to takeoff by maintenance personnel? TRANSPONDER AND INTERROGATOR SETS LEARNING OBJECTIVE : Recognize the components of the Transponder Set AN/APX-72, Interrogator Set AN/APX-76(V), and Transponder Set AN/APX-100(V). Components of Transponder Set AN/APX-72, Interrogator Set AN/APX-76, and Transponder Set AN/APX-100(V) are discussed here. TRANSPONDER SET AN/APX-72 The AN/APX-72 transponder set generates and transmits a 1090 MHz reply signal only upon receipt and successful validation of an interrogation signal. Although the IFF transponder system is capable of responding to interrogations in any of the five modes of operation, it responds only if the operator has manually enabled a particular mode. The five major components of the AN/APX-72 are discussed here. RF TRANSMISSION LINE SWITCH SA-1769/A. The RF transmission switch (fig. 2-8) 2-7 AE/ATf02-07 Figure 2-7.—AN/APM-424(V)2 transponder test set. ANT 2 J4 J3 J1 J2 COM ANT 1 AE/ATf02008 Figure 2-8.—SA-1769/A RF transmission line switch.

couples interrogations and replies between two UHF L-band antennas and the transponder set. The switching between the antennas operates at approximately 38 Hz. This switching action prevents the antenna system from being blanked out during aircraft maneuvers. TRANSPONDER SET CONTROL C-6280/APX-72. The IFF control box (fig. 2-9) contains controls and indicators used for system operation and manual testing. Reply codes for Modes 1 and 3/A are selected with panel-mounted thumbwheel switches. These codes can be inserted or changed during flight, while Mode 2 and Mode 4 codes must be preset before takeoff. RECEIVER/TRANSMITTER RT-859/APX-72. The receiver/transmitter (fig. 2-10) will transmit a reply when RF interrogations are received from an IFF interrogator. The unit receives coded interrogations at 1030 MHz and transmits code replies at 1090 MHz. The transponder is capable of operating in five modes and superimposing special signals on the mode replies. Thumbwheel switches mounted on the front panel are used to preset code for Mode 2 operation prior to flight. TRANSPONDER TEST SET TS-1843/APX . The test set (fig. 2-11) provides on-ground or in-flight transponder system test capability. During normal operation it routes interrogations and reply pulses between the receiver/transmitter and the line switch. During manual test mode it generates interrogation test pulse pairs when Mode 1, 2, 3/A or C tests are enabled. These tests are selected from the transponder set control. Transponder reply codes are monitored and evaluated by the transponder test set. When the reply code pulses fall within specified limits, a monitor go signal is established, which causes the TEST monitor indicator on the control box to light. When reply code pulses do not meet proper specifications, a NO GO condition exists and causes the TEST monitor indicator to stay dark, indicating an error has occurred. COMPUTER KIT-1/TSEC . The transponder computer, figure 2-12, allows the IFF transponder to respond to Mode 4 interrogations. When enabled, the transponder computer decodes Mode 4 interrogations. If the interrogation contains the correct code, a coded Mode 4 reply is generated and routed to the transmitter-receiver for transmission. Mode 4 operating codes are classified and are inserted in the computer with an electronic keying device prior to takeoff. Q2-11. Where are Modes 1 and 3/A selected on the AN/APX-72 transponder set? Q2-12. What part of the APX-72 system provides for ground or in-flight transponder testing capability? Q2-13. The AN/APX-72 generates a signal of what frequency when interrogated? 2-8 EMER NORMSTBY OUT ON A B HOLD ZERO CODE O U T AUDIO LIGHT O U T IDENT O U T MIC MASTERTESTREPLY I F F MODE 4 O N O N M-1 M-2 M-VA M-C TEST OUT OUT OUT OUT MODE 1 MODE 3/A OFF MON RAD TEST AE/ATf0209 Figure 2-9.—C-6280/APX-72 transponder set control.

INTERROGATOR SET AN/APX-76(V) The interrogator system works in conjunction with the radar unit. The system transmits (interrogations) at 1030 MHz to transponders of airborne or surface crafts. Coded replies are received at 1090 MHz from the target transponders and processed by the receiver/transmitter, which is the major component of the IFF interrogator set. Upon receipt of coded reply signals the system 2-9 115V 5A 5A 5A MODE 2 ELAPSED TIME HOURS X 1000 SPARE 28V 0 1 7 0 RT-859/APX-72 28VDC.OR 115V. 10. 400-L28VDC JI POWER CONTROL & VIDEO ANT. J5 PRESS VALVE FUSE FUSE FUSE A B AE/ATf02010 Figure 2-10.—RT-859/APX-72 receiver/transmitter. 1J1 POWER 1J3 ANTENNA FRONT VIEW VSWR D8 RF OUT -DBM -(DBM + 13) = -DBV (50-OHM LOAD) RF IN DBW REAR VIEW Ae/Atf02011 Figure 2-11.—TS-1843/APX test set. AE/ATf02012 Figure 2-12.—KIT-1/TSEC transponder computer.

determines whether the target is friendly, hostile, or unknown. The IFF interrogator set is capable of providing challenges in five different modes (1, 2, 3/A, and C) and classified mode 4 (code A and B). In addition, the system incorporates interrogation sidelobe suppression (ISLS) and receiver (RSLS) functions to provide a narrow bandwidth. This permits selective aircraft interrogation in a narrow sector of the aircraft path and also aids in anti-jamming. Additionally, the IFF interrogator system includes a self-checking performance monitoring and fault isolation capability. The interrogator set consists of five major components. These five components are discussed in the following text. INTERROGATOR SET CONTROL PANEL C-7383/APX-76(V). The interrogator control panel, often referred to as the control box (fig. 2-13), provides selection of interrogation modes (1, 2, 3/A, 4A, or 4B) or STANDBY , and allows for SIF reply codes for Modes 1, 2, and 3/A operation. A momentary two-position toggle switch (TEST/CHAL CC) allows for loop testing the system, or to provide correct code challenge. The loop testing allows for the interrogation of the onboard transponder set by the IFF interrogator. Correct code challenge enables interrogations for which IFF display pulses are generated if the received SIF reply code is identical to the code switch settings. There are two indicators (FAULT and CHAL) that indicate the operating status of the system. The Mode 4 alarm may be overridden by using the toggle switch M4 ALARM OVERRIDE. RECEIVER/TRANSMITTER RT-868/APX-76(V). The receiver/transmitter (fig. 2-14) is capable of transmitting challenges in five different modes (1, 2, 3/A, C and 4). It converts these modes to modulation pulses, and then uses these pulses to modulate the internally generated 1030 MHz carrier to provide IFF interrogation pulses. Coded replies from target transponders are received and used to determine target nature (friendly, hostile, or unknown). The propagation delay between challenge pulses and receipt of coded replies provides a means of determining target range. SWITCH-AMPLIFIER SA-1568/APX-76(V) . The switch-amplifier (fig. 2-15) switches the 2-10 000 0 M4 ALARM OVERRIDE FAULT CHAL CODEMODE TESTA 1 P R ES S TOTE ST P R ES S TOTE ST A CHAL CC AE/ATf02013 Figure 2-13.—C-7383/APX-76(V) interrogator control box. XMTR RCVR VID FAULT PUSH TO TEST RF MON SUM ANT DIFF ANT J3 J4 -20.DB POWER ADJ J6 ON ON OFF OFF GTC AC DC TOTAL HRS RCVR GATESIGNAL J2 J1 POWER AND CONTROL TEST 1 2½ AE/Atf02014 Figure 2-14.—RT-868/APX-76(V) receiver/transmitter. AE/ATf02015 Figure 2-15.—SA-1568/APX-76(V) switch-amplifier.

interrogator's RF output from the sum antenna channel to the difference antenna channel for the duration of the ISLS pulse. During this time, the output is amplified by 4 to 7 dB. This amplification provides the required antenna output characteristics. Additionally, the unit contains internal performance monitoring circuits that perform self-tests for each challenge period. If a malfunction is determined, the unit will generate an interrogator switch-amplifier fault signal. ELECTRONIC SYNCHRONIZER SN-416/APX-76(V). The synchronizer (fig. 2-16) contains performance monitoring circuits that check the timing triggers and pre-triggers, gates, and a test video. If any of these signals are incorrect, the synchronizer generates a fault signal, which causes the FAULT flag indicator to go red. The synchronizer fault is also applied to the fault indicator circuit. The fault indicator circuit also receives status inputs from the interrogator receiver/transmitter, switch-amplifier, and the interrogator computer (KIR-1/TSEC). If any of the inputs reflect a NO-GO status, the fault indicator circuit generates a fault indication signal to light the FAULT indicator lamp on the interrogator control. COMPUTER KIR-1/TSEC . The interrogator computer is used for Mode 4 security and decoding for secure operation. Mode 4 A or B codes are loaded prior to flight by maintenance personnel, who use special electronic keying devices. Q2-14. The interrogator works in conjunction with what other aircraft system? Q2-15. What component of the interrogator set is used to select SIF reply codes for Modes 1, 2, and 3/A? Q2-16. What component monitors the timing triggers, pre-triggers, gates, and test video and causes the fault indicator lamp on the interrogator control to illuminate when a fault has been detected within the system? TRANSPONDER SET AN/APX-100(V) The AN/APX-100 transponder set generates and transmits a 1090 MHz reply signal only upon receipt and successful validation of an interrogation signal similar to the AN/APX-72 transponder. This system uses two antennas (upper and lower) to receive interrogation signals. After the two antennas receive the signal they are routed through the RF distribution subassembly to the top and bottom receivers respectively. Each of the transponders’ receivers convert the RF signal into two separate video signals. Both videos are then compared in the diversity processor, and the stronger of the two signals is used to provide the coder assembly with digital pulses. The decision made by the processor as to which is the stronger video is indicated to the RF distribution subassembly so that it may cause the transmitted reply to be radiated from the same antenna. The coder assem- bly decodes the digital pulses by using a digital delay line, which identifies the spacing between the pulses and recognizes the interrogation mode that has been received. For all modes, with the exception of Mode 4, settings of the switches on the control panel cause a reply train in the appropriate mode to be encoded by the coder assembly and sent to the modulator. If decoding identifies the interrogation received as Mode 4 and the control panel has enabled Mode 4, the signal is sent to the Mode 4 module were it is then passed to the transponder computer. The resulting Mode 4 reply is then returned to the IFF transponder and then sent to the modulator. If the IFF transponder is unable to respond to the Mode 4 interrogations, an audio tone will be generated in the pilot’s headset and the IFF caution light will illuminate. The modulator/transmitter subassembly generates the RF reply pulses. These pulses are routed by the RF distribution subassembly to the upper or lower antenna depending upon the diversity decision made by the processor. The transponder set comes in two configurations. One is of the panel (or console) mounted construction; while the other is a rack (electronic bay) mounted that requires a remote, panel mounted control, and a MT-4811, electrical mounting base. These components are discussed briefly in the following text. 2-11 AE/ATf02016 Figure 2-16.—SN-416/APX-76(V) synchronizer.

RECEIVER/TRANSMITTER RT-1248/APX-100. The IFF transponder set RT-1248/APX-100 (fig. 2-17) is of the panel-mounted configuration and consists of a receiver/transmitter/control contained in a single weapon replaceable assembly. The system receives, decodes, and replies to interrogation by challenging aircraft, ships, or shore installations. Modes of interrogation include aircraft identification, flight status, and altitude. The transponder responds to normal IFF challenges or SIF challenges. When operated in conjunction with the KIT-1/TSEC computer, the IFF transponder set will receive interrogations and transmit replies in secure Mode 4. RECEIVER/TRANSMITTER RT-1157/APX-100. The RT-1157/APX-100 is referred to as the rack-mounted configuration, which consists of three separate assemblies, as shown in figure 2-18. The system has the same operating characteristics as the RT-1248/APX-100, but was designed for use in different aircraft platforms. The major difference with this system is that the control box is located away from the receiver/transmitter. ANTENNA AS-3557/A . The AS-3557/A (fig. 2-19) is a dual element blade antenna used for the IFF and VHF/UHF communication systems. The IFF system uses the L-band elements of the upper and lower forward communication antennas. Figure 2-20 displays where these antennas are typically located on the 2-12 MODE 2 MODE 2 TEST MODE 4 /MON MASTER TOP TEST TEST M-VA M-C VAC AUDIO REPLY IDENT ALT KIT ANT I F F N O G O A N T D I V AE/ATf02017 Figure 2-17.—RT-1248/APX-100 receiver/transmitter. MODE 2 MODE 2 TEST MODE 4 /MON MASTER TOP TEST TEST M-VA M-C VAC AUDIO REPLY IDENT ALT KIT ANT I F F N O G O A N T D I V J2 J1 ANT MODE 2 J3 J4 TT BOT ANT AE/ATf02018 Figure 2-18.—RT-1157/APX-100 receiver/transmitter.

aircraft. The ANT SEL-IFF switch on the ANT SEL control panel assembly controls the antenna selection. Setting ANT SEL-IFF switch to BOTH enables diversity (automatic antenna selection) operation. Diversity operation compares signal strength received from both antennas and selects the antenna that has the greater signal strength. Selecting UPPER or LOWER forces the IFF system to operate on the antenna selected. Q2-17. The APX-100 transponder set is available in what number of configurations? Q2-18. To respond to Mode 4 interrogations, the IFF transponder set system requires what addi- tional components? Q2-19. What APX-100 system is referred to as the rack-mounted configuration? 2-13 Figure 2-19.—Antenna AS-3557/A. Figure 2-20.—Typical IFF antenna position on F/A-18A/B/C/D.