(±0.25), 73.95 (±0.30), and 94.25 (±0.35) micro- seconds from the leading edge of the initial framing pulse. In mode 3/A, one reply pulse train containing the code 7700 is transmitted, followed by the three sets of framing pulse pairs with no information pulses. The location of these framing pulse pairs is the same as in modes 1 and 2. Special Position Indicator.— The special position indicator (SPI) allows the air traffic controller to differentiate aircraft above or below a given altitude. This puke is present in the reply pulse train in mode C when a D4 pulse (18.85 [±0.05] microseconds from the initial framing pulse) is used. The SPI pulse is located 24.65 (±0.01) microseconds from the initial framing pulse. This pulse is sometimes called the mode C caboose pulse. This SPI pulse is the initial framing pulse for the second reply pulse train.    The SPI pulse is the same pulse used in the mode 2 and 3/A IDENT function. X-Pulse Function.— Special aircraft, such as drones, are identified by the transmission of the X-pulse. This pulse (10.15 [±0.05] microseconds from the initial framing pulse) appears in a normally unused position. This pulse is generated when the IFF transponder control box is modified by grounding a single external control lead. With this modification, all replies in modes 1, 2, and 3/A will include this pulse, along with the normal framing and information pulses. SELF-TEST MODE.— When the TEST switches on the IFF transponder control box are enabled, an interrogation pulse pair is applied from the test set to the receiver-transmitter. Upon receiving these interrogation pulses, the RT replies to the test set. The test set then analyzes the replies for bracket-spacing frequency, power, and antenna circuit VSWR. The test set will give a GO/NO-GO indication on the control box. There is a switch for each of the modes 1,2, 3/A, and C. MONITOR OPERATION.— When the RAD TEST-OUT-MON switch on the IFF transponder control box is set to the MON position, the transponder performance is checked. The test set will monitor the performance by detecting the replies generated in response to external interrogation signals. If a GO condition is detected, the TEST indicator on the transponder control box will illuminate. For a NO-GO condition, the TEST indicator remains off. INTERROGATOR SET AFUAPX-76A(V) The interrogator set is used in conjunction with the aircraft’s radar set. It provides air-to-air modes 1, 2, 3/A, 4, SIF, and IFF mode 4 challenges. The IFF interrogator receives a basic trigger from the radar set, and a delayed radar trigger is returned to the radar after generation of the IFF interrogator trigger. The IFF replies are decoded and combined with the displayed radar return video. The interrogator set uses receiver side-lobe suppression (RSLS) and interrogator side-lobe suppression (ISLS). The video output is developed in the IFF interrogator, and is applied to the radar set for display. Interrogator Set Major Components The interrogator set consists of six major components. These six are discussed in the following text. ANTENNA AS-2719/AP.— The interrogator set uses an antenna that is hard mounted to the radar set antenna The IFF portion of the antenna is imbedded in the upper part of the antenna dish, and consists of 10 IFF elements. INTERROGATOR SET CONTROL C-7383/ APX-76A(V).— The interrogator control box (fig. 3-22) provides operating mode selection of the SIF, or mode 4 code. The control box contains five thumbwheel switches to select the desired interrogation mode, or standby, and the desired code. A momentary two-position toggle switch (TEST/CHAL CC) allows for loop-testing the IFF, or to provide correct code challenge. The loop-testing allows for the interrogation of the onboard transponder set (AN/APX-72) by the IFF interrogator. Correct code challenge enables interrogations for which IFF display pulses are generated if the received Figure 3-22.-C-7383/APX-76A(V) interrogator control box. 3-20