IDENT Function

airspeed-altitude computer. All framing and information reply pulses are 0.45 (±0.1) microsecond wide. All modes are transmitted by the RT at a frequency of 1090 MHz. Mode 1.— The reply pulse train consists of from zero to five information pulses framed by two framing pukes. The spacing between the information pulses is in multiples of 2.9 (±0.05) microseconds from the initial framing pulse. The position where the sixth information pulse would be (17.4 [±0.05] micro- seconds from the initial framing pulse) is not used. There are 32 different codes available for use from the specified five information pulses. Mode 2 and 3/A.— When transmitted, these reply trains contain from zero to 12 information pulses, plus the two framing pulses. The information pulse spacing is in multiples of 1.45 (±0.05) microseconds from the initial framing pulse. The position where the seventh information pulse would be (10.5 [±0.05] microseconds from the initial framing pulse) is normally not used. There are a possible 4,096 different codes available from the specified 12 information pulses. Mode C.— When the Airspeed-Altitude Com- puter is connected in the system, the reply train consists of from 1 to 11 information pulses and 2 framing pulses. The information pulses are spaced in multiples of 1.45 (±0.05) microseconds from the initial framing pulses. The positions where the seventh and the ninth pulses are normally located (10.5 [±0.05] and 13.05 [±0.05] microseconds from the initial framing pulse) are not used. When there is a pulse in the thirteenth position (18.85 [±0.05] microseconds from the initial framing pulse), there is a special position indicator pulse also generated in the train. This pulse is located 24.65 (±0.05) micro- seconds from the initial framing pulse. There is a total of 2,048 possible codes available from the specified 11 information pulses. When the airspeed-altitude computer is not on-line, the reply train will be the framing pulses only. Mode 4.— This reply train is determined by the KIT-1A/TSEC. IDENT Function.— The identification-of- position (IDENT) function is used in modes 1, 2, and 3/A. The IDENT function, which can be selected by the pilot for transmission for approximately 20-second intervals, is used to distinguish between aircraft displaying the same coding. When used in mode 1, the reply pulse train containing the code in use is transmitted twice for each trigger puke received. The second train is 24.65 (±0.05) microseconds from the leading edge of the first framing pulse of the first reply train. The IDENT function in modes 2 and 3/A uses the special position indicator (SPI) pulse. The reply pulse train containing the code in use is followed by a pulse for each trigger received. This SPI pulse is 24.65 (±0.05) microseconds from the leading edge of the first framing pulse of the reply train. Emergency Function.— The EMERGENCY signals are selected by the pilot to indicate an in-flight emergency. The emergency function is used with modes 1, 2, and 3/A. For modes 1 and 2, the reply pulse train containing the code in use is transmitted once for each trigger pulse received, followed by three sets of framing pulse pairs with no information pukes. The three sets of framing pulses are located 24.65 (±0.1), 44.95 (±0.15), 49.30 (±0.20), 69.60 Figure 3-21.-Normal reply pulse characteristics. 3-19