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
