User's Manual

ROCKWELL COLLINS
COMPONENT MAINTENANCE MANUAL with IPL
TDR-94, PART NO 622-9352
(4) Mode S Transponder Replies
(a) Typically, the ground-based interrogator tracks an airplane throughout its assigned
airspace. The mode S transponder equipped airplane responds to mode S
interrogations with the ATCRB S 4096 code and the mode S reply format. During
each scan, the SSR interrogations are in both mode A and mode C. The mode S
transponder reply uses a technique known as pulse position modulation (PPM). This
is illustrated in Figure 13/GRAPHIC 34-50-96-99B-080-A01. A pulse transmitted
in the first half of the interval represents a logic 1, while a pulse transmitted in the
second half represents a logic 0. In addition to the message data contained within
each reply transmission, the m ode S interrogation also contains a 24-bit discrete
address. This uniquely identifies the reply and provides for a large number of
airplanes, each with its own distinct address. The mode S reply format is described
in the following paragraphs.
Mode S, T
ransponder Reply Code Pulse Format
Figure 13/GRA PHIC 34-50-96-99B-080-A01
(5) Mode S Reply Descr iption
(a) The reply pulse pattern is shown in Figure 13/GRAPHIC 34-50-96-99B-080-A01.
The reply data block is for m ed by PPM encoding of the reply data. The first pulse
occurs 128 μs after the start of the P4 interrogation pulse. The mode S transponder
reply to an ATCRBS interrogation is identical to the reply transmitted by a non mode
S transponder (see Figure 7/GRAPHIC 34-50-96-99B-074-A01). However, t he reply
to a mode S interrogation can assume several different formats as summarized
in Figure 14/GRAPHIC 34-50-96-99B-081-A01. The fields are described in the
following paragraphs.
1
Announced Address (AA)
The 24-bit AA field contains the airplane address in the clear (no special
coding) and is used in DF number 11.
2
Altitude Code (AC) Field
The 13 bit AC field contains the altitude code and is used in downlink format
34-50-96
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May 18/06