User's Manual

SYSTEM DESCRIPTION AND INSTALLATION MANUAL
T
3
CAS/Part No.9005000
2 Air to Air Communication
Traffic Alert and Collision Avoidance System and Mode S
Transponders are an integral part of the Traffic Alert and Collision
Avoidance System (TCAS). TCAS-equipped aircraft are airborne
interrogators, communicating with other TCAS-equipped aircraft
through their Mode S Transponders for coordination of collision
avoidance maneuvers. TCAS aircraft acquire other Mode S
Transponder-equipped aircraft by receiving squitter transmissions
(unsolicited All-Call type replies, transmitted pseudo-randomly every
0.8 to 1.2 seconds), and thereafter by special addressed
interrogations. Although either Mode S or ATCRBS Mode C
Transponders aid TCAS-equipped aircraft in avoiding collisions,
coordination of collision avoidance maneuvers is possible between
two aircraft only if both are Mode S and TCAS equipped.
3 Mode S Messages
a Interrogation and Reply Formats
Mode S features have been added to the ATCRBS already
in place. This procedure ensures that the older airborne
transponders and the ground-based interrogators used in the
ATCRBS are still functional. The Mode S signal formats
used for this combined system operation are ATCRBS/Mode
S All-Call [Mode A and Mode C], Mode S interrogation,
Mode S SLS, and Mode S reply.
b Interrogation Pulses and Timing
The uplink Mode S format for the interrogation pulse group
consists of pulses designated P1, P2, and P3. The time
spacing between the P1 and P3 pulses determines the type
of interrogation (Mode A aircraft identification or Mode C
altitude reporting). Pulse P2, which follows P1 by 2
microseconds, is used for Side Lobe Suppression (SLS) in
the ATCRBS. The amplitude of P2 is recognized by the
airborne transponder as either a main beam or SLS
interrogation. With Mode S interrogation, the basic P1 and
P3 pulse system is extended to include a P4 pulse, which
follows P3 by 2 microseconds. The P4 pulse uses the same
spacing as between P1 and P2. However, P4 has an
additional feature in that its pulse width can be either 1.6
microseconds or 0.8 microseconds, whereas the P1 and P3
pulse widths are always 0.8 microseconds.
c Replies to All-Call Interrogations and Pulse Width
In operation, when a standard ATCRBS Transponder
receives this interrogation of P1, P3, and P4 pulses, it
responds with the ATCRBS reply, which consists of 14
pulses that carry the identity code or the altitude code. The
P4 pulse is ignored since the ATCRBS Transponder circuit is
designed so that it does not recognize the P4 pulse. The
response is dependent upon the presence and length of P4.
When a standard P1 and P3 interrogation is received from
Pub. No. 8600200-001, Revision 004
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04 Nov 2014
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