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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
34-45-00-011
TCAS - FUNCTIONAL DESCRIPTION
TCAS - FUNCTIONAL DESCRIPTION
EFFECTIVITY
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TCAS - FUNCTIONAL DESCRIPTION
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BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
General
TCAS transmits these two types of interrogation signals:
* Whisper-shout for ATCRBS transponders
* Mode S.
The TCAS finds and monitors all of the airplanes within range
that have ATCRBS or mode S transponders. The TCAS
operates with its mode S transponders to interrogate traffic
airplanes and calculate if they are a threat. The TCAS does not
track airplanes that do not have a transponder. The TCAS tracks
airplanes with transponders that do not reply with mode C
information. These airplanes send framing pulses in response
to a mode C interrogation. The TCAS uses the framing pulse
replies to give range and bearing to the target. TCAS cannot
give RAs for targets that do not report altitude.
The TCAS can calculate maneuver commands to prevent
possible collisions with target airplanes.
If the traffic airplane also has TCAS, the two airplanes can
agree on maneuvers that give a safe distance between them.
Whisper-Shout Interrogation
The TCAS uses an ATCRBS all-call whisper-shout interrogation
to look for airplanes that have an ATCRBS transponder. The
whisper-shout procedure changes the strength of the
interrogation pulse in many steps. This lets TCAS interrogate
intruders at different ranges. Intruders that reply are put on the
TCAS roll call list and TCAS monitors them.
The airplanes that are near receive the lower power (whisper)
interrogations. The airplanes that are far away do not receive
these low power signals so they do not respond.
The TCAS transmits a suppression pulse as the interrogation
power increases. The airplanes that are farther away receive
the higher power (shout) interrogations. The airplanes that are
near receive the interrogation and the suppression pulse so
they do not reply.
Mode S Interrogation
Mode S transponders transmit a squitter signal once each
second. The signals contain a 24-bit airplane address. The
TCAS listens for mode S squitter signal transmissions from
airplanes that have mode S transponders. When the TCAS
receives an address, it puts that airplane on a roll call list. The
TCAS then interrogates the mode S transponder of the traffic
airplane with the discrete 24-bit airplane address on the roll call
list.
TCAS - BASIC OPERATION
EFFECTIVITY
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
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TCAS Data Calculation
When the TCAS interrogates a target, the reply signal from the
target usually contains the airplane altitude and any information
requested by TCAS.
The TCAS computer uses the round-trip time of the reply signal
to calculate the range and range rate of the target. It uses the
direction of the reply to calculate bearing of the target. If the
TCAS computer receives altitude data, it calculates these
parameters of the traffic airplane:
* altitude
* altitude rate
* relative altitude.
The TCAS computer uses the target data to calculate if the
target is on a possible collision or near collision course with its
flight path. If it is, the TCAS computer calculates traffic
coordination maneuver data. This data and the position data
go to the DEUs to make the TCAS displays. If TCAS advises that
a maneuver be made it also sends an aural command to the
flight crew.
Air-to-Air Coordination
If the target also has TCAS, the two TCAS computers
communicate with each other using the mode S transponders.
Both TCAS computers calculate an escape path if it is
necessary. Both TCAS computers send the escape path to the
mode S transponder in the other airplane. This sets up the airto-
air mode S data link between the airplanes.
TCAS - BASIC OPERATION
EFFECTIVITY
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
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