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Beam Steering and Attenuator Circuits

The beam steering circuits send the signals to the four antenna elements. During transmit, the beam steering circuit controls the phase relationships of the four RF outputs. The whisper/ shout attenuator makes the output power smaller or larger based on the control signals from the transmitter.

During receive, the phase of the received signals is different in each element. These beam steering circuit sends the signals to the receiver.

The TCAS computer receives a discrete input from the landing gear lever switch. When the landing gear lever is down, the TCAS computer uses all four antenna elements equally for the bottom antenna. Thus, the bottom antenna operates as an omnidirectional antenna.


The TCAS computer has BITE. The BITE continuously monitors TCAS for system faults and interface faults during normal operation. Also, when you start a self-test, the BITE makes test signals and sends them to the signal processor and receiver/ transmitter circuits. The BITE monitors for system faults and interface faults during the self-test.

When the BITE detects a fault through the continuous fault monitor, the fault goes into the fault memory. It also goes to the I/O circuits for output to the CDS DEUs and the FDAU.

When the BITE detects a fault during a self-test, the fault data goes to the I/O and then to these systems:



TCAS computer status LEDs if test is done from computer front panel.


Directional Antenna

The directional antenna has four elements spaced 90 degrees apart. Each element is independent of the others and has it own coaxial connector.







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.

Date: 2016-03-03; view: 541

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Training Information Point | Proximate Traffic and Other Traffic
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