747-400 MAINTENANCE MANUAL
VERTICAL SEE A
STABILIZER TIP
FWD
LIGHTNING 
DIVERTER 
STRIP 
VERTICAL 
STABILIZER TIP 

VOR/LOC ANTENNA  SCREWS  FWD
SCREWS (2 PLACES) CONNECTOR COAX 
A 
VOR/LOC Antenna Installation Figure 401 

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400 MAINTENANCE MANUAL
 ATC SYSTEM - DESCRIPTION AND OPERATION
______________________________________
 1.  General_______
 A.  The Air Traffic Control Radar Beacon System, (ATC system), supplies airplane track, altitude and identification data to an ATC ground station. The ATC transponder responds to an interrogation from an ATC ground station. The ATC system responds to ground station interrogations in one of three different modes: mode-A, mode-C and mode-S. Mode-A and Mode-C provide identification and altitude information. Mode-S provides selective airplane identification and data link capabilities. After a ground station interrogation, the transponder automatically transmits a pulse coded reply signal in one of the above modes. The mode of reply is determined by the mode of interrogation.
 B.  Two ATC systems are installed on the airplane each with its own transponder. Only one transponder operates at a time. A dual ATC control panel provides independent and isolated control for both systems. Control information is sent to either transponder by signal discretes and an ARINC 429 bus. There are two ATC antennas on the airplane. The antennas are shared by the two transponders. Two rf relays switch both antennas from one transponder to the other.
 C.  Each transponder receives altitude data from two air data computers (ADC's) at a time. The altitude data is sent to the transponders by ARINC 429 data buses.
 D.  The transponder transmits a squitter signal, alternating between the top and bottom antennas, approximately every second. The squitter signal is coded with the airplane's mode S address.
 E.  The ATC and DME systems operate in the same L-band frequency range. A suppression circuit is connected between the ATC transponders and the DME interrogators to prevent simultaneous transmissions.
 F.  TCAS equipped airplanes: A suppression circuit is connected between these three systems (ATC, DME, TCAS) to prevent simultaneous transmissions.
 G.  The ATC system operates in three different modes: Mode-A, mode-C and mode-S. The mode of operation is determined by the mode of interrogation from a ground station and the current configuration of the altitude reporting function.
 H.  The left ATC system receives 115-volt ac, 400-Hz power from the standby BUS through the ATC L circuit breaker. The right ATC system receives 115-volt, 400-Hz power from bus No. 2 through the ATC R circuit breaker. Both top and bottom antenna RF transfer relays receive 28-volt dc power through ATC ANTENNA SW circuit breaker. All circuit breakers are located on the overhead circuit breaker panel (P7).
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747-400
MAINTENANCE MANUAL

 2.  ATC Control Panel (Fig. 1)
_________________
 A.  The ATC control panel is located on the aft electronic control panel (P8). The control panel contains switches and controls which provide reply codes and mode selection for two ATC/Mode S transponders. The control panel is divided into two operational sides left (No. 1) and right (No. 2). Each side operates independently of the other. Each side provides its respective transponder with tuning and control signals for system operation.
 B.  The control panel has these switches and indicators:
 (1)  
Four code switches which select the four-digit ATC identification code.

 (2)  
A four-digit code display window which shows the selected identification code.

 (3)  
A transponder select switch which selects the left or right transponder.

 (4)  
An IDENT switch which causes the active transponder to transmit a special pulse identifier with the next ground station interrogation.
　
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