
ATC TRANSPONDERS
The ATC transponder system is part of the air
traffic control radar beacon system (ATCRBS).
The aircraft transponder provides aircraft
range, altitude, and identification informa-
tion to ground radar controllers. Although the
aircraft is equipped with dual transponders,
only one may be used at a time.
The transponder system provides ATC ground
radar controllers with either mode A, C, or S
information. Mode A provides controllers with
aircraft identification. Mode C provides baro-
metric altitude and identification. Mode S
provides ground controllers and traffic and
collision avoidance system (TCAS) equipped
aircraft location, altitude, and aircraft identi-
fication information.
The dual transponder system consists of an
antenna, a transponder, an air data computer,
and a control unit (Figure 16-8). The anten-
nas are mounted on the underside of the
forward fuselage.
Located in the underfloor avionics bay, the
transponder receives ground radar interroga-
tion pulses on a frequency of 1030 MHz. The
transponder decodes the interrogation pulses
to determine whether to reply mode A, C, or
S information. Based on the interrogation, the
transponder replies the information requested
on 1090 MHz. The air data computer supplies
barometric altitude information for mode C and
S operations.
The ATC transponder control unit allows se-
lection of transponder 1 or 2. The flight crew
input ATC assigned identification codes using
the code selector knobs. The control unit fea-
tures an IDENT pushbutton that flashes aircraft
information when pressed. A test pushbutton
initiates a self-test of the transponder system.
CL 601-3A/R
16-9
FOR TRAINING PURPOSES ONLY
FlightSafety Canada
L t é e
L t d .
CL-600-2B16 PILOT TRAINING MANUAL
2
ATC
TX
Collins
A
C
T
1
IDENT
TEST
PRE
ALT
STBY
OFF
ON
MODE SELECTOR
OFF
—REMOVES POWER FROM
THE TRANSPONDER.
STBY
—ENABLES RECEPTION
AND DISABLES TRANSMISSION.
ON
—ENABLES RECEPTION
AND TRANSMISSION.
ALT
—ENABLES RECEPTION
AND TRANSMISSION AND
TRANSMITS BAROMETRIC
ALTITUDE IN RESPONSE TO
A MODE C REQUEST.
ACT ANNUNCIATOR
COMES ON WHILE THE CODES ARE
CHANGED AND FLASHES WHEN THE
REPLY CODE DOES NOT MATCH THE
DISPLAYED CODE.
TX ANNUNCIATOR
COMES ON EACH TIME THE
TRANSPONDER REPLIES TO
AN INTERROGATION.
ALTITUDE DISPLAY
DISPLAYS THE ENCODER
ALTITUDE OR THE TWO-DIGIT
FAULT CODE.
DIMMING SENSOR
AUTOMATICALLY
CONTROLS THE
BRIGHTNESS OF
THE DISPLAYS.
TEST PUSHBUTTON
INITIATES THE TRANSPONDER
SELF-TEST ROUTINE.
PRE PUSHBUTTON
ENABLES THE SELECTED
PRESET CODE TO BE
STORED IN THE
NONVOLATILE MEMORY
AND RECALLS STORED
CODE FOR DISPLAY.
CODE DISPLAY
DISPLAYS THE SELECTED CODE
OR DIAGNOSTIC READOUT IN
CASE OF FAILURE.
INNER CODE SELECTOR
SELECTS THE THIRD AND
FOURTH DIGITS OF THE
REPLY CODE.
OUTER CODE SELECTOR
SELECTS THE FIRST AND
SECOND DIGITS OF THE
REPLY CODE.
IDENT PUSHBUTTON
INITIATES TRANSMISSION OF A SPECIAL
IDENT PULSE TO THE GROUND RADAR.
1/2 TRANSPONDER
SELECTOR
IN POSITION 1, ENABLES
THE ATC 1 TRANSPONDER.
IN POSITION 2, ENABLES
THE ATC 2 TRANSPONDER.
CENTER PEDESTAL
Figure 16-8. ATC Transponder Control Unit