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 transmitting data to TCAS computers to assist pilot in avoiding airborne
collisions.
Fig 23-1 Air Traffic Control
[5] Figure 23-2 shows the ATC control panel, this panel allows the pilots to
select the proper mode of operation, to select codes for reply and to test and
monitor system operation.
Fig. 23-2 ATC Control panel
 The five-position ATC Transponder select switch allows the selected
transponder to be placed in the STANDBY, ON, or TEST mode. The STBY
mode inhibits replies. The ON mode enables replies. The TEST mode sends a
discrete to the transponder enabling the transponder self-test function.
 The code select controls are 4 code select
knobs. The left-hand outer knob selects the
thousands and the left-hand inner knob
selects the hundreds. The right-hand inner
knob selects the tens and the right-hand outer
knob selects the unit. The 4-digit octal code
ranges from 0000 to 7777, and appears in the
liquid crystal display window.
 The IDENT switch is a momentary
pushbutton identification switch that causes
the transponder to transmit a special pulse
that enables the ground station to identify the
airplane on the ground station scope.
Fig.23-3 ATC interrogation signal
The ALT RPTG switch is a 3-position switch that enables the selection of air
data source (1 or 2 ) for coded altitude information replies (mode C ). The
switch also allows the pilot to turn off mode C replies.
 The transponder fault display monitors the selected system's operation. The
display will light if a fault is detected.
[6] The ATC transponder receives pulse-coded signals from air traffic control
ground stations and TCAS equipped airplanes. These signals are interrogations.
The transponder responds to the interrogations with pulse-coded signals. The
ground secondary surveillance radar (SSR) and the airborne TCAS system
interrogate the airborne transponder in one or two modes. The type of
interrogation is determined by the spacing between two pulses (P1 and P3)
transmitted on a carrier frequency of 10300.2MHz. (see figure 23-3) Each
interrogation contains a third pulse at the same frequency which is not
transmitted by the SSR but by an omnidirectional antenna located with the SSR.
The third pulse P2 is transmitted 2 microsecond (10 second ) after the P1 pulse.
By comparing the relative amplitude of the P1 and P2 pulses, the airborne
transponder can determine whether or not the airplane is in the mair lobe of the
SSR .In figure 23-3, airplane A in the main lobe will receive a P1 pulse of a
higher amplitude than P2 (at least 6 dB higher required for response), and
recognize the interrogation as coming from the main beam. The situation is
reversed for airplane B, and the P2 Pulse will be of the same or greater
amplitude than the P1 pulse. The transponder in airplane B will not reply .
[7] The airplane transponder only responds to Mode A ( identity ) and Mode C
( altitude ) interrogations. The reply consists of up to 14 pulses, each with a 0.45
microsecond pulse width and pulse interval of multiple of 1.45 microseconds,
twelve of the pulses carry the information ( identity or altitude). Two others,
called framing pulses F1 and F2 , come before and after the twelve information
pulses, (see figure 23-4) for identifying the beginning and the end of each reply
signal.
Fig. 23-4 ATCRBS reply signal
[8] For mode A reply, the coded pulses represent a four digit octal code entered
on the ATC control panel by the pilot. The sum of the subscripts of the
-6
transmitted " A" pulses designates the value of the most significant code digit,
the sum of the subscripts of the transmitted " B" pulses designates the second
digit. " C "pulses, the third digit, etc. Therefore, 0000 is transmitted with all
pulses missing in A, B, C, D groups, 7777 is transmitted with all pulses present.
In mode A replies, after the framing pulse F2, an identification pulse ( IP ) can
occur. If the pilot presses the IDENT button on the ATC control panel, the IP
will be transmitted for 18 seconds and the ground radar operator will see an
enhanced target on the radar scope.
[9] For mode C reply, the coded pulses represent altitude data from the air data
computer. The pulses within mode C reply correspond to a specific altitued
value.
EXERCISE
A. Find out the explanations of the following words in your " English-Chinese
technical dictionary"
1. interrogator [ P1-83 ]
2. decibel (db) [ P1-40 ]
B. Translate paragraph [1] and paragraph [8] (in lesson 23 ) into Chinese.
C. Explain the following abbreviations
　
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