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INSTRUMENT FLIGHT RULES WORKBOOK
CHAPTER ONE
110.
TRANSPONDER
Air traffic control has been greatly improved with the introduction of radar. However, the age-
old problem of distinguishing individual aircraft remained. This problem of identification led to
the development of an electronic system that permitted aircraft to identify themselves
automatically. In the military, this system is called IFF/SIF (Identification, Friend or
Foe/Selective Identification Feature). Its civilian counterpart is the Air Traffic Control (ATC)
Radar Beacon System. The Collins TDR-950 is installed in the T-34C and has no IFF/SIF
capability.
The theory of operation is similar to TACAN. With TACAN DME, the aircraft computes
distance by interrogating a ground station which in turn replies. The transponder, however, does
just the reverse. A ground facility, such as an Air Route Traffic Control Center, interrogates the
aircraft's transponder which in turn replies with its radar identification code, previously set by the
pilot.
SBY
ON
ALT
1
2
0
0
OFF
TST
DIM
REPLY
IDENT
Figure 1-19 Transponder Control Panel
The transponder has two separate modes of operation. They are Mode 3 (known as Mode A in
civilian aircraft) and Mode C. When using Mode 3, a pilot will be assigned a four-digit number,
"squawk" or "code." The transponder is a 4096 code transponder, which means it can reply on
any one of 4096 discrete codes that can be assigned by ATC. (NOTE: 7 is the largest number
found on the transponder.) A discrete code gives an aircraft a radar "fingerprint" and is manually
selected by the pilot. Even on VFR flights, pilots who are held in radar contact by ATC can
receive traffic advisory information.
Some codes have been reserved for special uses. Code 1200 can be used by any aircraft flying
VFR. Code 7700 has been reserved to alert ATC that the pilot is declaring an emergency. Code
7600 is reserved for aircraft experiencing two-way radio failures. Code 7500 is used to indicate
that the aircraft is being hijacked.
In airspace over the continental United States above 10,000 feet MSL, excluding airspace at and
below 2500 feet AGL, aircraft are required to have a transponder that is equipped with automatic
altitude reporting capability (Mode C). This system transmits the aircraft's altitude with
reference to Mean Sea Level +300 feet to the interrogating radar ground station. Transponders
substantially increase the capability of radar to see an aircraft, and Mode C enables the controller
to rapidly determine potential traffic conflicts.
INTRODUCTION TO AIRBORNE NAVIGATION AND COMMUNICATIONS
EQUIPMENT AND PRINCIPLES OF OPERATION 1-25


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