Transponder - P-3050039

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

ALT

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