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A 737-300/400/500MAINTENANCE MANUAL
ATC TRANSPONDER
B
ATC System Component Location
Figure 1
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A 737-300/400/500MAINTENANCE MANUAL
C. ATC transponder No. 1 receives 115 volts ac from bus No. 1 when the No. 1 ELEX AC on P6-1, load control center right and ATC-1 circuit breakers on P18-1, load control center left are closed. ATC transponder No. 2 receives 115 volts ac from bus No. 2 when the No. 2 ELEX AC on P18-1, load control center left and ATC-2 circuit breakers on P6-1, load control right are closed.
4. ATC Antenna
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A. Each ATC antenna is an omnidirectional blade-type antenna connected by a coaxial cable to an ATC transponder. Each antenna receives or transmits rf energy. The antennas are located on the bottom centerline of the fuselage at station 305 (No. 1) and at station 355 (No. 2).
5. Operation
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A. Basic ATC Transponder Principles
(1)
The transponder is an integral part of the Air Traffic Control Radar Beacon System (ATCRBS). In the airplane, its function is to transmit a coded response to a coded interrogation transmitted by the air traffic control ground station.
(2)
There are two types of radar at each of these ATC ground stations. The first, called the Primary Surveillance Radar (PSR), operates on the normal radar principles of receiving energy reflected from the airplane under surveillance. The second, called the Secondary Surveillance Radar (SSR), operates on the coded reply from the airborne transponder. Both radars are used in conjunction to develop the total air traffic situation and to display it on a single radar scope.
(3)
The SSR interrogates the transponder in one of two modes. These are referred to as mode A, or mode C interrogations. The type of interrogation is determined by the spacing between two pulses (P1 and P3) transmitted by the SSR on a carrier of 1030 |2 MHz. Each interrogation contains a third pulse at the same frequency which is not transmitted by the SSR but by an omnidirectional antenna which is located at the ground radar station. This pulse is transmitted 2 usec after the P1 pulse and is referred to as P2. Mode A and mode C interrogation characteristics are shown in Fig. 2.
(4)
The purpose of P2 is to allow the airborne transponder to determine whether the interrogation come from the main beam or a side lobe of the SSR. If the transponder has been interrogated by a side lobe, no reply is generated. A reply to a side lobe interrogation would give the ground radar operator an erroneous position reading of the airplane carrying the transponder. The transponder determines by an amplitude comparison between P1 and P2 if the interrogation is by a side lobe. If P1 is larger than P2, the interrogation is a valid main beam interrogation. If P2 is equal to or larger than P1, the interrogation is from the side lobe of the SSR.
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A
737-300/400/500MAINTENANCE MANUAL
(5) The transponder replies to mode A and mode C interrogations with a coded pulse group on a carrier frequency of 1090 |3 MHz. The transmitted message consists of four octal digits (0-7) which gives the transponder the capability of 4096 different identification numbers and altitude data from -1000 to +128,000 ft. In a mode A reply the coding of the pulses represents an identification number of the airplane carrying the transponder. The coding of a mode A reply can consist of up to fifteen pulses. Twelve of these pulses carry the identification number. Two others, called framing pulses, come before and after the twelve information pulses. The last is a special identification pulse to aid the radar operator. The mode C reply is basically the same as a mode A reply; however, in a mode C reply, the coding of the pulses represents altitude data. The data to be transmitted is inserted in the transponder on the appropriate input lines on the main connector and originate at the altitude digitizer. The special identification numbers and the reply coding characteristics for the full 15 pulse reply are shown in Fig. 2.
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