(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 came 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.

(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 give 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 pulse is never used in a mode-C reply. The control head identification numbers and the reply coding characteristics for the full 15-pulse reply are shown in Fig. 2.

5C8 
34-53-0 Page 4  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Jun 20/85 

5C8 
Jun 20/85  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  34-53-0 Page 5 

K76103
ATC System Pulse Codes  5C8 
34-53-0  Figure 2  Jun 20/85 
Page 6 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

B.  Functional Description
(1)  
Receiver

(a)  Received signals from the antenna are routed through the Diplexer/Low Pass Filter and applied to the Preselector/Mixer. In the Preselector the signal is passed through a bandpass filter tuned to 1030 MHz. The filter output is applied to the mixer diode and mixed with the 970-MHz local oscillator (LO) signal. The LO signal is filtered in the preselector, by a 2-pole bandpass filter prior to mixing. The 60-MHz IF signal from the mixer is applied to the IF amplifier/detector module where it is amplified and detected. The output of the IF amplifier/detector is applied to the video processor where it is amplified, pulse shaped, cleaned up by a noise suppression circuit, and elimination of P2 pulse is performed if the received signal is from the ATC ground station main lobe. The signal is then sent to the decoder/encoder circuitry.

(2) 
 Decoder/Encoder

(a)  The video processor output is applied to the decoder/encoder along with the 2.7586 MHz 2-phase clock from the decoder/encoder (D/F) oscillator. The decoder/encoder then decodes the received message, determines whether mode A or mode C data is requested and then develops the appropriate serial data output to be applied to the transmitter.

(3)  
Transmitter

(a)  The modulator accepts the decoder/encoder serial data outputs, adjusts the amplitude and width of the pulses and supplies the proper signal to the transmitter modulator circuit. The modulation signal modulates the transmitter oscillator which is a 1090-MHz cavity controlled pulsed oscillator. The output of the transmitter oscillator is approximately 1.5 watts peak RF power. The pulsed RF signal is applied to the exciter module where the signal is amplified to 150 watts peak RF power. This 150-watt signal is then used to drive the power amplifier module where the RF signal is amplified to 500 watts peak RF power. The power amplifier output is then sent to the Diplexer to be routed to the ATC transponder antenna.
　
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