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R  and P3. These pulses are transmitted by the directional 
R  antenna of the Secondary Surveillance Radar (SSR) and spaced 
R  according to the SSR mode of operation. A control pulse P2 is 
R  radiated by the omnidirectional antenna of the SSR two 
R  microseconds after the pulse P1 is transmitted from the 
R  directional antenna of the SSR. 
R  Amplitude of pulses P1 and P3 varies according to the position 
R  of the directional antenna. P1 and P3 are at higher amplitude 
R  than P2 when the aircraft flies in the antenna main beam. 
R  Amplitude of the pulse P2, also called side lobe suppression 
R  pulse, is constant whichever the position of the 
R  omnidirectional antenna. 
R  The aircraft receives the P1 pulse at a higher amplitude than 
R  P2. The aircraft transponder detects this amplitude difference 
R  and determines the interrogation to be a valid interrogation. 
R  When the transponder detects that P2 is within 6 dB of P1, in 
R  amplitude, it determines that the interrogation is not valid. 

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R  2_  Mode S all call interrogation
R  (Ref. Fig. 007) 
R  A mode S all call interrogation consists of three pulses 
R  labelled P1, P3 and P4. They are transmitted by a directional 
R  antenna. A control pulse P2 (Side Lobe Suppression Pulse) is 
R  transmitted following P1 by an omnidirectional antenna. 
R  The detection of pulse P1, P2 and P3 is the same as for mode A 
R  or C interrogation. 
R  The transponder initiates a reply until the presence of the 
R  pulse P4 is detected. Upon detection of the pulse P4, the 
R  reply is terminated. If the width of the pulse P4 is 0.8 
R  microseconds (mode A or mode C only all-call), no reply is 
R  transmitted. If the width of the pulse is 1.6 microseconds 
R  (mode S all-call), a mode S reply is generated 128 
R  microseconds after the leading edge of the pulse P4. 
R  3_  Mode S interrogation
R  (Ref. Fig. 008) 
R  A mode S interrogation consists of a preamble followed by a 
R  data block. 
R  The preamble consists of two equal transmitted pulses labelled 
R  P1 and P2 and spaced by two microseconds. 
R  The data block consists of a single pulse P6 with a synchro 
R  phase reversal modulation. 
R  Pulse P6 duration : short 15.5 microseconds, 
R  long  29.5 microseconds, 
R  Data block : 56 or 112 bits. 
R  The pulse P5, which is radiated by the omnidirectional antenna 
R  may be overlaid on P6 by the ground interrogator as a Side 
R  Lobe Suppression (SLS) signal in any mode S interrogation. The 
R  pulse P5 is spaced 0.4 microsecond from the synchro phase 
R  reversal. 
R  (b) Encoded reply 
R  1_  Mode A or C reply
R  A mode A or C reply consists of two framing pulses F1 and F2, 
R  separated by a 20.3 microseconds time interval. The encoding 
R  of the reply is done by means of the presence (1) or absence 
R  (0) of 13 reply pulses (12 pulses plus X pulse). 
R  2_  Mode S all call reply
R  A mode S all call reply consists of a four-bit preamble 
R  followed by a data block. 
R  The data block contains 56 position modulated pulses.

 

R  3_  Mode S reply
R  A mode S reply occurs after a mode S interrogation or a mode S 
R  all call interrogation (P4 duration 1.6 microseconds). 
R  A mode S reply consists of a four-bit preamble followed by a 
R  data block. The data block contains 56 (short) or 112 (long) 
R  position modulated pulses. 
R  4_  Mode S message content
R  The minimum data link transponder supports all surveillance 
R  functions, in addition to bidirectional air-to-air data 
R  exchange, ground-to-air data uplink (Comm A), air-to-ground 
R  data downlink (Comm B), and multisite message protocol. In 
R  addition, the transponder is capable of receiving Extended 
R  Length Messages (ELMs) from the ground. ELMS are received in 
　
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