TCAS Antenna Figure 007 
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R  TCAS Antenna 
R  Figure 007A 
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 TCAS Antenna Figure 007B 
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 A teflon gasket is located between the fuselage and the base of the antenna, in order to facilitate the removal of the antenna. An O-ring is provided to seal the antenna to the fuselage. The antenna is used to receive and provide directional information for 1090 MHz Mode S squitters, Mode S and Air Traffic Control Radar Beacon System (ATCRBS) replies. Proper phasing of the four antenna elements enables omni or directional transmission of 1030 MHz broadcast or coordination messages and ATCRBS or Mode S interrogations.
 **ON A/C ALL
 6. Operation_________
 (Ref. Fig. 008)
 A. Intruder Detection The TCAS detects A/C equipped with Mode S transponders by listening for squitter transmissions. Mode S transponders announce their presence by transmitting squitter messages once every second. The TCAS also detects A/C equipped with transponders that do not reply to Mode S interrogations but do reply to Mode C interrogations. The TCAS must actively search for Mode C equipped intruder aircraft because Mode C transponders do not transmit squitter messages. Once the presence of a Mode C intruder is confirmed, it is tracked by the TCAS. The TCAS is capable of tracking up to a combined total of 30 Mode S and Mode C intruders. Tracking is performed by repetitive TCAS interrogations in Mode S and Mode C format.
 (1) Interrogation of aircraft equipped with Mode A or Mode C transponders With respect to aircraft equipped with Mode A or Mode C transponders, the TCAS is active and transmits Mode C only all-call interrogations (P1, P3 and P4 pulses). The code is similar to the one used by the Mode A and Mode C ground stations. The P4 pulse informs those Mode S transponders that this interrogation is not addressed to them (Ref. Fig. 009) The nominal time interval between two interrogations is one second. But, to limit radio-electric interference in dense traffic areas, each interrogation consists of a series of interrogations of increasing strength to reach more remote aircraft (whisper-shout) with 1 ms time periods inside the series. (Ref. Fig. 010) The first transmission consists of relatively low power P1, P3 and P4 pulses only. Therefore, only the nearest aircraft will receive and reply to these interrogations. Then an S1 pulse is also transmitted. This pulse is at a lower amplitude, causing the close-in aircraft to interpret this as a side lobe from the transmitting station, requiring no reply. The purpose of the whisper-shout sequence is to reduce the number of aircraft replying to any one interrogation, thus limiting interference.
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 TCAS - Operation / Control and Indicating Figure 008
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 TCAS - Mode C Only All-Call Interrogation Figure 009
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 TCAS - Whisper-Shout Transmitter Sequence Figure 010
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 (2)
 Replies of aircraft equipped with Mode A or Mode C transponders Aircraft equipped with Mode C transponders reply by transmitting their altitude, octal encoded in four digits ABCD, with a value of 100 ft for the LSB in the ATCRBS format. Aircraft equipped with Mode A transponders reply by transmitting their Mode A identification code. In this case, intruder presentation on the ND is limited to a display of its position in range and bearing. (Ref. Fig. 011)

 (3)
 Interrogation of aircraft equipped with Mode S transponders

 (a)
 Transmission coding The TCAS uses the Mode S function for certain identification of intruders as a 24-bit address is definitively assigned to each aircraft by air traffic control. The interrogation comprises three pulses: P1, P2 and P6. P2 level is equal to or greater than the P1 level, which is the no-reply condition for the aircraft equipped with Mode A or C transponders. Therefore, only Mode S transponders reply to the interrogation. The useful information is contained in P6 divided into 56 or 112 chips. A chip is an unmodulated interval of 0.25 microseconds, preceded by possible phase reversals. The message formats contain a number of bits permitting a more complete and diversified information exchange than in Mode C. There are two distinct message formats:
　
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