TCAS - Surveillance and Display
 Figure 001A

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R  **ON A/C 201-207, 210-210, 
R  Post SB 34-1179 For A/C 201-207, 
R  Post SB 34-1238 For A/C 201-207, 
R  A. Presentation 
R  The TCAS II (Traffic Collision Avoidance System) is a system whose 
R  function is to detect and display aircraft in the immediate vicinity and 
R  to provide the flight crew with indications to avoid these intruders by 
R  changing the flight path in the vertical plane only. 
R  The TCAS periodically interrogates their transponders, computes their 
R  trajectories and constantly determines their potential threat. The 
R  acquisition of their signals is achieved by means of two transmit/receive 
R  antennas, one located on the underside of the fuselage and the other on 
R  the top. 
R  The system can establish individualized communications with each aircraft 
R  through ATC/Mode S transponders, thus permitting operation in dense 
R  traffic areas while avoiding an overload of radio-electric transmissions 
R  that would result from a general all-intruder response. 
R  The TCAS II system is designed to provide the air traffic control system 
R  with an additional possibility. It usually operates independently but may 
R  be controlled from ground stations. 
R  The TCAS has the capability to communicate with ATCRBS (Air Traffic 
R  Control Radar Beacon System) ground stations equipped with the Mode S 
R  system to indicate to them the vertical maneuver orders presented to the 
R  aircraft pilot. This information can facilitate the task of the ground 
R  station controller who, in turn, can modify the TCAS operating mode and 
R  cancel the avoidance orders if he deems it necessary for safety. 
R  The system maintains surveillance within a sphere determined by the 
R  transmit power and receiver sensitivity of the TCAS computer. The area in 
R  which a threat is imminent depends on the speed and path of the own A/C 
R  and the threat A/C. 
R  There is an area defined as TAU within the surveillance arc which 
R  represents the minimum time the flight crew needs to discern a collision 
R  threat and take evasive action. 
R  The TCAS detection capability covers an area of 35-40 NM in range and 
R  plus or minus 9900 ft in altitude but display range is authorized up to 
R  plus or minus 2700 ft in altitude. 
R  NOTE : The ABOVE/BELOW function on the ATC/TCAS control unit is used to____ 
R  increase the display range above 1200 ft and below -1200 ft. 
R  NOTE : The description of the system conforms to RTCA/DO-185A Change 7.0.____ 
R  (Ref. Fig. 001B) 

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 TCAS - Surveillance and Display R Figure 001B
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R B. Principle When an aircraft is airborne, its TCAS periodically transmits interrogation signals for all ATCRBS and Mode S transponder-equipped aircraft in the vicinity. These interrogations are received by the ATCRBS ground stations and by the transponders of the other aircraft. In response to these interrogations, the transponders of nearby aircraft return signals containing their altitude value. The TCAS computes the range between the two aircraft by measuring the elapsed time between transmission of the interrogation and reception of the reply. The altitude, altitude rate, range and range rate are determined by a periodic tracking of these exchanges and the data are used for intruder threat assessment. Each threat is treated individually but the TCAS determines the best collision avoidance possibility with respect to all aircraft in its vicinity, while establishing maneuver coordination with the other TCAS-equipped aircraft. The optimum maneuver is the one that ensures an adequate separation of trajectories with a minimum vertical trend variation.
R C. Advisories Visual and aural advisories are supplied by the TCAS computer whenever assessment of the relative position of two aircraft reveals a potential collision hazard. The Traffic Advisories (TA) indicate the position of nearby aircraft which are or may become a threat. Their display alerts the flight crew to the presence of intruders and facilitates their visual acquisition. The Resolution Advisories (RA) may be divided into two categories:
　
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