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The variations of range and altitude with successive interrogations
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The bearing of surrounding aircraft with the interferometry principle.
Each threat is treated individually but the TCAS determines the best solution of collision avoidance with respect to all aircraft in its vicinity. At the same time, the TCAS coordinates maneuvers with other TCAS-equipped aircraft. The best solution of collision avoidance is the maneuver that ensures an adequate separation of trajectories with a minimum vertical speed variation.
3.1.2.1. DETECTION PHASE
TCAS detects surrounding aircraft by regularly listening to acquisition squitters (refer to 2.1.3.8 – Extended Squitter for more details on squitters) from Mode S transponders of surrounding aircraft. When TCAS records some 24-bit addresses, TCAS starts a cyclical process repeated every second:
1.
Mode C interrogations to get altitudes of non-Mode S aircraft
2.
Mode S interrogations to get altitudes of Mode S aircraft via selective calls
3.
Squitter listening.
This principle applies to the detection of aircraft equipped with Mode A or Mode
C transponders. When the own aircraft TCAS interrogates in Mode C:
4.
An aircraft equipped with a Mode A transponder, the latter replies with its SQWK code. TCAS uses this reply for horizontal positioning purpose (i.e. bearing). TCAS does not process the SQWK code itself,
5.
An aircraft equipped with a Mode C transponder, the latter replies with its standard barometric altitude.
2 Mode A transponders does not transmit the barometric altitude. Therefore, the TCAS is not able to compute the relative altitude from a reply provided by a Mode A transponder, and only triggers a TA for aircraft equipped with Mode A transponders.
3.1.2.2. TRACKING PHASE
When TCAS detected surrounding aircraft, TCAS tracks them by series of interrogations and replies. These exchanges permit the update of the relative altitude, the range and the bearing for each aircraft, and to determine the variations of range and altitude. TCAS can track up to 40 aircraft (60 aircraft for the A380 TCAS function) simultaneously and displays the 8 most threatening aircraft.
3.1.3. TCAS AND MODE S
TCAS, Mode S transponders and Mode S ground stations use the same frequencies to transmit and receive messages. Thanks to this statement, data exchanged through the Mode S data link allow the coordination of avoidance maneuvers and the communication between aircraft equipped with TCAS and Mode S transponder.
3.1.3.1. COORDINATED MANEUVERS
The coordination of maneuvers prevents the flight path corrections ordered by each TCAS from resulting in a hazardous situation. It prevents from two aircraft maneuvering in the same direction (e.g. both aircraft climb) that could lead to a worse situation.
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