getting to grips with surveillance(45)

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In most cases of encounters between two TCAS-equipped aircraft, mutual identification is almost simultaneous. However, there is a sufficient delay to establish the priorities for the coordination process.
The first aircraft to detect a potentially hazardous situation computes an avoidance maneuver sense, and communicates it to the other aircraft. The other aircraft takes the information into account and in turn computes an avoidance maneuver in the opposite sense.
It may happen that two aircraft simultaneously detect and simultaneously transmit coordination messages with avoidance maneuvers in the same sense. In this particular case, the aircraft with the highest 24-bit address reverses  the sense of its avoidance maneuver.
In coordinated maneuvers, only one RA reversal is triggered when changes in the encounter geometry occur. Therefore, the initial RA is reversed when:
- 
The initial RA has been displayed for at least 9 seconds, or

- 
The aircraft, with the lowest 24-bit address, has a vertical speed greater than 2 500 ft/min (upwards or downwards), and flies in the opposite sense of its initial RA.

 

This delay provides the two aircraft with sufficient time to respond to the initial RA.
For TCAS compliant with TCAS II Change 7.1 (refer to 3.1.6 – TCAS II Change 7.1), if the aircraft with the highest 24-bit address does not follow its TCAS order, the coordination of maneuvers gives the priority to the other aircraft
(i.e. with the lowest 24-bit address).
The coordination of maneuvers may be phased as follows:
1.  
Detection: The own aircraft TCAS listens to squitters.

2.  
Acquisition: The own aircraft TCAS receives a squitter, and interrogates the transponder of the intruder identified by the 24-bit address contained in the squitter. The transponder of the intruder replies with several data including its barometric altitude.

3.  
Tracking: The own aircraft TCAS tracks the intruder with regular interrogations.

4.  
Coordination: If the intruder becomes a threat, the own aircraft TCAS computes an avoidance maneuver to avoid a risk of collision. The two aircraft initiates a coordination procedure with the exchange of a coordination interrogation and a coordination reply.

 

3.1.3.2. COMMUNICATION WITH ATC GROUND STATIONS
When the TCAS triggers an RA, the TCAS is able to report it to Mode S ground stations. This report informs the ATC controller that the reporting aircraft had performed an avoidance maneuver.

 

3.1.4. COLLISION THREAT EVALUATION

Aircraft are categorized (i.e. OTHER, PROXIMATE, TA and RA) according to two criteria: the vertical separation or relative altitude (difference of barometric altitudes) and the range between aircraft. Figure 3-2: Vertical separation and range
Regular interrogations of surrounding aircraft permit to determine the variations of the vertical separation and of the range. These variations are called vertical rate and range rate.
　
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