2 Coordination reply
_ After acquisition of this message, the intruder's transponder replies with a Long Special Surveillance DF 16 type message, containing the information previously transmitted to it by its own TCAS:
 ---------------------------------------------------------------------| BITS | FIELD | INDICATION | |------ |-------|---------------------------------------------------| | 6 | VS | Vertical Status - indicates whether aircraft is | | | | on ground or airborne | | 9-11 | SL | With RI, SL indicates the sensitivity level at | | | | which the interrogated aircraft's TCAS is | | | | operating | | 14-17 | RI | Reply Information | | 20-32 | AC | Altitude Code - contains aircraft altitude | | | | encoded in 100 ft increments if bit 28 equals 0, | | | | and in 25 ft increments if bit 28 equals 1 | | 33-40 | VDS | V Definition Subfield defines the contents of the| | | | data and coding in the field MV (Comm V) composed| | | | of bits 41 to 88 | | 41-54 | ARA | Active Resolution Advisory - indicates the RA | | | | type currently generated by the TCAS | | 55-58 | RAC | Resolution Advisory Complement - Indicates the RA| | | | complement type currently received from other | | | | TCAS-equipped aircraft | | 59-88 | | Not used | ---------------------------------------------------------------------
R 1EFF : ALL 1 34-43-00Page 64 1 1 May 01/05 1 1 1CES 1

 (d) Correspondence of radio messages with ARINC 429 words The coordination information exchanged between two aircraft transits via ARINC 429 buses linking the transponder to the TCAS. The following table gives the correspondence between the two types of link:
 ---------------------------------------------------| 1 - TRANSPONDER TO TCAS | |-------------------------------------------------| | | RADIO MESSAGE | ARINC MESSAGE | |---------|-------------------|-------------------| | FIELD | MESSAGE | BITS | LABEL | BITS | |---------|---------|---------|---------|---------| | MTB | UF16 | 42 | 271 | 09 | | CVC | | 43-44 | | 10-11 | | VRC | | 45-46 | | 12-13 | | CHC | | 47-49 | | 14-16 | | HRC | | 50-52 | | 17-19 | | HSB | | 56-60 | | 20-24 | | VSB | | 61-64 | | 25-28 | ---------------------------------------------------
---------------------------------------------------| 2 - TCAS TO TRANSPONDER | |-------------------------------------------------| | | RADIO MESSAGE | ARINC MESSAGE | |---------|-------------------|-------------------| | FIELD | MESSAGE | BITS | LABEL | BITS | |---------|---------|---------|---------|---------| | SL | DF16 | 09-11 | 274 | 23-25 | | RI | | 14-17 | | 26-29 | | ARA | | 41-54 | 273 | 12-25 | | RAC | | 55-58 | | 26-29 | ---------------------------------------------------
D. Principles of Computation In the TCAS, target aircraft are categorized depending on specific criteria varying in function of altitude. The TCAS essentially uses two types of information to perform this classification:
 -
the relative altitude between two aircraft, known by the difference of their barometric altitudes

 -
the distance or range separating them. (Ref. Fig. 018) Acquisition of these two parameters at regular intervals (tracking) enables their variations to be calculated:

 -altitude rate
R 1EFF : ALL 1 34-43-00Page 65 1 1 May 01/05 1 1 1CES 1

 TCAS - Definition of Main Parameters Figure 018
 1EFF : 1 1R 1CES  ALL  1 11 1 34-43-00 Page 66 May 01/05

 -range rate. Assessment of the potential threat represented by an intruder depends on two criteria determined with respect to a point in the traffic area called Closest Point of Approach (CPA). This is the point of minimum distance between the two aircraft, assuming that their trajectories do not deviate. The two criteria are:
 -vertical separation at CPA
 -time left before reaching CPA.
 (1) Projected vertical separation The threat is evaluated by calculating the vertical separation between the two aircraft at the closest point of approach. (Ref. Fig. 019) The TCAS computer processes the current altitude and altitude rate of the intruder to predict whether it will be within limits considered dangerous at the closest point of approach. Intruder 1 penetrates the altitude zone delimited by the upper and lower thresholds but when it reaches CPA, it will be outside this zone. Therefore no advisory will be issued. However, when intruder 2 reaches CPA it is still inside this zone and therefore an advisory will be issued.

R **ON A/C 001-049, 101-105, 151-199,
 (2) Time to intercept (TAU) The TCAS does not need to locate the CPA in space, but rather it needs to know the time to intercept for two aircraft. For example, if two aircraft are approaching on the same axis on a collision course, this time is the ratio of distance between them to the sum of their speeds.
 D
 T = ---------
V1 + V2 More generally, the TCAS uses range and range rate measurement to compute this time:
　
中國航空網 www.k6050.com
航空翻譯 www.aviation.cn
本文鏈接地址：A320飛機維護手冊 AMM NAVIGATION 導航3(111)