If two aircraft detect each other at exactly the same time and simultaneously transmit coordination messages containing incompatible deviation senses it is, by convention, the aircraft that has the highest Mode S address that cancels its trajectory correction. A time delay in the display of orders on the display units avoids opposing orders.
 (2) Communications protocol Communications between two aircraft comprises three phases:
 (a)
 Detection phase The TCAS receives squitter messages transmitted by the transponder of the intruder aircraft. These are DF11-type messages transmitted periodically at one second intervals and intended to enable its detection and identification. This message essentially uses two fields:

 -an AA (Address Announced) field containing the Mode S address which must be acknowledged
 -A PI (parity interrogator Identity) field which is the result of a parity encoding using a method of polynominal multiplication. On reception, the Address Announced field is multiplied by the same polynominal and the result is compared with the received value.

 (b)
 Surveillance phase After detection, a surveillance phase starts. This phase is composed of an acquisition part and a tracking part.

 1_ Acquisition interrogation
 When the TCAS receives a squitter and acquires the Mode S
 address of the intruder it enters into contact by transmitting
 a UFO-type message (Short special surveillance interrogation)
 with the following specific fields:
 - bit 9, RL = 0: reply message length requested short (56
 bits)
 - bit 14, AQ = 1: acquisition-type message indication.
 2_ Acquisition reply
 The intruder's transponder replies to this request by a DFO
 message containing the following information:
 - bit 6, VS: Vertical Status, = 1 if the aircraft is on the
 ground, = 0 if the aircraft is airborne
 - bits 9 to 11, SL: Sensitivity level indicates in which
 sensitivity level its TCAS is operating
 - bits 14 to 17, RI: combinations of bits, from values 8 to
 15, specify the maximum speed the aircraft can reach. The
 other combinations are not used 

R 1EFF : ALL 1 34-43-00Page 62 1 1 May 01/05 1 1 1CES 1 -bits 20 to 32, AC: aircraft altitude code indicating the barometric altitude.

 3 Tracking interrogation
_ After its acquisition, the intruder is tracked by UFO-type interrogations with the following field values:
 -RL = 0: reply message length requested short
 -AQ = 0: not an acquisition message.
 4 Tracking reply
_
 The intruder's transponder replies with a DFO message
 indicating altitude and TCAS sensitivity level by a
 combination of fields SL and RI:

 -SL: bits 9 to 11
 -RI: bits 14 to 17, combination values 0 to 7. Values 8 to 15 are not used
 -AC: bits 20 to 32, aircraft altitude code.
 (c) Coordination phase If the intruder becomes a threat, the TCAS programs a deviation maneuver to avoid a risk of collision. A coordination procedure is initiated between the two aircraft with an exchange of the following messages:
 1 Coordination interrogation
_ The TCAS transmits a UF16 Long Special Surveillance message whose fields contain the following indications:
 ---------------------------------------------------------------------| BITS | FIELD | INDICATION | |-------|--------|--------------------------------------------------| | 9 | RL | = 1: reply message length requested long | | 14 | AQ | = 0: non-acquisition type interrogation | | 33-40 | UDS | U Definition Subfield - defines the other data | | | | in the MU field (Comm-U), composed of bits 42 | | | | to 88 | | 42 | MTB | Indicates multiple threat processing | | 43-44 | CVC | Cancel Vertical resolution advisory Complement - | | | | used to cancel an RA complement sent earlier to | | | | an intruder | | 45-46 | VRC | Vertical Resolution advisory Complement - used to| | | | transmit an RA vertical complement to the intru- | | | | der requesting it not to modify its trajectory | | | | (don't climb, don't descend) | | 47-49 | CHC | Cancel Horizontal resolution advisory Complement-| | | | not used in TCAS II | | 50-52 | HRC | Horizontal Resolution advisory Complement - not |
R 1EFF : ALL 1 34-43-00Page 63 1 1 May 01/05 1 1 1CES 1 ---------------------------------------------------------------------| BITS | FIELD | INDICATION | |-------|--------|--------------------------------------------------| | | | used in TCAS II | | 53-55 | | not used | | 56-60 | HSB | Encoded Sense bits for Horizontal resolution | | | | advisory complement - not used in TCAS II | | 61-64 | VSB | Encoded Sense Bits for Vertical resolution advi- | | | | sory complement - parity code to protect the 4 | | | | vertical command bits (43-46) | | 65-88 | MID | Interrogator TCAS-equipped aircraft Mode S | | | | address. | ---------------------------------------------------------------------
　
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