-all Mode S interrogations (UPLINK format) are binary differential phase shift keying (DPSK) signals
-Mode S replies (DOWNLINK format) are formed by pulse position modulation (PPM) encoding the reply data. (Ref. Fig. 012) The Mode S reply is preceded by a preamble containing four pulses of specific duration and intervals intended to guarantee received message validity. Any messages whose preamble is not in complete conformity with the model are rejected by the TCAS. This information is encoded in PPM mode with, for each bit, a logic level one if the first half of the interval is at 1 and a logic level zero if it is at zero.
(b)
Squitters The Mode S transponder participates actively in its own detection by transmitting signals, at one second intervals, intended to inform nearby aircraft of its presence.
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TCAS - Mode C Reply Figure 011
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TCAS - Mode S - Interrogation and Reply Figure 012
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This transmission, called squitter transmission, consists of a Format DF = 11 message, containing the Mode S 24-bit address assigned to the aircraft, whereas all the bits of the message PI field at zero indicates a squitter.
(c) Mode S communication messages The Mode S ATC communications system supporting personalized exchanges between ATC ground stations and Mode S transponder-equipped aircraft comprises:
-a set of 25 standard messages for station-aircraft uplinks (Uplink format) identified UFxx
-another set of 25 standard messages for downlinks (Downlink format) identified DFxx. The message may be long or short and contain either 56 or 112 bits. Each message consists of specific fields with bit combinations that have specific meanings. Two fields, however, have an identical definition for all messages:
-
the "message-type" field consisting of bits 1 to 5 whose coding translates the format decimal value into binary
-
the address field containing the last 24 bits of the message (bits 33 to 56 or bits 89 to 112 depending on whether the message is short or long). These 24 bits contain the Mode S address of the transmitter. The TCAS uses this bidirectional link capability to communicate with other TCAS equipped aircraft to coordinate avoidance maneuvers. It may also dialog with ground stations: these stations have the possibility of monitoring and modifying its action. In the Mode S message set, the TCAS uses only six UF-type messages and seven DF-type messages. These messages are:
-UF0, UF4, UF5, UF16, UF20, UF21
-DF0, DF4, DF5, DF11, DF16, DF20, DF21. The following two figures give the list of messages used by the TCAS for communications with other aircraft and with ground stations. They are followed by the definition of the fields used in the messages.
NOTE : The TCAS only uses short messages (56 bits).
____
(Ref. Fig. 013, 014)
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TCAS - Message Uplink Format Figure 013
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TCAS - Message Downlink Format Figure 014
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DEFINITION OF UPLINK FORMAT MESSAGE FIELDS
-------------------------------------------------------------------------------| DESIGNATOR | FIELD | INDICATION | |-------------|----------------|----------------------------------------------| | AP | ADDRESS PARITY | Coded address with parity check | | AQ | ACQUISITION | Indicates if it is an interrogation message; | | | | 1 = interrogation | | DI | DESIGNATOR | Specifies type of information contained in SD| | | IDENT | field | | MA | MESSAGE | Used by ground station to transmit a TCAS SL | | | Comm-A | command to a TCAS-equipped aircraft | | MU | MESSAGE | Used by TCAS to transmit to other aircraft RA| | | Comm-U | coordination information (under fields UDS, | | | | MTB, CVC, VRC, CHC, HRC, HSB, VSB) | | PC | PROTOCOL | Operating commands to the transponder | | RL | REPLY LENGTH | Indicates if message is short (0) or long (1)| | RR | REPLY REQUEST | Length and content of reply information | | | | requested by the interrogator | | SD | SPECIAL | Contains control codes affecting the | | | DESIGNATOR | transponder protocol | -------------------------------------------------------------------------------
DEFINITION OF DOWNLINK FORMAT MESSAGE FIELDS
-------------------------------------------------------------------------------| DESIGNATOR | FIELD | INDICATION | |-------------|----------------|----------------------------------------------| | AA | ADDRESS | Mode S address in the clear in 24 bits | | | ANNOUNCED | | | AC | ALTITUDE CODE | Information indicating aircraft altitude | | AP | ADDRESS PARITY | Coded address with parity check | | CA | CAPABILITY | Transponder capability | | DR | DOWNLINK | Requests extraction of downlink message by | | | REQUEST | the interrogator (existing RA) | | FS | FLIGHT STATUS | Flight status of the aircraft: ground, | | | | flight, alert, SPI | | ID | IDENTIFICATION | Contains the Mode A identification code | | | CODE | | | MB | MESSAGE | Indicates Advisory content to the ground | | | Comm-B | station | | MV | MESSAGE | Contains ARA, RAC, VDS subfields used for | | | Comm-V | coordination | | RI | REPLY | Type of reply and airspeed capability | | | INFORMATION | | | SL | SENSITIVITY | TCAS current sensitivity level | | | LEVEL | |
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