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Mode S interrogation message formats
 All valid Mode S interrogation are listed here.
 The first 5 bits of the message indicate a UF message format.
 Items in brackets, e.g (AP:24) represent the NAPN field code,
 and the number of bits in the field N24N.
 Undescrored numbers represent unused fields and should not be
 processed.

 -TCAS Surveillance: UF=0 (0 0000) 3_ (RL:1) 4_ (AQ:1) 18_ (AP:24)
 -
Surveillance, Altitude Request: UF=4 (0 0100) (PC:3) (RR:5) (DI:3) (SD:16) (AP:24)

 -
Surveillance, Identity Request: UF=5 (0 0101) (PC:3) (RR:5) (DI:3) (SD:16) (AP:24)

 -Mode S All-Call Interrogation: UF=11 (0 1011) (PR:4) (II:4) 19_ (AP:24)
 -Long TCAS Interrogation (COMM-U): UF=16 (1 0000) 3_ (RL:1) 4_ (AQ:1) 18_ (MU:56) (AP:24)
 -
COMM-A Interrogation, Altitude: UF=20 (1 0100) (PC:3) (RR:5) (DI:3) (SD:16) (MA:56) (AP:24)

 -
COMM-A Interrogation, Identity: UF=21 (1 0101) (PC:3) (RR:5) (DI:3) (SD:16) (MA:56) (AP:24)

 -Uplink Extended Length Message (UELM)/COMM-C Interrogation: UF=24 (1 1) (RC:2) (NC:4) (MC:80) (AP:24)
 NOTE : PC, RR, DI and SD subfields are undefined for UF=20/21
____ broadcast interrogations.
Mode S messages downlink formats
 All valid Mode S replies are listed here.
 The first 5 bits of the message indicate a UF message format.
 Items in brackets, e.g (AP:24) represent the NAPN field code,
 and the number of bits in the field N24N.
 Underscrored numbers represent unused fields are shall be set
 to zero.

 -TCAS Surveillance: DF=0 (0 0000) (VS:1) 2_ (SL:3) 2_ (RI:4) 2_ (AC:13) (AP:24)
 -Surveillance, Altitude:
R 1EFF : 106-149, 221-299, 301-399, 401-499, 1 34-52-00Page 56 1 1 Aug 01/05 1 1 1CES 1 DF=4 (0 0100) (FS:3) (DR:5) (UM:6) (AC:13) (AP:24)

 -Surveillance, Identity:
 DF=5 (0 0101) (FS:3) (DR:5) (UM:6) (ID:13) (AP:24)

 -Mode S All-Call Reply, ATCRBS/Mode S All-Call Reply, Mode S Squitter: DF=11 (0 1011) (CA:3) (AA:24) (PI:24)
 -Long TCAS Reply (COMM-V): DF=16 (1 0000) (VS:1) 2_ (SL:3) 2_ (RI:4) (AC:13) (MV:56) (AP:24)
 -Extended Squitters:
 DF=17 (1 0001) (CA:3) (AA:24) (ME:56) (PI:24)

 -
COMM-B Reply, Altitude: DF=20 (1 0100) (FS:3) (DR:5) (UM:6) (AC:13) (MB:56) (AP:24)

 -
COMM-B Reply, Identity: DF=21 (1 0101) (FS:3) (DR:5) (UM:6) (ID:13) (MB:56) (AP:24)

 -Downlink Extended Length Message (DELM)/COMM-D:
 DF=24 (1 1) 1_ (KE:1) (ND:4) (MD:80) (AP:24)

 3 Mode S interrogation data field definition
_ The following table gives the definition of uplink format message fields:
R 1EFF : 106-149, 221-299, 301-399, 401-499, 1 34-52-00Page 57 1 1 Aug 01/05 1 1 1CES 1 -------------------------------------------------------------------------------| DESIGNATOR | FIELD | INDICATION | -------------------------------------------------------------------------------| AP | address parity | 24-bit address and parity overlaid | | | | | | AQ | acquisition | differentiates acquisition from non-acquisi-| | | | tion interrogations | | | | | | DI | designator | identifies the coding contained in the SD | | | identification | field | | | | | | II | interrogator | identifies the interrogator | | | identification | | | | | | | MA | message | 56-bit MA field contains messages directed | | | Comm A | to the A/C | | | | | | MC | message | 80-bit MC field contains one segment of a | | | Comm C | sequence of segments transmitted in ELM mode. | | | | | | MU | message | 56-bit MU messages contains information used | | | Comm U | in air-to-air exchanges | | | | | | NC | number of C | number of segments transmitted in ELM mode | | | segments | | | | | | | PC | protocol | operating commands of the transponder | | | | | | PR | probability | contains commands to the transponder to | | | of reply | specify the reply probability to the mode S | | | | only all-call interrogations (Allows | | | | operators to acquire closely spaced A/C). | | | | | | RC | reply | designates the transmitted segment as initial,| | | control | intermediate or final | | | | | | RL | reply | indicates if message is short or long | | | length | | | | | | | RR | reply | length and content of the reply information | | | request | requested by the interrogator | | | | | | SD | special | contains control codes affecting the link | | | designator | protocol | -------------------------------------------------------------------------------

R 1EFF : 106-149, 221-299, 301-399, 401-499, 1 34-52-00Page 58 1 1 Aug 01/05 1 1 1CES 1 The table below gives the definition of downlink format

 message fields:
　
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