User's Manual

SYSTEM DESCRIPTION AND INSTALLATION MANUAL
T
3
CAS/Part No.9005000
Table 1-25: Downlink Format Messages
Downlink Format
Field Description
Message Format with Number of Bits
DF=0 [00000]
Short Air-Air Surveillance
VS:1, CC:1, X:1, SL:3, X:2, RI:4, X:2, AC:13,
AP:24 X:Pad
DF=4 [00100]
Surveillance, Altitude Reply
FS:3, DR:5, UM:6, AC:13, AP:24
DF=5 [00101]
Surveillance, Identity Reply
FS:3, DR:5, UM:6, ID:13, AP:24
DF=11 [01011]
All-Call Reply
CA:3, AA:24, PI:24
DF=16 [10000]
Long Air-Air Surveillance
VS:1, X:2, SL:3, X:2, RI:4, X:2, AC:13, MV:56,
AP:24 X:Pad
DF=17 [10001]
Extended Squitter (ADS-B)
CA:3, AA:24, ME:56, PI:24
DF=20 [10100]
Comm-B, Altitude Reply
FS:3, DR:5, UM:6, AC:13, MB:56, AP:24
DF=21 [10101]
Comm-B, Identity Reply
FS:3, DR:5, UM:6, ID:13, MB:56, AP:24
Table 1-26: Uplink Format Fields
Designator
Field
Description
AP
Address Parity
24-bit discrete address with parity check bits
overlaid
AQ
Acquisition
Designates formats UF=0, 16 as acquisition
transmissions or non-acquisition.
CL
Code Label
Identifies the contents of the IC field
DI
Designator Identification
Identifies the coding contained in the SD field
DS
COMM-B Data Selector
Contains the identity of the ground-initiated
COMM-B register
IC
Interrogator Code
Contains either the II Code or SI Code
II
Interrogator Identification
Identifies the interrogator
MA
Message COMM-A
56-bit uplink field contains messages directed to
the aircraft
MU
Message COMM-U
56-bit uplink field contains information used in
air-to-air exchanges part of the long special
surveillance interrogation
NC
Number of C Segments
Number of segments transmitted in ELM mode
and part of a COMM-C interrogation
PC
Protocol
Operating commands to the transponder
PR
Probability of Reply
Contains commands to the transponder which
specify the reply probability to the Mode S only
All-Call interrogations
RC
Reply Control
Designates the transmitted segment as initial,
intermediate or final
Pub. No. 8600200-001, Revision 004
34-45-29
1-117
04 Nov 2014
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