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signifies alert, no SPI, aircraft is on the ground
signifies alert and SPI, aircraft is airborne or on the ground
signifies no alert and SPI, aircraft is airborne or on the ground
reserved
not assigized
Note.- The conditions which cause an alert are given in 3.1.2.6.10.1.1.
3.1.2.6.5.2 DR: Downlink request. This 5-bit (9-13) downlink field shall contain requests to downlink information.
Coding
0 signifies no downlink request
signifies request to send Comm-B message
reserved for ACAS
reserved for ACAS
signifies Comm-B broadcast message 1 available
signifies Cornrn-B broadcast mcssage 2 available
reserved for ACAS
reserved for ACAS
not assigned
see downlink ELM protocol (3.1.2.7.7.1)
Annex 10 - Aeronautical Telecommunications Volume ZV
Codcs 1-15 shall take precedence over codes 16-31.
Note.- Giving precedence to codes 1-15 permits the annotincement of a Comm-B message to interrupt the
announcement @a downlink ELM. This gives priority to the announcement of the shorter message.
3.1.2.6.5.3 UM: Utility message. This 6-bit (14-19) downlink field shall contain transponder communications status
information as specified in 3.1.2.6.1.4.1 and 3.1.2.6.5.3.1.
3.1.2.6.5.3.1 Subfields in UM for multisite protocols
WM FIELD STRUCTURE
The following subfields shall be inserted by the transponder into the UM field of the reply if a surveillance or Comm-A
interrogation (UF equals 4, 5, 20, 21) contains DI = 1 and RSS other than 0:
IIS: The +bit (14-17) interrogator identifier subfield reports the identifier of the interrogator that is reserved for
multisite communications.
IDS: The 2-bit (18, 19) identifier designator subfield reports the type of reservation made by the interrogator
identified in ITS.
Assigned coding is:
0 signifies no information
1 signifies IIS contains Comm-B II code
2 signifies TIS contains Comm-C II code
3 signifies IIS contains Comm-D II code.
3.1.2.6.5.3.2 Multisite reservution status. The interrogator identifier of the ground station currently reserved for
multisite Comm-B delivery (3.1.2.6.1 1.3.1) shall be transmitted in the IXS subfield together with code 1 in the IDS subfield
if the UM content is not specified by the interrogation (when DI = 0 or 7, or when DI - 1 and RSS = 0).
The interrogator identifier of the ground srarion currently rescrved for downlink ELM delivery (3.1.2.7.6.1), if any, shall
be transmitted in the IIS subfield together with code 3 in the IDS subfield if the UM content is not specified by the
interrogation and there is no current Comm-B reservation.
3.1.2.6.5.4 ACr Altitude code. This 13-bit (20-32) field shall contain altitude coded as follows:
a) Bit 26 is dcsignated as the M bit, and shall be 0 if the altitude is reported in feet. M equals 1 shall be reserved to
lndicatc that thc altitude reporting is in metric units.
h) If M equals 0, then bit 28 is designated as the Q bit. Q equals 0 shall be used to indicate that the altitude is reported
in 100-foot increments. Q equals 1 shall be used to indicate that the altitude is reported in 25-foot increments.
c) If the M bit (bit 26) and the Q bit (bit 28) equal 0, the altitude shall be coded according to the pattern for Mode C
replies of 3.1.1.7.12.2.3. Slarting with bit 20 the sequence shall be C1, Al, C2, AZ, C4, A4, ZERO, B 1, ZERO, B2,
D2, B4, D4.
d) If the M blt equals 0 and the Q bit equals 1, the 11-bit field represented by bits 20 to 25, 27 and 29 to 32 shall
represent a binary coded field with a least significanl bit (LSB) of 25 ft. The binary value of the positive decimal
intcgc~ " N shall be encoded to report pressure-altitude in the range [(25 N - 1 000) plus or minus 12.5 ft]. The
coding of 3.1.2.6.5.4 c) shall be used to report pressure-altilude above 50 187.5 ft.
Chapter 3 Annex 10 - Aeronautical Telecommunications
Note 1.- This coding mefhod is only able to provide values between minus 1 OOOft and plus 50 175fr.
Note 2.- The most sign,@cant bit (MSB) of thisfield is bit 20 as required by 3.1.2.3.1.3.
e) If the M bit equals 1, the 12-bit field represented by bits 20 to 25 and 27 to 31 shall be reserved for encoding altitude
in metric units.
f) 0 sflall be transmitted in each of the 13 bits of the AC field if altitude information is riot available or if the allitude
has been determined invalid.
3.1.2.6.6 COMM-B ALTITUDE REPLK DOWNLINK FORMAT 20
1 6 9 14 20 33 89
This reply shall be generated in response to an interrogation UF 4 or 20 with an RR field value greater than 15. The format
of this reply shall consist of these fields:
Field Reference
DF downlink format
FS flight status
DR downlink request
UM utility message
AC altitude code
　
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