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ACAS X MOPS
TCAS Sensitivity Level Control
For TCAS II systems, control of the TCAS sensitivity level can be accomplished by one
or more ground-based Mode S sensors through the transmission of Comm-A
interrogations, UF=20, 21, containing TCAS Sensitivity Level Command Messages
(§188.8.131.52.3.2.2) to the TCAS II aircraft. The interrogator identification information
required to correlate the sensitivity level command (SLC) with a particular originating
ground-based Mode S sensor site is contained in the IIS subfield (§184.108.40.206.3.1.17) of SD
of the same Comm-A.
ACAS X shall (1148) ignore any Sensitivity Level Command Messages it receives.
ACAS X shall (1149) not use the Sensitivity Level value obtained from a Sensitivity
Level Command Message to modify the value of ownship Sensitivity Level.
Communication With Other Ground Equipment
RA Broadcast Interrogations
ACAS X shall (1152) use the long special surveillance interrogation, UF=16, with
UDS=49 and a broadcast address (i.e., 24 ONES) to transmit RA Broadcast
Interrogations. RA Broadcast Interrogations shall (1153) be transmitted at full power
from the bottom antenna at jittered, nominally 1-second, intervals for the period that the
RA is active. Additionally, one interrogation, indicating RA termination, shall (1154) be
transmitted immediately after the RA is concluded. The RA Broadcast Interrogation
shall (1155) include the MU field as specified in §220.127.116.11.18.104.22.168 and shall (1156)
describe the most recent RA that existed during the preceding 1-second period.
Installations using directional antennas shall (1157) operate such that complete circular
coverage is provided nominally every 1 second and the same RA sense and strength is
broadcast in each direction.
Note: This allows RA activity to be monitored in areas where Mode S ground station
surveillance coverage does not exist by using special RA broadcast signal
receivers on the ground. RA broadcasts are normally destined for ground
equipment but are defined as uplink transmissions.
Mode S Crosslink Capability
Mode S crosslink capability provides the capability to request and receive ground-
initiated Comm-B register information from Mode S transponders able to provide this
information. The following paragraph describes the interaction required between an
airborne ACAS X which utilizes crosslink interrogations and a Mode S-equipped aircraft
to properly carry out the Mode S crosslink function. At a minimum, crosslink
interrogations would need to be accounted for by the Interference Limiting algorithms as
specified in §22.214.171.124, and additionally impact to ACAS X range performance would need
to be considered.
ACAS X identifies those Mode S transponders able to support the crosslink capability by
examining the CC field in short special surveillance format DF=0 replies. ACAS X can
then request the contents of a specific Comm-B register in a Crosslink-capable Mode S
transponder by setting RL=1 and designating the identity of the register in the BDS field
of a short special surveillance format UF=0 interrogation addressed to that transponder.
In response to this request, the Mode S transponder will reply with a long special
surveillance format DF=16 in which the MV field contains the request ed Comm-B
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