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ACAS X MOPS
In particular, in response to a UF=0 with RL=1 and BDS=8, the transponder will reply
with a DF=16 in which the MV field contains the aircraft identification message ( Ref. B:
Appendix 1, Section 4.0). In response to a UF=0 with RL=1 and BDS=0, the transponder
will reply with a DF=16 with an MV field of all 0s.
Extended Squitter With Aircraft Identification Message
ACAS X may optionally be implemented to receive and decode the content of the ME
field in format DF=17 Extended Squitters containing the aircraft identification message.
The aircraft identification message is identified by format type code 1, 2, 3 or 4 in the
first five bits of the message field, and the contents of the message are defined in Ref. G.
Compatibility With Own Mode S Transponder
The ACAS X equipment will operate in conjunction with and in close proximity to a
Mode S transponder. This imposes design requirements on the Mode S transponder that
are not covered in Ref. B so that ACAS X requirements stated herein can be met.
Similarly, the functions of ACAS X equipment shall (1158) not in any way cause the
Mode S transponder to fail to meet its performance requirements as specified in Ref. B.
Therefore, it is the responsibility of the manufacturer to identify all areas where
incompatibilities may arise and to design and construct his equipment so that the
requirements of Ref. B. and this document are met when both equipment are operating.
Two general areas of potential incompatibility are identified below as exam ples; others
may be found to exist in accordance with specific equipment designs.
a. The first area of possible incompatibility concerns the methods used to int erface the
transponder and the ACAS X equipment. It is not within the scope of this document
to control or test specific interface design details within ACAS X. It is the
responsibility of the manufacturer to verify that his equipment does in fact interface
in such a way that all overall logical, protocol, timing and other requirements or
constraints imposed on the combined operation of the two functional entities have
been fulfilled under any possible combination of operating conditions.
This may be accomplished by conducting all testing with both functions fully
operative and performing their normal functions by means of simulated inputs and/or
outputs. For example, when tests are under way to verify that the ACAS X
equipment requirements of this document are met, the Mode S transponder could be
periodically interrogated by simulated input signals near its MTL level.
b. The second area of potential incompatibility concerns electromagnetic interference.
Since each unit generates transmission signals at the receiver frequency of the other,
it is possible for signals from one to degrade the performance of the other.
All of the ACAS X requirements stated in this document shall (1159) be met when the
ACAS X equipment is operating in conjunction with an operating Mode S transponder
with the possible exception of those times that the Mode S transponder is active. The
active state of the Mode S transmitter is defined as either the time interval between the
leading edge of the first transmitted pulse of a reply minus 10 microseconds and the
trailing edge of the last transmitted pulse of that reply plus 10 microseconds, or the time
interval during which a mutual suppression occurs, whichever is greater.
Aircraft Suppression Bus
The ACAS X equipment shall (1160) issue a 70 ±1 microsecond suppression pulse to
other on-board aircraft equipment beginning at each interrogation transmitted from the
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