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be established late. If the number of other ACAS in the area
exceeds the limits given in the table, the interference limiting
requirements of Chapter 4, 4.3.2.2 require that the ACAS
transmitter power and receiver sensitivity be further reduced,
thereby resulting in a later establishment time. However, tht:
track probability is expected to degrade gradually as any of
these limits is excecdcd.
1.3.2.6 The table reflects the fact that the ACAS tracking
perfbrmance involves a compromise between closing speed
and traffic density. Although it may not be possible to maintain
a high probability of track when the traffic density and the
intruder closing speed are both simultaneously large, the
ACAS design is capable of reliable track establishment on
high-speed intruders when operating in relatively low-density
en-route airspace (typically characterized by densities of less
than 0.017 aircraft/km2, i.e. 0.06 aircraft/NM2) or when
operating in higher density, low-altitude terminal airspace
where the closing speeds are typically below 260 d s (500 kt)
for operational reasons.
1.3.2.7 The table also accounts for the fact that higher
closing speeds are associated with the forward direction than
with the side or back directions so that the ACAS surveillance
design is not required to provide reliable detection for the
highest closing speeds in the side or back directions.
1.3.3 SYSTEM RANGE
LIMITATIONS
The required nominal tracking range of the ACAS is 26 km
(14 NM). However, when operating in high density, the
interference limiting feature may reduce system range to
approximately 9.3 km (5 NM). A 9.3 km (5 NM) range is
adequate to provide protection for a 260 m/s (500 kt)
encounter.
Figure A-1. Illustration of ACAS functions
ATT-3
Resolution
advisory
L
-
Threat
detection
i
Surveillance
-
+ +
L
Traffic
4 advisory -
- Own
aircraft
tracking
Coordination
and
communication
-
Evaluatiol~
and
Range
test
L
---
-L L
tracking
- L
-
-
- Altitude - test
selection of
advisory
Other
ACAS
aircraft
-
Other
aircraft - - Ground
stations
Altitude
test
Annex.10 - Aeronautical Telecommunications Volume ZV
1.4 Control of interference to
the electromagnetic environment
1.4.1 The ACAS equipment is capable of operating in all
traffic densities without degrading the electromagnetic
environmenl. Each ACAS equipment knows the number of
other ACAS units operating in the local airspace. This
knowledge is used to ensure that no transponder is suppressed
by ACAS activity for more than 2 per cent of the time and to
ensure that ACAS does not contribute to an unacceptably high
fruit rate that would degrade ground SSR surveillance
performance. Multiple ACAS units in the vicinity
cooperatively limit their own transmissions. As the number of
such ACAS units increases, the interrogation allocation for
each of them decreases. Thus, every ACAS unit monitors the
number of other ACAS units within detection range. This
information is then used to limit its own interrogation rate and
power as necessary. When this limiting is in full effect, the
effective range of the ACAS units may not be adequate to
provide acceptable warning times in encounters in excess of
260 mls (500 kt). This condition is normally encountered at
low altitude where this closing speed capability is sufficient.
Whenever the ACAS aircraft i s on the ground, ACAS
automatically limits the power of its interrogations. This
limiting is done by setting the ACAS count (n,) in the
interference limiting inequalities to a value three times the
measured value. This value is selected to ensure that an ACAS
unit on the ground does not contribute any more interference
to the electromagnetic environment than is unavoidable. This
value will provide an approximate surveillance range of 5.6
krn (3 NM) in the highest density terminal areas to support
reliable ground ACAS surveillance of local airborne traffic and
a 26 km (14 NM) range in very low density airspace to provide
wide area surveillance in the absence of an SSR.
1.4.2 The presence of an ACAS unit is announced to
other ACAS units by the periodic transmission of an ACAS
interrogation containing a message that gives the address of
the ACAS aircraft. This transmission is sent nominally every
8 to 10 seconds using a Mode S broadcast address. Mode S
transponders are designed to accept message data from a
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