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maximum range of 22 km (12 NM). No target outside of this
range should be eligible to generate an RA. However, ACAS
is able to detect ACAS broadcast interrogations from ACASequipped
aircraft out to a nominal range of 56 km (30 NM).
3.1.1.4 The equipment should have the capacity for
surveillance of any mix of Mode A/C or Mode S targets up to
a total peak target capacity of 30 aircraft. ACAS equipment is
nominally capable of reliable surveillance of high-closingspeed
targets in a peak traffic density of up to 0.017 aircraft
per square km (0.06 aircraft per square NM) or approximately
27 aircraft in a 26 krn (14 NM) radius.
3.1.1.5 When the average traffic density exceeds the
above value, the reliable surveillance range decreases. ACAS
equipment is capable of providing reliable surveillance ol:
targets closing only up to 260 mls (500 kt) in an average traffic
density of 0.087 aircraft per square km (0.3 aircraft per square
NM). The surveillance range required for 260 m/s (500 kt)
targets is about 9.3 km (5 NM). It is possible to provide 9.3 km
(5 NM) surveillance in a short-term peak traffic density of
0.087 aircraft/km2 (0.3 aircrafVNM2) or more without
exceeding a total target capacity of 30. If the overall target
count ever exceeds 30 at any range up to 26 km (14 NM), the
long-range targets may always be dropped without
compromising the ability to provide reliable surveillance of
lower-speed targets. Thus a peak capability of 30 targets (any
mix of Mode AfC or Mode S) is adequate for ACAS and if the
number of Mode A/C plus Mode S targets under surveillance
exceeds 30, excess targets are to be deleted in order of
decreasing range without regard to target type.
3.1.2 SURVEILLANOCFE I NTRU~SRSW lTH
MODE A/C TRANSPONDERS
3.1.2.1 Surveillance of Mode A/C transponders is
accomplished by the periodic transmission of a Mode C-only
all-call (internode) interrogation (Chapter 3, 3.1.2.1.5.1.2).
This elicits replies from Mode A/C transponders, but not from
Mode S transponders, thus preventing the replies of Mode S
transponders from synchronously garbling the replies of Mode
NC transponders. Other techniques for reducing synchronous
garble are (I) the use of directional antennas to interrogate
only those aircraft in an azimuth wedge, and (2) the use of a
sequence of variable power suppressions and interrogations
(known as "whisper-shout") that interrogates only aircraft that
have similar link margins (see 3.2.2). The use of both of these
techniques together provides a powerful tooi for overcoming
the effects of synchronous garble.
3.1.2.2 Whisper-shout employs a sequence of
interrogations at different power levels transmitted during each
surveillance update period. Each of the interrogations in the
sequence, other than the one at lowest power, is preceded by a
suppression transmission, where the first pulse of the
interrogation serves as the second pulse of the suppression
transmission. The suppression transmission pulse begins at a
time 2 microseconds before the first pulse of the interrogation.
The suppression pulse is transmitted at a power level lower
than the accompanying interrogation so that the transponders
that reply are only those that detect the interrogation and do
not detect the suppression. To guard against the possibility that
some transponders do not reply to any interrogation in the
sequence, the suppression pulse is transmitted at a power level
somewhat lower than that of the next lowcr interrogation. The
time interval between successive interrogations should be at
least 1 millisecnnd. This ensures that replies from transponders
at long range are not mistaken for replies to the subsequent
interrogation. All interrogations in the sequence are
transmitted within a single surveillance update interval.
3.1.2.3 Responses to each Mode C-only all-call
interrogation are processed to determine the range and altitude
code of each reply. It is possible to detennine the altitude
codes for up to three overlapping replies if care is taken to
identify the location of each of the rcccivcd pulses.
3.1.2.4 After all of the replies are- received in response to
the whisperlshout sequence, duplicate replies should be
merged so that only one "report" is produced for each detected
aircraft. Reports may be correlated in range and altitude with
the predicted positions of known intruders (i.e., with existing
tracks). Since intruding aircraft are interrogated at a high rate
(nominally once per second), good correlation performance is
achieved using range and altitude. Mode A code is not needed
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