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the threat. Such computation would continue to assume that
the target aircraft is travelling at its reported maximum capable
speed directly toward the collision point.
3.2.1 POWERL EVELS
3.2.1.1 In the absence of interference and when using an
antenna whose pattern is identical to that of a quarter-wave
monopole above a ground plane, it is possible to provide
reliable air-to-air surveillance of transponders at ranges of
26 km (14 NM) by using a nominal effective radiated power of
54 clBm (250 W).
3.2.1.2 The transmitter output power is to be carefully
limited between transmissions because any leakage may
severely affect the performance of the Mode S transponder on
board the ACAS aircraft. The leakage power into the transponder
at 1 030 MHz is generally to be kept at a level below
-90 a m . If the physical separation between the transponder
antenna and the ACAS antenna is no less than 50 cm, the
coupling loss between the two antennas will exceed 20 dB.
Thus, ~f the RF power at 1030 MHz at the ACAS antenna
terminal does not exceed -70 dBm in the inactive state, and
if a minimum antenna spacing of 50 cm is adhered to, the
direct interference from the ACAS antenna to the transponder
antenna will not exceed -90 dBm. This requirement is to
ensure that, when not transmitting an interrogation, ACAS
does not radiate RF energy that could inkrfere with, or reduce
the sensitivity of, the SSR transponder nr other radio
equipment in nearby aircraft or ground facilities.
3.2.1.3 Measurcs must also be taken to ensure that direct
1030 MHz leakage from the ACAS enclosure to the
transponder enclosure is below -1 10 dBm when the two units
are mounted side-by-side in a typical aircrafl installation.
3.2.1.4 It is expected that the ACAS equipment be tested
side-by-side with Mode S transponders of equivalent
classification to ensure that each unit meets its sensitivity
requirenlents in thc presence of transmitter leakage from the
other.
3.2.2 CONTROOLF SYNCHRONOUS
INTERFERENCE BY WHISPER-SHOUT
3.2.2.1 To control Mode NC synchronous interference
and facilitate ACAS operation in airspace with higher traffic
densities, a sequence of interrogations at different power levels
may be 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 pulse
(designated SI) 2 microseconds preceding the PI pulse. The
combination of S1 and PI SeNeS as a suppression transmission.
S1 is transmitted at a power level lower than that of PI. The
minimum time between successive interrogations is to be 1
millisecond. All interrogations in the sequence should be
transmitted within a single surveillance update interval.
3.2.2.2 Because the suppression transmission in each step
is always at a lower power level than the following
interrogation, this technique is referred to as whisper-shout.
The intended mechanism is that each aircraft replies to only
one or two of the interrogations in .a sequence. A typical
population of Mode NC transponders at any given range may
have a large spread in effective sensitivity due to variation in
receivers, cable losses, and antenna shielding. Ideally, each
transponder in the population will respond to two
interrogations in the sequence, and will be turned off by the
higher power suppression transmissions accompanying higherpower
interrogations in the sequence. Given a situation in
which several aircraft are ncar enough to each other in range
for their replies to synchronously interfere, it is unlikely they
would all reply to the same interrogation and, as a result, the
severity of synchronous interference is reduced. Use of
whisper-shout also reduces the severity of the effects of
multipath on the interrogation link.
3.2.2.3 Figure A-2a defines a whisper-shout sequence
that is matched to the requirements for high-density Mode A/C
surveillance and Figure A-2b defines a whisper-shout sequence
that is matched to the requirements for low-density Mode NC
surveillancc. Five distinct subsequences are defined; one for
each of the four beams of the top-mounted antenna and one
for the bottom-mounted omnidirectional antenna. The
interrogations may be transmitted in any order. When the high
density sequence of Figure A-2a is truncated to limit
interference, the steps are dropped in the order shown in the
column Interference limiting priority. When the low-density
sequence of Figure A-2b is reduced in power to limit
interference, each interrogation and its related MTL value, as
ir~dicated in the last column, is reduced by 1 dB in the order
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