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developed for intruders equipped with Mode S transponders.
Because of Mode S selective address, there is no synchronous
garble associated with surveillance of Mode S transponders.
However, multipath must be dealt with and the surveillance of
Mode S transponders should be accomplished with as few
interrogations as possible to minimize interference.
3.1.3.2 The Mode S modulation formats are inherently
more resistant to multipath than are the Mode AIC modulation
formats. However, the greater length of the Mode S
3.1.3.5 After ACAS has determined the altitude of a
detected Mode S aircraft, it compares the altitude of this
aircraft to its own altitude to determine whether or not the
target can be ignored or should be interrogated to determine its
range and range rate. If the measured range and the estimated
range rate indicate that it is (or could soon be) a collision
threat, the intruder should be interrogated once per second and
the resulting track data fed to the collision avoidance
algorithms. An aircraft at longer range should be interrogated
only as often as necessary to maintain track and ensure that it
will be interrogated once per second before it becolnes a
collision threat.
3.1.3.6 The use of passive detection in combination with
altitude comparison and a less frequent interrogation of nonthreat
intruders reduces the Mode S interrogation rate
ailtomatically when the local densities of other ACAS aircraft
are very high. Therefore, a higher interrogation power level is
available to improve surveillance performance.
Annex I0 - Aeronautical Telecommunications Volume ZV
3.1.3.7 MODE S SURVEILLANCE INITIATION
3.1.3.7.1 The equipment is intended to provide Mode S
surveillance with a minimum of Mode S interrogations. The
identity of Mode S targets is determined by passively
monitoring transmissions received with DF = 11 or DF = 17.
Error detection and correction is applied to the received
squitters to reduce the number of addresses to be processed.
The altitude of the Mode S targets from which a squitter has
been received is determined by passive monitoring
lransmissions received with DF = 0 (short air-air surveillance
replies, Chapter 3, 3.1.2.8.2) or DF = 4 (surveillance altitude
replies, Chapter 3, 3.1.2.6.5) or active selective interrogations
(air-air surveillance interrogation, Chapter 4, 4.3.8.4) and
monitoring the corresponding air-air surveillance replies. The
equipment monitors squitter and altitude replies whenever it is
not transmitting, or receiving replies to, Mode S or Mode C
interrogations. Each received reply is examined to determine
what further action should be taken.
3.1.3.7.2 To reduce the number of unnecessary
interrogations, a squitter target is not interrogated if so few
squilters and alt~tuller eplies are rcceived from it that no threat
is indicated. Targets that might be a threat are called valid
targets. The equipment is not intended to interrogate a target
unless the altitude information indicates that it is within
3 050 m (10 000 ft) of own altitude. The ACAS aircraft
interrogates targets from which it does not receive altitude
information but does continue to receive error-free squitters. In
order to establish timely acquisition of targets that transition
the 3 050 m (10 000 ft) relative altitude boundary, the altitude
of targets that are beyond 3 050 m (10 000 ft) of own altitude
are monitored using unsolicited DF = 0 or DF = 4 replies, or
in the absence of such replies, by periodically interrogating to
elicit a DF = 0 reply.
3.1.3.7.3 The following is an example of one acceptable
means of processing squitters and altitude replies to reduce
unneeded interrogations:
a) When a valid squitter is first received, a running sum
initialized 'at 0 is associated with it. During each
succeeding surveillance update interval the sum is
decremented by 1 if no squitters or altitude replies with
a particular address are received, and the sum is
incremented by 16 for each reception of either a squitter
or an altitude reply. The process continues until the sum
equals or exceeds 20. When the sum becomes less than
or equal to -20, the address is removed from the system.
When it equals or exceeds +20, the target is declared to
be valid.
b) When a target has been declared to be valid, it is
interrogated unless its altitude differed from the ACAS
altitude by more than 3 050 m (10 000 ft). Otherwise, its
altitude is monitored using DF = 0 or DF = 4 replies, or
in the absence of such replies, by interrogating once
every lOseconds to elicit a DF = 0 reply.
c) When any of these conditions are satisfied, the running
　
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