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sum continues to be incremented and decremented even
though its value may exceed 20.
3.1.3.8 MODE S RANGE ACQUISITION
3.1.3.8.1 The equipment should transmit an acquisition
interrogation (UF = 0, 16, AQ = 1, Chapter 3, 3.1.2.8.1.1) to
determine the range of each valid target with relative altitude
as defined above, or from which inadequate altitude
information has been received.
3.1,.3.8.2 If an acquisition interrogation fails to elicit a
valid reply, additional interrogations should be transmitted.
The total number of acquisition interrogations addressed to a
single target must not exceed three within a single surveillance
update period. The first acquisition interrogation is to be
transmitted using the top antenna. If two acquisition
irlterrogations to a target fail to elicit valid replies, the next two
acquisition interrogations to that target are to be transmitted
using the bottom antenna. If in the acquisition attempt in the
first surveillance update period, valid replies are not received,
ACAS transmits a total of nine acquisition interrogations
distributed over the first six successive surveillance update
periods. If acquisition interrogations fail to elicit replies within
six surveillance update intervals, the acquisition process is to
cease until enough additional squitterslfruit are received
indicating that a successful acquisition is likely. One means of
accomplishing this is to process subsequent squitterslfruit as
described in 3.1.3.7, but with the increment 16 replaced by 8.
If a second failure to acquire occurs, the process is repeated
with an increment of 4. After any subsequent failure, an
increment of 2 is used.
3.1.3.8.3 If additional attempts are made to acquire the
target, they conform to the pattern described above except that:
a) On the second and third attempt, only nne interrogation
is to be made during a single surveillance update
interval; and in the absence of valid replies, six
interrogations are to be transmitted during the first six
surveillance update intervals.
b) Any further attempts consist of a single interrogation
during the entire six update intervals.
3.1.3.8.4 When a valid acquisition reply is received, the
VS field in the reply is examined to determine the vertical
status of a target. If a target is determined to be on the ground,
its vertical- status is periodically monitored by interrogating as
often as necessary to ensure timely acquisition when airborne.
When a valid acquisition reply is received from an airborne
target, onc or more interrogations are to be transmitted to the
target within two surveillance update intervals in order to
confirm the reliability of the altitude data and the altitude
quantization bit. When two replies have been received from an
airborne target that have altitude values within 150 m (500 ft)
Attachment Annex 10 - Aeronazctical Telecommunications
of each other and within 3 050 m (10 000 ft) of own altitude
and have identical quantization bit values, periodic surveillance
intcrrogations (designated as "tracking" interrogations) arc to
be initiated for that target.
3.1.3.8.5 The range of the target is used with its estimated
range rate to determine its potential threat to ACAS. If the
target is not an immediate potential threat, it can be
interrogated less frequently than if it were a potential threat for
which an advisory would soon most likely be issued. Each
1-second surveillance update interval, the potential threat lcvel
(TAU) of the target is calculated as follows:
TAU = - (r - SMOD~I~)I;,
where r is the tracked range, r is the estimated relative range
rate and SMOD is a surveillance distancc modifier which is
equivalent to 5.6 km (3 NM). If the estimated relative range
rate is either a negative value of less than -6 kt or positive
(either a slow convergence or the aircraft are diverging), the i
value used to calculate TAU is -6 kt. An SMOD value of 5.6
km ensures that ACAS will always use the nominal I-second
interrogation cycle in situations where the value of TAU can
change rapidly, such as in a parallel approach. A target with a
TAU value of equal to or less than 60 seconds is interrogated
at the nominal rate of once every second, A target with a TAU
value greater than 60 seconds is interrogated at a rate of once
every five seconds if the altitude of the targct and own aircraft
are both less than 5 490 m (I8 000 ft) and at a rate of at least
once every five seconds if the altitude of the target or own
aircraft is greater than 5 490 m (18 000 ft).
3.1.3.9 MODE S SURVEILLANCE EXTENSION
3.1.3.9.1 The equipment passes position reports for a
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