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Mode S target to the collision avoidance algorithms only if all
replies used for threat assessment after the initial range acquisition
occur within range and altitude windows centred on
range and altitude predicted from previous reply history, the
altitude quantization bit matches h e previous value, and the
VS field in the short special surveillance reply indicates the
target to be airborne at least once during the previous three
surveillance update cycles. The range and altitude windows are
the same as those used for Mode AIC tracking in 3.1.2.9.2 and
3.1.2.9.3 respectively.
3.1.3.9.2 If a tracking interrogation fails to elicit a valid
reply, additional interrogations are transmitted. The total
number of tracking interrogations addressed to a single target
is not expected to exceed five during a single surveillance
update period or sixteen distributed over six successive
surveillance update periods. The first tracking interrogation is
transmitted using the anlenna that was used in the last
s~ccessful interrogation of that target. If two successive
tracking interrogations fail to elicit vslirl replies from a target,
the next two interrogations to that target are transmitted using
the other antenna.
3.1.3.10 Missitig Mode S replies. The equipment
continucs to pass to the collision avoidance algorithms
predicted position rcpods for Modc S targets for six
surveillance update intervals following the rcccipt of the last
valid reply to a tracking interrogation if the target is
interrogated once every second or for eleven 1-second
surveillance update intervals following receipt of the last valid
reply to a tracking interrogation if the target is interrogated
once every five seconds. The equipment does not pass position
reports for Mode S targets for more than six surveillance
update intervals following the receipt of the last reply to a
tracking interrogation whose rate is once every second or for
more than eleven 1-second surveillance update intervals
following receipt of the last reply to a tracking interrogation
whose rate is once every five seconds unless the target again
satisfies the range acquisition criteria of 3.1.3.7. The Mode S
address of a dropped track is retained for four additional
seconds to shorter1 the reacquisition process if squitters are
received.
3.1.3.1 1 Mode S overload. The equipment passes position
reports for all Mode S targets regardless of the distribution of
targets in range, provided the total peak target count does not
exceed 30.
3.1.3.12 Mode S power programming. The transmit power
level of Mode S tracking interrogations to targets (but not airto-
air coordination interrogations) is to be automatically
reduced as a function of range for targets withi11 18.5 krn
(10 NM) as follows:
where PT is the adjusted power level, P, is the nominal
power level (typically 250 W), which is transmitted to targets
at ranges of 18.5 km (10 NM) or more, and r is the predicted
range of the target. The actual transmitted power is the lesser
of PT and the limit imposed by the interference limiting
inequalities of Chapter 4, 4.3.2.2.2.2.
3.1.3.13 Mode S track capacity. When the aircraft density
is nominally 0.087 Mode S aircraft per km2 (0.3 aircraft per
NM~i)n the vicinity of the ACAS aircraft, there will be about
24 aircraft within 9.3 krn (5 NM) and about 142 aircraft within
56 km (30 NM) of the ACAS aircraft. Thus, the ACAS
equipment is expected to have capacity for at least 150 aircraft
addresses.
3,1.3.14 USE OF BEARING E S TFO R
MODE S SURVEILLANCE
3.1.3.14.1 Bearing estimation capability is not required
for high-density Modc S surveillance. However, if bearing
estimates are available, it is seen that the use of directional
Mode S interrogations significantly reduces the transmitter
power requirement of the equipment. Directional Mode S
interrogations may also be used in the absence of bearing
information, provided the intcrference limits are not exceeded.
Annex 10 - Aeronautical Telecommunications Volume IV
3.1.3.14.2 Bearing estimates may also be used in
conjunction with knowledge of own airspeed to reduce the
overall Mode S interrogation rate. The following is one
possible way of achieving such a reduction.
3.1.3.14.3 Instead of calculating time-to-endanger based
on the conservative assumption that the two aircraft are on a
head-on collision course, the time-to-endanger can be
increased by taking into account the threat bearing and the
limited turn-rate of own aircraft and allowing for the time that
would be required for own aircraft to turn in the direction of
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