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processed. they are combined with the tracks that are newly
initiated during the current scan, and then all the tracks are
examined pairwise to determine if a given pair of tracks is
likely to represent the same intruder. If:
a) the ranges differ by at most 150 m (500 ft)
b) the range rates differ by at most 4.6 mls (8.9 kt)
C) either
I ) the altitudes differ by at most 30 m (100 ft), or
2) the altitude rates differ by at most 3 mls (10 ftls) and
both tracks were initiated during the same scan,
only one of the tracks is retained, preference being given to the
track showing the larger number of replies since initiation.
3.1.2.9.6 Image track processing. Those tracks that could
have been formed by replies specularly reflected from the
ground are referred to as image tracks. A track is identified as
an image if there exists a track at shorter range (referred to as
the real track) such that:
f) If the track is above 3 050 m (10 000 ft), the term
contained within the second pair of brackets is a) the difference between the real altitude and the image
multiplied by four. altitude is less than or equal to 60 m (200 ft) for altitudeAffachment
Annex 10 - Aeronauticul Telecommunications
reporting ta~gets, or both the image track and the real lransmission makes it more likely to be overlapped by
track are non-alhtude-reporting; and multipath. The use of top-mounted antennas and variable
receiver thresholds (to protect the Mode S reply preamble)
b) the difference between the measured image range rate incrcases the multipath resistance to an acceptable level for
and the calculated image range rate i, is less than or reliable air-air surveillance. The use of antenna diversity
equal to 21 m/s (40 kt), where the calculated image transponders on ACAS aircraft provides an additional
range rate is either (for the single-reflection case): reliability margin for coordination between pairs of conflicting
ACAS aircraft.
I
i;= (-f 1 [i+ L [((2ri-,12 -12t ( Z ~ - Z ? ) ' ( ~i~)+ L r , - r )
+ ~ ~ - ( Z ~ - Z ) ( Z O - Z ) ] ]
or (for the double-reflection case):
where;
ri is the image range,
r is the real range,
Z is the real altitude, for altitude reporting targets or
Z is set to own altitude for non-altitude reporting
targets, and
Z, is own altitude
3.1.3.3 Mode S interrogation rates are kept low by passive
detection of ~ransporlder~ rar~slnissio~arlisd by interrogating
once per second only those intruders that could become
immediate threats. Intruders that are not likely to become
immediate threats should be interrogated less frequently
(i.e. once every 5 seconds). Passive address acquisition
prevents unnecessary interference with other elements of the
SSR and ACAS system. ACAS listens to Mode S all-call
replies @F = 11, acquisition squitter transmissions, Chapter 3,
3.1.2.8.5.1 or DF = 17, extended squitter transmissions,
Chapter 3, 3.1.2.8.6.1). These may occur in response to Mode
S ground station all-call interrogations or as spontaneous
transmission (called squitters) at intervals ranging from 0.8 to
1.2 seconds for the acquisition squitkr, and at shorter intervals
for the extended squitter. Reception of squitters may be
alternated between the top and bottom antennas. If reception is
switched, it will be necessary to control the switching times to
avoid undesirable synchronism with the squitters transmitted
by Mode S antenna diversity transponders.
3.1.3.4 The 24-bit aircraft address in the ssuitter is
If a track is identified as an image, it may be retained, but it protected by error coding to ensure a high probability of
cannot be flagged as established for use by the threat detection obtaining a correct address. Since the squitter transmission does
logic. not contain altitude information, ACAS attempts to obtain
altitude passively from Mode S replies generated in response to
3.1.2.10 Missing Mode A/C reports. The equipment ground interrogations or interrogations from other ACAS
continues to pass to the collision avoidance algorithms predicted aircraft. If altitude is not received shortly after address
position reports for Mode AIC targets for six surveillance update detection, the Mode S aircraft is actively interrogated to ob~ain
intervals following the receipt of the last valid correlating reply. altitude.
The equipment does not pass position reports for more than six
surveillance update intervals following the receipt of the last
valid correlating reply unless the target again satisfies the
surveillance initiation criteria of 3.1.2.8.
3.1.3 SURVEILLANCE OF
INTRUDERS WITH MODE S TRANSPONDERS
3.1.3.1 Efficient air-air surveillance techniques have been
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