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to transmit ACAS RAs to Mode S ground stations. This
information can then be used by air traffic services to monitor
ACAS RAs within an airspace of interest.
1.26 FUNCTIONS PERFORMED
BY ACAS
1.2.6.1 The functions executed by ACAS are illustrated in
Figure A-1. To keep the illustration simple, the functions "own
aircraft tracking" and "intruder aircraft tracking" have been
represented once in Figure A-1, under "surveillance".
However, the trackers that are intended to support the collision
avoidance function may not be suitable to support the
surveillance function. Separate tracking functions may be
required to adequately support both the collision avoidance
and the surveillance functions.
1.2.6.2 Surveillance is normally executed once per cycle;
however, it may be executed more frequently or less frequently
for some intruders. For example, surveillance may be executed
less frequently for some non-threatening intruders to respect
interference limiting inequalities or it may be executed more
frequently for some inlruders to improve the azinluth estimate.
1.2.6.3 Parameters used in the implementation of the
ACAS functions are adjusted automatically or manually to
maintain collision avoidance protection with minimal
interference to normal air traffic control (ATC) operations.
1.3 Intruder characteristics
1.3.1 TRANSPONDEEQRU IPAGE
OF INTRUDER
ACAS provides RAs on aircraft equipped with altitude
reporting Mode AIC or Mode S transponders. Some aircraft
are equipped with SSR transponders but do not have altitude
encoders. ACAS cannot generate KAs in conflicts with such
aircraft because, without altitude information, a collision threat
assessment cannot be made. ACAS equipment can generate
only TAs on such aircraft, describing their ranges, range rates
and bearings. Aircraft equipped with Mode A only
transponders and those not equipped with or not operating
Mode AIC or Mode S transponders cannot be tracked by
ACAS.
1.3.2 INTRUDCELROS ING SPEEDS AND
TRAFFIC DENSITIES
1.3.2.1 ACAS equipment designed for operation in high
density iirspace is capable of providing overall surveillance
performance on intruders as defined in Chapter 4, 4.3.2 and
Table 4- 1.
1.3.2.2 The conditions enumerated in Table 4-1, which
define two distinct density regions in the multi-dimensional
condition space that affects ACAS performance, were
extrapolated from airborne measurements of the performance
of a typical ACAS. The airborne measurement data indicated
that the track establishment probability will not drop abruptly
when any of the condition bounds is exceeded.
1.3.2.3 The performance is stated in terms of probability
of tracking a target of interest at a maximum closing speed in
a given traffic density at least 30 seconds before the point of
closest approach. The maximum trafic density associated with
each of the two density regions is defined as:
where n(r) is the maximum 30-second time average of the
count of SSR transponder-equipped aircraft (riot counting own
aircraft) above a circular area of radius r about the ACAS
aircraft ground position. In the airborne measurements, the
radii were different for the two density regions. In the highdensity
measurements the radius was 9;3 km (5 NM). In the
low-density measurements the radius was 19 krn (10 NM).
Traffic density outside the limits of the circular area of
constant density may be assumed to decrease inversely
proportional to range so that the number of aircraft is given by:
Atfachment Annex 10 - Aeronautical Telecommunications
where r, is the radius of the constant density region.
1.3.2.4 When the density is greater than 0.017 aircraft/krn2
(0.06 aircraft/N~~th)e, nominal radius of uniform density r, is
taken to be 9.3 krn (5 NM). When the density is equal to or less
than indicated above, r, is nominally 18.5 km (10 NM).
1.3.2.5 The table is based on an additional assumption that
at least 25 per cent of the total transponder-equipped aircraft in
the highest density 0.087 aircraftlkm2 (0.3 aircraftNvl2)
airspace are Mode S equipped. If fewer than 25 per cent are
Mode S equipped, the track probability for Mode A/C aircraft
may be less than 0.90 because of increased synchronous garble.
If the traffic density within r, exceeds the limits given in the
tablc or if thc traffic count outside of r, continues increasing
faster than r, the actual track establishment probability for
Mode AIc aircraft may alsn he less than 0.90 because of
increased synchronous garble. If the closing speed exceeds the
given limits, the tracks for Mode A/C and Mode S aircraft may
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