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broadcast interrogation without replying. The announcement
messages received by the ACAS aircraft's Mode S transponder
are monitored -by, the interference limiting algorithms to
develop an estimate of the number of ACAS units in the
vicinity.
2. FACTORS AFFECTING
SYSTEM PERFORMANCE
about 2.8 km (1.5 NM) of each other generate replies that
persistently and synchronously overlap each other at the
interrogating aircraft. The number of overlapping replies is
proportional to the density of aircraft and their range from
ACAS. Ten or more overlapping replies might be received in
moderate density terminal areas. It is possible to decode
reliably only about three overlapping replies. Hence, there is a
need to reduce the number of transponders that reply to each
interrogation. Whisper-shout and directional transmit
techniques are available for controlling such synchronous
-garble (see 3.2 and 3.3). ?bey are both needed in ACAS
equipment operating in the highest traffic densities.
2.2 Multipath from
terrain reflections
2.2.1 SSR transponders use quarter-wave monopole
antennas mounted on the bottom of the aircraft. A stub antenna
of this sort has a peak elevation gain at an angle of 20 to 30
degrees below the horizontal plane. This is suitable for groundair
surveillance, but the direct air-air surveillance path may
operate at a disadvantage relative to the ground reflection path,
particularly over water.
2.2.2 If the ACAS unit uses a bottom-mounted antenna,
there are geometries for which the reflected signal is
consistently stronger than the direct signal. However, when a
top-mounted antenna is used for interrogation, its peak gain
occurs at a positive elevation angle and the signal-to-rnultipath
ratio is improved. Thus, when ACAS transmits from the topmounted
antenna, the effects of multipath are reduced
significantly. Even when a top-mounted antenna is used, the
multipath will still occasionally exceed the receiver threshold.
Thus, there is need to reject low-level multipath. ACAS can
achieve this rejection through the use of variable receiver
thresholds (see 3.4).
2.3 Altimetry data quality
2.3.1 MEASUREMENETR RORS
2.3.1.1 The vertical separation between two conflicting
aircraft is measured as the difference between own altitude and
the intruder's altitude as reported in its Mode C or Mode S
reply. If the ACAS aircraft is an air carrier, it will normally
have accurate altimetry; an intruding aircraft might have less
accurate altimetty.
2.3.1.2 Errors in altimetry can cause two types of effects:
first, if the aircraft are on a near collision course, errors could
2.1 Synchronous garble indicate safe passage, and the impending near mid-air collision
might not be resolved by ACAS; second, if the aircraft are on
When a Mode C interrogation is transmitted, all the a near collision course, but are separated in altitude, errors
transponders that detect it reply. Since the reply duration is could lead to an ACAS manoeuvre in the wrong direction
21 microseconds, aircraft whose ranges from ACAS are within which could induce an even closer encounter.
Attachment Annex 10 - Aeronautical Telecommunications
2.3.1.3 ACAS attempts to achieve a difference of at least
90 m (300 ft) between aircraft at closest approach based on
reported altitude. Thus, if the combination of intruder and
ACAS altimetry errors approached 90 m (300 ft), there would
be finite risk of inadequate vertical separation despite the
presence of ACAS. Studies of the expected altimetry errors of
both ACAS and non-ACAS aircraft at altitudes from sea level
to R. 400 have concluded that the risk is essentially negligible
if both aircraft are equipped with -high accuracy altimetry
systems that can achieve root-sum-square (RSS) errors of
approximately 15 m (50 ft). It was further concluded that if an
ACAS with high accuracy altimetry operates in a traffic
environment consisting of typical general aviation aircraft
(with RSS errors of approximately 30 m (100 ft), normally
distributed), then altimetry errors will occasionally lead to
inadequate ACAS RAs. However, this will not occur often
enough to seriously interfere with the effectiveness of the
system. Performance was considered LO be inadequate if both
aircraft in an encounter had a low accuracy altimetry system.
This led to thc requirement that ACAS possess a high accuracy
system.
2.3.2 ALTITUDE BIT FAILURE
If the Mode C or Mode S altitude reports from the intruding
aircraft or the altitude data for own aircraft contain bit errors,
ACAS may develop erroneous estimates of the corresponding
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