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continue to be satisfied, the 1 dB change is made and no
further changes in either the Mode A/C or Mode S parameters
are made for the ensuing 8 seconds, except as described in
3.2.3.8.3 above.
3.2.4 INTERROGATION JIlTER
Mode AIC interrogations from ACAS equipment are
intentionally jittered to avoid chance synchronous interference
with other ground-based and airborne interrogators. It is not
necessary to jitter the Mode S surveillance interrogations
because of the inherently random nature of the Mode S
interrogation scheduling process for ACAS.
3.3 Antennas
3.3.1 USE OF DIRECTIONAL INTERROGATIONS
3.3.1.1 A directional antenna is recommended for reliable
surveillance of Mode A/C targets in aircraft densities up to
0.087 aircraft per square km (0.3 aircraft per square NM). The
recommended antenna system consists of a four-beam antenna
mounted on top of the aircraft and an omnidirectional antenna
on the bottom. A directional antenna may also be used instead
of the omnidirectional antenna on the bottom of the aircraft.
The directional antenna sequentially generates beams that
point in the forward, aft, left, and right di~ections.T ogether
these provide surveillance coverage for targets at all azimuth
angles without the need for intermediate pointing angles.
3.3.1.2 The directional antenna typically has a 3-dB beam
3.2.3.8'4 3' The third step is to add a width (BW) in azimuth of 90 +lo degrees for all elevation
step to those tentatively scheduled, when it is not prevented by angles between +20 and -15 degrees. The
an 8-second freeze, and the following conditions are satisfied: beamwidth is to be limited by transmission of a P7*
suppression pulse following each PI interrogation pulse by 2
a) inequalities (I), (2) and (3) are satisfied and will microseconds. The P2 pulse is transmitted on a separate
continue to be satisfied after the step is added; and control pattern (which may be omnidirectional).
Annex 10 - Aeronaulical Telecommunications Volume ZV
3.3.1.3 There is need for timely detection of aircraft
approaching with low closing speeds from above and below.
Detection of such aircraft suggests a need for sufficient
antenna gain within a k10 degree elevation angle relative to the
ACAS aircraft pitch plane. An ACAS directional antenna
typically has a nominal 3 dB vertical beamwidth of 30 degrees.
3.3.1.4 The shape of the directional antenna patterns and
the relative amplitude of the P, transmissions is controlled
such that a) a maximum suppression transponder located at
any azimuth angle between 0 and 360 degrees and at any
elevation angle between +20 and -15 degrees would reply to
interrogations from at least one of the four directional beams
and b) a minimum suppression transponder would reply to
interrogations from no more than two adjacent directional
beams. A maximum suppression transponder is defined as one
that replies only when the received ratio of PI to Pz exceeds
3 dB. A minimum suppression transponder is defined as one
that replies when the received ratio of PI to P2 exceeds 0 dB.
3.3.1.5 The effective radiated power (ERP) from each
antenna beam (forward, left, right, aft, omni) is expected to he
within &2 dB of its respective nominal value as given in
Figure A-2a.
3.3.1.6 A forward directional transmission, for which
TRY = 49 dBm and BW = 90° has a power gain product at
beam centre of approximately,
This is 1 dB greater than the nominal and allows for adequate
coverage at the cross-over points of the directional beams. The
TRP of the side and aft beams is reduced relative to the front
beam to account for the lower closing speeds that occur when
aircraft approach from these directions. Mode AIC
surveillance performance will generally improve as the
directivity (and hence the number of beams) is increased for
the top-mounted antenna. However, the use of a directional
antenna on the bottom would provide only marginal
improvement in detectability and would, if used at full power,
degrade the overall performance of the equipment by
increasing the false track rate due to ground-bounce multipath.
3.3.2 DIRECTION FINDING
The angle-of-arrival of the transmissions from the replying
transponders can be determined with better than l@degree
RMS accuracy by means of several simple and practical
direction-finding techniques. These techniques typically
employ a set of four or five monopole radiating elements
mounted on the aircraft surface in a square array with quarterwave
spacing. The signals from these elements may be
combined so as to generate from two to four distinct beams
which may be compared In phase or amplitude to provide an
　
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