Home' RTCA Documents for Review : ACAS X MOPS DRAFT Vol.1 Contents 139
ACAS X MOPS
Note: Single Mode C interrogations are susceptible to uplink mode conversion due to
multipath and may result in a mixture of Mode A and Mode C replies from an
intruder that are separated by 13 μsec. A switch from a single Mode C
interrogation to a whisper-shout sequence is based on a successful range
correlation between two consecutive replies in order to reduce the possibility of a
switch due to a fruit reply. Mode C tracks are established only through use of
whisper-shout interrogations in order to prevent multipath induced mode
Control of Synchronous Garble by Directional Interrogation
A minimum four-beam directional interrogation antenna mounted on the top of the
aircraft is required for reliable ACAS X surveillance of ATCRBS targets in aircraft
densities up to 0.3 aircraft per square NM. The required azimuth resolution of the
directional antenna for reply degarbling depends on the specific whisper -shout sequence
used. A high-resolution whisper-shout sequence will reduce the need for high resolution
in azimuth and vice-versa. The following discussion of antenna requirements and
definition of antenna characteristics will enable the manufacturer to determine the
degarbling benefits to be gained as a function of azimuth resolution.
126.96.36.199.4.2.1 Directional Interrogation Beamwidth Control
In order to reduce synchronous garble, the interrogation beamwidth shall (1289) be
limited by transmission of a P2 sidelobe suppression pulse following each P1 interrogation
pulse by 2 microseconds. The P2 pulse shall (1290) be transmitted on a separate control
pattern and shall (1291) have the same shape as the other pulses specified in §188.8.131.52.
The azimuth and elevation patterns of all beams shall (1292) be as specified in §184.108.40.206.
Interrogation beamwidth limiting, using the P2 suppression pulse, shall (1293) be
controlled to prevent transponder reply gaps between adjacent directional beams. The
minimum required reply beamwidth is defined as the maximum amount of beamwidth
limiting that still causes a maximum-suppression transponder located at any azimuth
angle and between +20 degrees and -15 degrees elevation to reply to interrogations from
at least one of the directional beams. A maximum-suppression transponder is defined as
one that, when interrogated by a whisper-shout interrogation sequence, replies only when
the received ratio of P1 to P2 exceeds the whisper-shout bin-width as defined in
§220.127.116.11.4.1.2. When the relative P2 power level is adjusted to satisfy the above
conditions, the required reply beamwidth is optimally minimized resulting in a maximum
amount of degarbling.
18.104.22.168.4.2.2 Directional Interrogation Radiated Power
The ACAS X effective radiated power (ERP) level in any azimuth direction shall (1294)
be sufficient to satisfy the system requirements of §2.2.2. Effective radiated power is
defined as the product of the net power delivered to the antenna terminal times the gain of
the antenna at 0 degrees elevation and at the azimuth of interest, where antenna gain is
specified relative to a matched quarter-wave stub.
For a directional interrogation top antenna, the ERP at the azimuth peak of each beam
shall (1295) be selected to maintain adequate range coverage, as defined by the product
of the appropriate closing speed and time to closest-point-of-approach, at the crossover
point of two adjacent beams.
For a four-beam directional top antenna with a 90 degree azimuth beamwidth, the ERP
associated with the largest whisper-shout step in the forward beam shall (1296) be at
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