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estimate of the direction of amval of the received signal. This
level. of direction-finding accuracy is adequate to provide the
pilot with TAs to effectively aid the visual acquisition of
intruding aircraft.
3.3.3 DIRECTIONATLR ANSMISSION FOR
CONTROL OF SYNCHRONOUS GARBLE
3.3.3.1 The use of directional interrogation is one
technique for reducing synchronous garble. The directional
interrogation can reduce the size of the interrogation region.
Coverage must be provided in all directions. Hence, multiple
beams are used to elicit replies from all aircraft in the vicinity
of the ACAS-equipped aircraft. Care must be taken to overlap
the beams so that gaps in coverage do not exist between
beams.
3.3.3.2 The antenna may be a relatively simple array
capable of switching among typically four or eight discrete
beam positions. For four beam positions, the antenna beamwidrh
is expected to k on the nrder of 100". The effective
antenna beamwidth for interrogating Mode A/C transponders
can be made more narrow than the 3-dB beamwidth by means
of transmitter side-lobe suppression.
The top-mounted directional antenna is to be located on the
aircraft ccntce line and as far forward as possible. The ACAS
antennas and the Mode S transponder antennas are to be
mounted as far apart as possible on the airframe to minimize
coupling of leakage energy from unit to unit. The spacing must
never be less than 0.5 rn (1.5 ft), as this spacing results in a
coupling loss of at least 20 dB.
3.4 Receiver and processur
A sensitivity equivalent to that of a Mode S transponder
(minimum triggering level of -74 dBm) will provide adequate
link margin to provide reliable detection of near co-altitude
aircraft in level flight at a range of 26 km (14 NM) provided
those aircraft are themselves equipped with transponders of
nominal transmit power.
3.4.2 CONTROLO F
RECEIVER THRESHOLD
3.4.2.1 ACAS receivers use a variable (dynamic)
threshold to control the effects of multipath. When the first
pulse of a reply is received, the variable receiver threshold
technique raises the receiver threshold from the minimum
Attachment Annex 10 - Aeronautical Telecommunications
triggering level (MTL) to a level at a fixed amount (e.g. 9 dB)
below the peak level of the received pulse. The receiver
threshold is maintained at this level for the duration of a Mode
A/C reply, at which time it returns to the MTL. When multipath
returns are weak compared to the direct-path reply, the
first pulse of the direct-path reply raises the receiver threshold
sufficiently so that the multipath returns are not detected.
3.4.2.2 Variablc rcccivcr thrcsholds havc historically bccn
avoided in Mode AIC reply processors because such
thresholding tends to discriminate against weak replies.
However, when used in conjunction with whisper-shout
interrogations, this disadvantage is largely overcome. On any
given step of the interrogation sequence it is possible for a
strong reply to raise the threshold and cause the rejection of a
weaker overlapping reply. However, with whisper-shout
interrogations, the overlapping replies received in response to
each interrogation are of approximately equal amplitudes since
the whisper-shout process sorts the targets into groups by
signal st~ngth.
3.4.2.3 The ACAS receiver MTL used in the reply
listening period following each whisper-shout interrogation
relates to the interrogation power in a prescribed manner. In
particular, less sensitive MTLs ate used with the lower
interrogation powers in order to control the Mode A/C fruit
rate in the ACAS receiver while still maintaining a balance
between the interrogation link and the reply link so that all
elicited replies are detected.
3.4.3 PULSE PROCESSING
3.4.3.1 A relatively wide dynamic range receiver faithfully
reproduces the received pulses. Provisions may be included for
locating the edges of received pulses with accuracy, and logic
may be provided for eliminating false framing pulses that are
synthesized by code pulses from real replies. The processor is
capable of resolving pulses in situations where overlapped
pulse edges are clearly distinguishable. It is also capable of
reconstructing the positions of hidden pulses when overlapping
pulscs of nearly the same amplilude cause ~llcfi ~lluwi~pu~lgsc s
to be obscured. The reply processor has the capacity for
handling and correctly decoding at least three overlapping
replies. Means are also provided for rejecting out-of-band
signals and for rejecting pulses with rise times exceeding 0.5
microsecond (typically, DME pulses).
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