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When the value of any of the smoothed limits is exceeded, the
appropriate action is required to limit interference within one
surveillance update interval. Means are to be provided tor
gradually restoring the surveillance sensitivity when the
environment subsequently improves enough Lo allow the
interference limits to be relaxed.
3.2.3.7.7 ACAS cross-link interrogations are included in
the summation of Mode S interrogations in the left-hand terms
of the interference limiting inequalities.
3.2.3.8 IMPLEMENTATION OF A n P I c u
IhTERFEREhrCE LIMITING PROCEDURE
3.2.3.8.1 The following describes one possible
implementation of an interference limiting procedure. It varies
the system parameters appearing in inequalities (I), (2) and (3)
to maximize and maintain approximate equality between the
estimated surveiIlance ranges for Modc S and Mode A/C
targets. In evaluating these inequalitics, 8-second averages of
the Modc S parameters are used, and currcnt or anticipated
values of the Mode A/C parameters are used. The procedure is
illustrated in the flow chart of Figure A-4.
Annex 10 - Aeronautical Telecommunications Volume ZV
ELIMINATE W-S
STEPS TO SATISFY
INEQUALITY (3)
SET ON OTHER +RETURN
NO
--+ DROP 1 W-S STEP J - L - ADD 1 W-S STEP
Figure Ad. Interference limiting flow diagram
- t
FOR MODE S
REDUCE POWER I dl3
INCREASE MTL 1dB
FOR MODE S
INCREASE POWER 1 dB
REDUCE MTL 1 dB
RETURN
1 , SET8SFREEZE
ON OTHER CHANGES - RETURN
Attachment Annex 10 - Aeronautical Xelecomrnunications
3.2.3.8.2 Step 1. The first step in the control process is to
reduce the number of whisper-shout steps tentatively
scheduled for use during the present scan if either:
a) inequality (3) is violated; or
b) inequality (1) or (2) is violated and the Mode S
surveillance range of the last scan does not exceed the
Mode A/C surveillance range that would result from use
of the scheduled whisper-shout sequence.
Whisper-shout steps are eliminated in the order dictated by the
design of the Mode AIC processor and the number of steps
eliminated is just large enough to ensure that neither of the
above conditions is satisfied. The value of the number of
whisper-shout steps tentatively scheduled for use is initialized
at the number used on the last scan.
The relative magnitudes of the ode S and Mode AIC
surveillance ranges are determined from the estimated
effective radiated power (EW) seen by targets with Mode S
and Mode A/C transponders located directly ahead of the
ACAS aircraft. The ERP in a given direction is determined by
the product of the power input to the antenna, and the antenna
pattern gain in that direction. If the transponder sensitivities
were identical, the Mode S range would be more or less than
the Mode A/C range according to whether the Mode S
transmitted power was more or less than the Mode A/C
transmitted power. Since Mode A/C transponders may have
somewhat lower sensitivities than Mode S transponders, the
Mode A/C range is assumed to be greater than the Mode S
range if, and only if, the Mode AIC power exceeds the Mode S
power by 3 dB.
3.2.3.8.3 Step 2. The second step in the controlling process
is to reduce the Mode S interrogation power for acquisition by
1 dB, and to increase the MTL for Mode S squitter listening by
1 dB from the values last used, if inequality (1) or (2) is
violated and the Mode S surveillance range of the last scan
exceeds the Mode AIC surveillance range that would result
from use of the scheduled whisper-shout sequence.
Once such a change has been made, the only change allowed
during the ensuing 8 seconds is a reduction in the number of
whisper-shout steps if needed to satisfy inequality (3). This 8-
second freeze allows the effect of the Mode S changes to
becomc apparent since the 8-second averages used in
inequalities (1) and (2) then will be determined by the
behaviour of the system since the change.
b) the Mode S surveillance range of the last scan exceeds
the Mode A/C surveillance range that would result from
use of the scheduled sequence; and
As many steps are added as possible without violating a) or b)
above.
3.2.3.8.5 Step 4. Finally, if condition a) of 3.2.3.8.4
above is satisfied, but condition b) is not, an estimate is made
of the effects of increasing the Mode S interrogation power for
acquisition by 1 dB and reducing the MTL for Mode S
squitterslfruit by 1 dB. If the estimate indicates that inequalities
(I) and (2) will not continue to be satisfied, the 1 dB
change is not made. If the estimate indicates that they will
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