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"victim" transponder, within the limits of uniform-in-range to
uniform-in-area. The left-hand side of the inequality allows an
ACAS unit to increase its interrogation rate if it transmits at
less than 250 W since low power transmissions are detected
by fewer transponders. Each normalized power value within
the summation in the left-hand side of this inequality contains
an exponent a which serves to match the inequality to the
localized ACAS distribution. The value of a defines the local
ACAS aircraft distribution curve and is derived from own
ACAS measurement of the distribution and number of other
ACAS within 56 krn (30 NM) range. As the ACAS
distribution varies from uniform-in-area (a = 1) to
uniform-in-range (a = 0.5), the density, and therefore the
electromagnetic impact, of ACAS aircraft in the vicinity of a
"victim" transponder becomes greater. This increased potential
for ACAS interference is offset by the greater degree of
interference limiting that results from using an exponent of
less than one in the normalized power values of the inequality.
The denominator of the first term on the right-hand side of this
inequality accounts for other ACAS interrogators in the
vicinity and the fact that all ACAS units must limit their
interrogation rate and power in a similar manner so that, as the
number of ACAS units in a region increases, the interrogation
rate and power from each of them decreases and the total
ACAS interrogation rate for any transponder remains less than
280 per second.
3.2.3.3 Within an airspace in which ACAS aircraft are
distributed between the limits of uniform-in-range to
uniform-in-area, and provided that the "victim" is taken off the
air for 35 microseconds by suppression or reply dead time
whenever it receives an ACAS interrogation, the total "off"
time caused by ACAS interrogations will then never exceed 1
per cent. Measurements and simulations indicate that the total
"off' time can be higher than 1 per cent in high-density
terminal areas because of ACAS aircraft distributions that are
beyond the region defined by uniform-in-area to
uniform-in-range and because of a Mode S transponder
recovery time to certain interrogations that is expected to be
greater than 35 microseconds. The second term on the
right-hand side of this inequality limits the maximum value of
the interrogation power-rate product for ACAS 11, regardless
of n,, in order to allow a portion of the total interference
limiting allocation to be used by ACAS I. The term, which is
matched to the ACAS distribution by the value of a in the
denominator, ensures that an individual ACAS II unit never
transmits more average power than it would if there were
approximately 26 other ACAS JI nearby distributed
uniformly-in-area or approximately 6 other ACAS I1 nearby
distributed nearly uniformly-in-range.
3.2.3.3.1 High-density terminal areas will suffer from
higher loads due to violation of the I per cent estimate at
approximately 14.8 - 18.5 km (8 - 10 NM) from touch down.
To ensure sufficient surveillance performance for both ACAS
and ground surveillance systems in such areas, ACAS flying
below 610 m (2 000 ft) AGL include also ACAS I1 and ACAS
JII operating on ground in the calculation of nb and n,. This
value was chosen for practical reasons:
a) the use of a radio altimeter allows sufficicnt
measurement accuracy at and below 610 m (2 000 ft);
and
b) it assumes aircraft are approaching on an ILS glide path.
In that case, 610 m (2 000 ft) AGL corresponds to a
distance of approximately 11.2 km (6 NM) from an
airport.
New approach procedures (e.g. based on MLS or GNSS) may
require additional considcrations to limit interference. And
even with ILS apprnach, it is recommended to establish
procedures switching ACAS I1 and ACAS UI to "stand-by"
while the aircraft is not on an active runway.
Annex 10 - Aeronautical Tclecornrnunications Volume ZV
3.2.3.4 Inequality (2) ensures that the transponder on
board the ACAS aircraft will not be turned off by mutual
suppression signals from the ACAS unit on the same aircraft
more than 1 per cent of the time.
3.2.3.5 Inequality (3) ensures that a "victim Mode NC
transponder will not generate more than 40 Mode A/C replies
in a one-second peiod in response to interrogations from all
the ACAS intermgaton within its detection range. Like
inequality (1) it includes terms to account for reduced transmit
power, to account for the other ACAS interrogators in the
vicinity, and to limit the power of a single ACAS unit. Forty
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