Home' RTCA Documents for Review : ACAS X MOPS DRAFT Vol.1 Contents 58
ACAS X MOPS
PA(k) is the total radiated power (in watts) from the antenna for the k-th Mode C
is a variable derived from surveillance of Active CAS aircraft and is used to match
inequality 1 to the measured distribution of nearby Active CAS aircraft.
The factor B is approximately 1.2 for a four-beam directional antenna.
The three inequalities are associated with the following physical mechanisms: 1)
reduction in "on" time of other transponders caused by Active CAS
interrogations, 2) reduction in "on" time of own transponder caused by mutual
suppression during transmission of interrogations, and 3) ATCRBS fruit caused
by Active CAS Mode C interrogations. Inequality 1 ensures that a "victim"
transponder will never detect more than 280 Active CAS interrogations in a one-
second period from all the Active CAS interrogators within 30 NM. The left-
hand side of the inequality allows an ACAS X unit to increase its interrogation
rate if it transmits at less than 250 W since low-power transmissions are detected
by fewer transponders. The denominator of the first term on the right-hand side
of this inequality accounts for other Active CAS interrogators in the vicinity and
the fact that all Active CAS units must limit their interrogation rate and power in
a similar manner so that, as the number of Active CAS units in a region
increases, the interrogation rate and power from each of them decreases and the
total Active CAS interrogation rate for any nearby transponder remains less than
280 per second. If the victim is taken off the air for 35 microseconds by
suppression or reply dead time whenever it receives an Active CAS interrogation,
the total "off" time caused by Active CAS interrogations will then never exceed
1%. The term 11/2 on the right-hand side ensures that an individual ACAS X
unit with a four-beam directional interrogation antenna and operating in a
uniform-in-area Active CAS distribution (i.e., =1) never transmits more
average power than it would if there were approximately 26 other Active CAS
units nearby or if operating in a uniform-in-range distribution (i.e., =0.5) never
transmits more average power than it would if there were approximately 6 other
Active CAS units nearby.
Inequality 2 ensures that the transponder on board ownship will not be turned off
by mutual suppression signals from the Active CAS unit on the same aircraft
more than 1% of the time.
Inequality 3 ensures that a victim ATCRBS transponder will not generate more
than 40 ATCRBS replies in a one-second period in response to interrogations
from all the Active CAS interrogators within its detection range. Like inequality
1 it includes terms to account for reduced transmit power, to account for the
other Active CAS interrogators in the vicinity, and to limit the power of a single
Active CAS unit. 40 ATCRBS replies per second is approximately 20% of the
reply rate for a transponder operating without Active CAS in a busy area of
multiple ATCRBS ground sensor coverage.
The three interference limiting inequalities are designed to check if the current
Mode C and Mode S surveillance are aligned with the goals of the ACAS X
contribution to the communication environment as a system, which are stated in
note 2 above.
The inequalities are not meant to restrict individual
interrogations, but rather modify Mode C and Mode S surveillance parameters
so that the inequalities can be satisfied in the future. The procedures above
describe how to modify Mode C and Mode S surveillance for interference
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