Home' RTCA Documents for Review : ACAS X MOPS DRAFT Vol.1 Contents 140
ACAS X MOPS
least +52 dBm in order to provide the necessary coverage at the crossover points of the
two adjacent beams. The ERP associated with the largest whisper -shout step in each of
the side beams shall (1297) be -4 dB relative to the forward beam and the ERP associated
with the largest whisper-shout step in the rear beam shall (1298) be -9 dB relative to the
forward beam in order to account for the lower closing speeds in those directions.
In terms of effective radiated power level, the largest interrogation transmitted from the
bottom omni-directional antenna shall (1299) be no larger than -18 dB relative to the
largest whisper-shout step in the forward beam of the top antenna in order to minimize
multipath interference and to reduce the impact of interference limiting.
188.8.131.52.4.2.3 Degarbling Performance as a Function of Azimuth Resolution
The degarbling improvement factor associated with directional interrogations can be
estimated by determining the "average beamwidth" which is defined as that azimuth
beamwidth over which an average transponder will reply to an interrogation. Since the
minimum and maximum reply beamwidths are bounded by P1/P2 = bin-width and 0 dB,
the average transponder can be considered to reply when its received P1 to P2 ratio
equals one-half of the bin-width. Although the average directional beamwidth is a good
indicator of the quantity of valid replies received, it does not predict the total quantity
actually observed through experience. Use of whisper -shout means that transponders are
being interrogated near their threshold level. This fact occasionally causes the P2 pulse
and P4 pulse of an interrogation that occurs outside of the main beam to trigger the
transponder into replying with a delayed Mode C reply. These unwanted replies, which
account for 20% of the valid replies, add to the synchronous garble problem and must be
accounted for. An "effective beamwidth" is defined which is 1.2 times the average
beamwidth described above. The degarbling improvement factor associated with a
specific directional antenna system relative to an omni antenna is then:
ATCRBS and Mode S Fruit Rejection
The equipment shall (1300) provide means for rejection of ATCRBS and Mode S replies
received in response to interrogations by other ACAS X interrogators and by ground-
based beacon interrogators.
Effective methods of rejecting ATCRBS and Mode S fruit are represented by the
surveillance requirements in §184.108.40.206.4 and the associated implementations suggested in
Appendix A, Sections A.5 through A.8.
Surveillance Tracking Requirements
In order to support the system performance specified in §2.2.2, ACAS X shall (1301)
provide the following surveillance functions in order to generate position reports on
Mode C and Mode S targets that are:
a. Closing at speeds of up to 1200 kt and at relative altitude rates of up to 10,000 ft/min.
b. Within 14 NM of ACAS X.
c. Within ±3000 ft and whenever possible within ±10,000 ft altitude relative to ACAS
The maximum closing rate capability may be increased beyond 1200 kt and/or the
surveillance range capability may be increased beyond 14 NM provided that the number
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