Publication: Magyar Közlöny
Issue: MK-2009-104 (Year: 2009, Number: 104)
Era: 2004-2010
Section: 
Paragraph Index: 2493

b) one fourth of the ratio of the number of ACAS aircraft, nb, within 11.2 km (6 NM) to the number of ACAS aircraft, nc, within 5.6 km (3 NM). A uniform-in-area distribution of ACAS aircraft within 56 km results in an α value of 1.0 and a uniform-in-range distribution results in a value of 0.5. Since decreasing values of α result in greater power reduction and therefore shorter surveillance ranges, the minimum value of α is constrained to 0.5 in order to preserve adequate surveillance range for collision avoidance in the highest density terminal areas. Additional constraints are imposed on the value of α1 to account for special situations in which the measured local ACAS distribution is: 1) based on numbers so small as to be inconclusive (nb = 1), in which case α1 is constrained to 1; 2) is inconsistent with a relatively high overall ACAS count (nb ≤ 4, nc ≤ 2, na > 25), in which case α1 is constrained to 1; or 3) is inconsistent with a relatively low overall ACAS count, (nc > 2, nb > 2nc, na < 40), in which case α1 is constrained to 0.5. 3.2.3.7 INTERFERENCE LIMITING PROCEDURES 3.2.3.7.1 At the beginning of each surveillance update interval, na, nb and nc are to be determined as indicated above. na is then used to evaluate the current right-hand limits in inequalities (1) and (3). Smoothed values of the Mode S variables in the inequalities are also to be calculated. nb and nc are used to compute the value of α1 according to the following expression: α1 = 1/4 [nb/nc] Annex 10 — Aeronautical Communications Volume IV 22/11/07 ATT-20 na and nb are used to compute the value of a α2 according to the following expression: [ ] a b Log n n Log α = In addition: IF [(nb ≤ 1) OR (nb > 4nc) OR (nb ≤ 4 AND nc ≤ 2 AND na > 25)] THEN α1 = 1.0; IF [(nb < 2nc) OR ((nc > 2) AND (nb > 2nc) AND (na < 40))] THEN α1 = 0.5; IF (na > 25nb) THEN α2 = 1.0; IF (na < 5nb) THEN α2 = 0.5; the value of α is the minimum of α1 and α2. 3.2.3.7.2 All air-to-air coordination interrogations and RA and ACAS broadcast interrogations are transmitted at full power. Air-to-air coordination interrogations and RA and ACAS broadcast interrogations are not included in the summations of Mode S interrogations in the left-hand terms of these inequalities. Whenever an RA is posted, surveillance interrogations to that intruder may be transmitted at full power to allow for maximum link reliability. Because the frequency of RAs is very low, these transmissions do not result in a measurable increase in interference. 3.2.3.7.3 If the smoothed value of the left-hand side of either inequality (1) or (2) equals or exceeds the current limit and own ACAS aircraft are operating below a pressure-altitude of 5 490 m (18 000 ft), both the Mode S and Mode A/C surveillance parameters are to be modified to satisfy the inequalities. If the left-hand side of inequality (3) exceeds the current limit and own ACAS aircraft are operating below a pressure-altitude of 5 490 m (18 000 ft), Mode A/C surveillance parameters are modified to satisfy the inequalities. 3.2.3.7.4 Mode A/C surveillance can be modified by sequentially eliminating steps from the whisper-shout sequence described in 3.2.2. Each whisper-shout step is uniquely associated with a receiver MTL setting. Thus, the receiver sensitivity in Mode A/C surveillance periods will be automatically tailored to match these power reductions. 3.2.3.7.5 The overall surveillance sensitivity for Mode S targets can be reduced by reducing the interrogation power and by increasing the receiver MTL during all Mode S squitter listening periods. This will indirectly reduce the Mode S interrogation rate by reducing the target count. Many Mode S interrogations are acquisition interrogations transmitted to targets of unknown range. It is thus not effective to directly control the Mode S interrogation rate simply by dropping long-range targets from the track file. 3.2.3.7.6 For airborne ACAS, the Mode A/C and Mode S surveillance power and sensitivity reductions are to be accomplished such that equality between the surveillance ranges for Mode S and Mode A/C targets exists in the forward beam. In order to provide a reliable 11.2 km (6 NM) surveillance range in all directions for nb, the maximum allowed interference limiting power reduction in any beam for an airborne ACAS unit is 10 dB for Mode S and 7 dB for Mode A/C. Mode A/C surveillance power and sensitivity reductions for ACAS on the ground are to be accomplished such as to achieve equal whisper-shout capability in each beam. This requires that Mode A/C power and sensitivity reduction be accomplished in the forward beam until it is equivalent to the side beams and then in the forward and side beams until they are equivalent to the rear beam. In order to provide a reliable 5.6 km (3 NM) surveillance range in all directions for surveillance prior to departure, the maximum allowed interference limiting power reduction for an ACAS unit on the ground is as follows:

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