Publication: Magyar Közlöny
Issue: MK-2007-70 (Year: 2007, Number: 70)
Era: 2004-2010
Section: Melléklet a 2007. évi XLVI. törvényhez
Paragraph Index: 4813

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 squitters/fruit by 1 dB. If the estimate indicates that inequalities (1) and (2) will not continue to be satisfied, the 1 dB change is not made. If the estimate indicates that they will continue to be satisfied, the 1 dB change is made and no further changes in either the Mode A/C or Mode S parameters are made for the ensuing 8 seconds, except as described in 3.2.3.8.3 above. 3.2.4 INTERROGATION JITTER Mode A/C interrogations from ACAS equipment are intentionally jittered to avoid chance synchronous interference with other ground-based and airborne interrogators. It is not necessary to jitter the Mode S surveillance interrogations because of the inherently random nature of the Mode S interrogation scheduling process for ACAS. 3.3 Antennas 3.3.1 USE OF DIRECTIONAL INTERROGATIONS 3.3.1.1 A directional antenna is recommended for reliable surveillance of Mode A/C targets in aircraft densities up to 0.087 aircraft per square km (0.3 aircraft per square NM). The recommended antenna system consists of a four-beam antenna mounted on top of the aircraft and an omnidirectional antenna on the bottom. A directional antenna may also be used instead of the omnidirectional antenna on the bottom of the aircraft. The directional antenna sequentially generates beams that point in the forward, aft, left, and right directions. Together these provide surveillance coverage for targets at all azimuth angles without the need for intermediate pointing angles. 3.3.1.2 The directional antenna typically has a 3-dB beam width (BW) in azimuth of 90 ±10 degrees for all elevation angles between +20 and –15 degrees. The interrogation beamwidth is to be limited by transmission of a P2 side-lobe suppression pulse following each P1 interrogation pulse by 2 microseconds. The P2 pulse is transmitted on a separate control pattern (which may be omnidirectional). 2007/70/II. szám Annex 10 — Aeronautical Telecommunications Volume IV 28/11/02 ATT-22 3.3.1.3 There is need for timely detection of aircraft approaching with low closing speeds from above and below. Detection of such aircraft suggests a need for sufficient antenna gain within a ±10 degree elevation angle relative to the ACAS aircraft pitch plane. An ACAS directional antenna typically has a nominal 3 dB vertical beamwidth of 30 degrees. 3.3.1.4 The shape of the directional antenna patterns and the relative amplitude of the P2 transmissions is controlled such that a) a maximum suppression transponder located at any azimuth angle between 0 and 360 degrees and at any elevation angle between +20 and –15 degrees would reply to interrogations from at least one of the four directional beams and b) a minimum suppression transponder would reply to interrogations from no more than two adjacent directional beams. A maximum suppression transponder is defined as one that replies only when the received ratio of P1 to P2 exceeds 3 dB. A minimum suppression transponder is defined as one that replies when the received ratio of P1 to P2 exceeds 0 dB. 3.3.1.5 The effective radiated power (ERP) from each antenna beam (forward, left, right, aft, omni) is expected to be within ±2 dB of its respective nominal value as given in Figure A-2a. 3.3.1.6 A forward directional transmission, for which TRP = 49 dBm and BW = 90o has a power gain product at beam centre of approximately, This is 1 dB greater than the nominal and allows for adequate coverage at the cross-over points of the directional beams. The TRP of the side and aft beams is reduced relative to the front beam to account for the lower closing speeds that occur when aircraft approach from these directions. Mode A/C surveillance performance will generally improve as the directivity (and hence the number of beams) is increased for the top-mounted antenna. However, the use of a directional antenna on the bottom would provide only marginal improvement in detectability and would, if used at full power, degrade the overall performance of the equipment by increasing the false track rate due to ground-bounce multipath. 3.3.2 DIRECTION FINDING The angle-of-arrival of the transmissions from the replying transponders can be determined with better than 10-degree RMS accuracy by means of several simple and practical direction-finding techniques. These techniques typically employ a set of four or five monopole radiating elements mounted on the aircraft surface in a square array with quarterwave spacing. The signals from these elements may be combined so as to generate from two to four distinct beams which may be compared in phase or amplitude to provide an estimate of the direction of arrival of the received signal. This level of direction-finding accuracy is adequate to provide the pilot with TAs to effectively aid the visual acquisition of intruding aircraft. 3.3.3 DIRECTIONAL TRANSMISSION FOR CONTROL OF SYNCHRONOUS GARBLE 3.3.3.1 The use of directional interrogation is one technique for reducing synchronous garble. The directional interrogation can reduce the size of the interrogation region. Coverage must be provided in all directions. Hence, multiple beams are used to elicit replies from all aircraft in the vicinity of the ACAS-equipped aircraft. Care must be taken to overlap the beams so that gaps in coverage do not exist between beams. 3.3.3.2 The antenna may be a relatively simple array capable of switching among typically four or eight discrete beam positions. For four beam positions, the antenna beamwidth is expected to be on the order of 100o. The effective antenna beamwidth for interrogating Mode A/C transponders can be made more narrow than the 3-dB beamwidth by means of transmitter side-lobe suppression. 3.3.4 ANTENNA LOCATION The top-mounted directional antenna is to be located on the aircraft centre line and as far forward as possible. The ACAS antennas and the Mode S transponder antennas are to be mounted as far apart as possible on the airframe to minimize coupling of leakage energy from unit to unit. The spacing must never be less than 0.5 m (1.5 ft), as this spacing results in a coupling loss of at least 20 dB. 3.4 Receiver and processor 3.4.1 SENSITIVITY A sensitivity equivalent to that of a Mode S transponder (minimum triggering level of –74 dBm) will provide adequate link margin to provide reliable detection of near co-altitude aircraft in level flight at a range of 26 km (14 NM) provided those aircraft are themselves equipped with transponders of nominal transmit power. 3.4.2 CONTROL OF RECEIVER THRESHOLD 3.4.2.1 ACAS receivers use a variable (dynamic) threshold to control the effects of multipath. When the first pulse of a reply is received, the variable receiver threshold technique raises the receiver threshold from the minimum PG = TRP = 55 dBm BW/360o 2007/70/II. szám Attachment Annex 10 — Aeronautical Telecommunications ATT-23 28/11/02 triggering level (MTL) to a level at a fixed amount (e.g. 9 dB) below the peak level of the received pulse. The receiver threshold is maintained at this level for the duration of a Mode A/C reply, at which time it returns to the MTL. When multipath returns are weak compared to the direct-path reply, the first pulse of the direct-path reply raises the receiver threshold sufficiently so that the multipath returns are not detected. 3.4.2.2 Variable receiver thresholds have historically been avoided in Mode A/C reply processors because such thresholding tends to discriminate against weak replies. However, when used in conjunction with whisper-shout interrogations, this disadvantage is largely overcome. On any given step of the interrogation sequence it is possible for a strong reply to raise the threshold and cause the rejection of a weaker overlapping reply. However, with whisper-shout interrogations, the overlapping replies received in response to each interrogation are of approximately equal amplitudes since the whisper-shout process sorts the targets into groups by signal strength. 3.4.2.3 The ACAS receiver MTL used in the reply listening period following each whisper-shout interrogation relates to the interrogation power in a prescribed manner. In particular, less sensitive MTLs are used with the lower interrogation powers in order to control the Mode A/C fruit rate in the ACAS receiver while still maintaining a balance between the interrogation link and the reply link so that all elicited replies are detected. 3.4.3 PULSE PROCESSING 3.4.3.1 A relatively wide dynamic range receiver faithfully reproduces the received pulses. Provisions may be included for locating the edges of received pulses with accuracy, and logic may be provided for eliminating false framing pulses that are synthesized by code pulses from real replies. The processor is capable of resolving pulses in situations where overlapped pulse edges are clearly distinguishable. It is also capable of reconstructing the positions of hidden pulses when overlapping pulses of nearly the same amplitude cause the following pulses to be obscured. The reply processor has the capacity for handling and correctly decoding at least three overlapping replies. Means are also provided for rejecting out-of-band signals and for rejecting pulses with rise times exceeding 0.5 microsecond (typically, DME pulses). 3.4.3.2 If a Mode S reply is received during a Mode C listening period, a string of false Mode C fruit replies may be generated. The ACAS equipment is expected to reject these false replies. 3.4.4 ERROR DETECTION AND CORRECTION 3.4.4.1 ACAS avionics intended for use in airspace characterized by closing speeds greater than 260 m/s (500 kt) and densities greater than 0.009 aircraft per km2 (0.03 aircraft per NM2) or closing speeds less than 260 m/s (500 kt) and densities greater than 0.04 aircraft per km2 (0.14 aircraft per NM2) requires a capability for Mode S reply error correction. In these high densities, error correction is necessary to overcome the effects of Mode A/C fruit. Mode S error correction permits successful reception of a Mode S reply in the presence of one overlapping Mode A/C reply. 3.4.4.2 Error correction decoding is to be used for the following replies: DF = 11 all-call replies, DF = 0 short air-air surveillance replies, and DF = 16 long air-air surveillance replies (both acquisition and non-acquisition). In addition, passive monitoring of DF = 4 short surveillance altitude replies requires error correction decoding. 3.4.4.3 If two or more acquisition replies requiring error correction are received within the Mode S range acquisition window, it may be impractical to apply error correction to more than the first received reply. Acquisition replies other than the first do not need to be corrected when this occurs. 3.4.5 RECEIVER SIDE-LOBE SUPPRESSION ACAS equipment that interrogates directionally may use receiver side-lobe suppression techniques to eliminate replies (fruit) generated by nearby aircraft that are outside the interrogated sector. This reduces the number of replies processed during the surveillance update period. 3.4.6 DUAL MINIMUM TRIGGERING LEVELS If the MTL of the receiver used by ACAS is lowered to obtain longer range operation with extended squitter, provision must be made to label squitter receptions that were received at the MTL that would have been used by an unmodified ACAS receiver. Squitter receptions that are received at the conventional MTL or higher are fed to the ACAS surveillance function. Squitter receptions that are received below the conventional MTL are not used for ACAS surveillance but are routed directly to the extended squitter application. This filtering by MTL is necessary to prevent ACAS from attempting to interrogate aircraft that are beyond the range of its active surveillance capability. This would increase the ACAS interrogation rate without providing any improved surveillance performance. Use of the conventional MTL for the ACAS surveillance function preserves the current operation of ACAS surveillance when operating with a receiver with an improved MTL. 3.5 Collision avoidance algorithms Note.— The guidance material on the collision avoidance logic of ACAS II is organized in two sections. This section addresses the Standards in the ACAS SARPs and elaborates on 2007/70/II. szám Annex 10 — Aeronautical Telecommunications Volume IV 28/11/02 ATT-24 important concepts using the design features of a specific implementation of the ACAS logic as examples. Section 4 provides further details on the algorithms and parameters used by this particular ACAS implementation. As a consequence of this arrangement, paragraphs in this section often refer to paragraphs in the next one. 3.5.1 GENERAL 3.5.1.1 The ACAS algorithms operate in a cycle repeated nominally once per second. At the beginning of the cycle, surveillance reports are used to update the tracks of all intruders and to initiate new tracks as required. Each intruder is then represented by a current estimate of its range, range rate, altitude, altitude rate, and perhaps, its bearing. Own aircraft altitude and altitude rate estimates are also updated. 3.5.1.2 After the tracks have been updated, the threat detection algorithms are used to determine which intruders are potential collision threats. Two threat levels are defined: potential threat and threat. Potential threats warrant TAs while threats warrant RAs. 3.5.1.3 The resolution algorithms generate an RA intended to provide vertical separation from all threats identified by the threat-detection algorithms. Coordination with each equipped threat occurs as part of the process of selecting the RA. Pairwise coordination with each equipped threat is necessary to establish which aircraft is to pass above the other and thus guarantee avoidance manoeuvres that are compatible. 3.5.2 THREAT DETECTION 3.5.2.1 Collision threat detection is based on simultaneous proximity in range and altitude. ACAS uses range rate and altitude rate data to extrapolate the positions of the intruder and own aircraft. If within a short time interval (e.g. 25 seconds hence) the range of the intruder is expected to be “small” and the altitude separation is expected to be “small”, the intruder is declared a threat. Alternatively, the threat declaration may be based on current range and altitude separations which are “small”. The algorithm parameters which establish how far into the future positions are extrapolated, and which establish thresholds for determining when separations are “small”, are selected in accordance with the sensitivity level at which the threat detection algorithms are operating. 3.5.2.2 Each sensitivity level defines a specific set of values for the detection parameters used by the algorithms. These include threshold values for the predicted time to closest approach, the minimum slant range, and the vertical separation. Through the process of sensitivity level control, these parameters are assigned different values to account for the smaller aircraft separations that occur in dense terminal airspace. Sensitivity level may be selected automatically using the altitude of own aircraft, or may be selected by command from a Mode S ground station, or by a manual pilot switch (see 3.5.12). 3.5.2.3 The values used for threat detection parameters cannot be optimum for all situations because ACAS is handicapped by its lack of knowledge of intruder intent. The result is that a balance has to be struck between the need to give adequate warning of an impending collision and the possible generation of unnecessary alerts. The latter may result from encounters that are resolved at the last moment by intruder manoeuvres. A feature of ACAS that helps in this respect is the variability of the protected volume of airspace. This volume is automatically coupled in size to the relative speed between the two aircraft, and is automatically aligned in a direction parallel to the relative velocity vector. Bearing plays no part in this process. Each encounter gives rise to a protected volume tailored to that encounter. In a multi-aircraft situation there is an individual protected volume for the ACAS aircraft paired with each threat. 3.5.3 PROTECTED VOLUME An intruder becomes a threat when it penetrates a protected volume enclosing own aircraft. The protected volume is defined by means of a range test (using range data only) and an altitude test (using altitude and range data). Application of these tests delivers a positive or a negative result (implying that the threat is inside or outside the appropriate part of the protected volume). An intruder is declared a threat when both tests give a positive result. 3.5.3.1 PROTECTED VOLUME TERMS’ 3.5.3.1 DESCRIPTION Collision plane. The plane containing the range vector and the instantaneous relative velocity vector originating at the intruder. Critical cross-sectional area. The maximum cross-sectional area of the protected volume in a plane orthogonal to the major axis. Instantaneous relative velocity(ies). The modulus of the current value of relative velocity. Linear miss distance (ma). The minimum value that range will take on the assumption that both the intruder and own aircraft proceed from their current positions with unaccelerated motions. Linear time to closest approach (ta). The time it would take to reach closest approach if both the intruder and own aircraft proceed from their current positions with unaccelerated motions. 2007/70/II. szám Attachment Annex 10 — Aeronautical Telecommunications ATT-25 28/11/02 Given that the only information available to ACAS to make range predictions are range and range rate estimates, both the linear miss distance and the linear time to closest approach are unobservable quantities. The unobservable quantities, linear miss distance and linear time to closest approach, are related to the observable quantities range r and range rate by the following equality: Major axis. In the context of the protected volume, the line through the ACAS II aircraft which is parallel to the instantaneous relative velocity vector. Range convergence. The aircraft is deemed to be converging in range if the range rate is less than or equal to zero. 3.5.4 RANGE TEST 3.5.4.1 The protected volume resulting from the range test used in the ACAS implementation described in Section 4 can be defined in terms of the maximum dimensions of a realizable implementation of the test which is illustrated by Figure A-5. This shows a section through the protected volume generated by a range test in the plane containing both aircraft and the instantaneous relative velocity vector. The protected volume is that which would be produced by rotating the solid curve about the x axis. Note that the length of the major axis is a function of the relative speed, s. For the realizable range test, the radius of the maximum cross section through the protected volume in a plane normal to the instantaneous relative velocity vector is mc. This represents the maximum miss distance for which an alert can be generated if the relative velocity at the time of entry to the protected volume is maintained to closest approach. The length of the major axis is the principal feature determining warning time while mc controls the projected miss distance which is likely to generate an alert. Ideally, the warning time would be T seconds and mc would be such that only intruders projected to have miss distances less than Dm (the radius of the broken-line circle in Figure A-5) would qualify for an alert. The significance of Dm, when specified as in the ACAS implementation described in Section 4, is that, to a good approximation, it represents the lateral displacement experienced by an aircraft over the time T when turning with a constant acceleration of g/3 (bank angle = 18o). Thus an encounter with a projected miss distance of Dm when the time to closest approach is T can result in a collision if either aircraft is manoeuvring with an acceleration of g/3. In the absence of adequate bearing rate or range acceleration data, ACAS cannot achieve the ideal. Figure A-6 shows the maximum value for mc (i.e. as a function of relative speed and sensitivity level). When the relative speed is very low, as can occur in a tail-chase, the protected volume produced by the range test becomes a sphere of radius Dm centred on the ACAS aircraft. 3.5.4.2 Essentially, the range test gives a positive result if, when approximately T seconds remain before closest approach, the relative velocity vector can be projected to pass through a circle of radius mc centred on the ACAS aircraft and placed in the plane normal to the relative velocity vector. Since the value of mc is very large compared to the value for adequate vertical separation, the use of the range test alone would generate a large number of unnecessary alerts. It is therefore necessary to tailor the range test protected volume to more modest proportions using altitude data. Inevitably, this reduces the immunity to manoeuvres in the vertical plane. 3.5.4.3 The constraints on the range test are designed to give a nominal warning time of T seconds allowing for a manoeuvre producing a displacement of Dm normal to the relative velocity vector. It may be demonstrated that, for an encounter having a reasonably large relative velocity, the relative acceleration produced by a turning aircraft is nearly normal to the relative velocity vector. For low relative speed there can be a substantial component of acceleration in the direction of a relative velocity. Erosion of the warning time due to this component is compensated by having a minimum length for the major axis of the protected volume which is greater than sT. 3.5.5 ALTITUDE TEST 3.5.5.1 The objective of the altitude test is to filter out intruders that give a positive result for the range test but are nevertheless adequately separated in the vertical dimension. The altitude test is used to reduce alert rate in the knowledge that the standard vertical separation distances for aircraft are normally much less than the standard horizontal separation distances. An inevitable result is that the acceleration protection, nominally provided by the range test in all planes, is largely restricted to the horizontal plane. Also, even in the absence of relative acceleration, the altitude test can delay warnings if some vertical separation at closest approach is predicted to exist. A view in elevation of the relative motion of two aircraft is shown in Figure A-7. AOB represents a plane normal to the relative velocity vector and containing the ACAS aircraft. The intruder may be horizontally displaced from the ACAS so it is not necessarily in the plane of the diagram. The essential feature of the altitude test is that it aims to give a positive result if the projected vertical miss distance is less than Zm. In the ACAS implementation described in Section 4, Zm varies with altitude in steps from 180 m (600 ft) to 240 m (800 ft). 3.5.5.2 Since the main interest is in intruders with projected miss distances less than Dm, an ideal altitude test (in combination with an ideal range test) would give a positive result if, inter alia, the relative velocity vector were projected to pass through the critical area shown by the solid outline in Figure A-7. In practice, the altitude test and the range test outlined in 3.5.1.2 tend to be satisfied if the vector passes ta = (r2 – ma 2) r· rr· – ( ) mˆ c 2007/70/II. szám Annex 10 — Aeronautical Telecommunications Volume IV 28/11/02 ATT-26 Figure A-5. Section through protected volume in the instantaneous collision plane ACAS X D sT/2 + (D + s T ) 2 ½ m s Y Inst. Intruder rel. vel. Major axis m mc 2007/70/II. szám Attachment Annex 10 — Aeronautical Telecommunications ATT-27 28/11/02 through the larger area defined by the broken outline. Those intruders passing through the shaded areas are likely to give rise to unnecessary alerts. 3.5.5.3 The altitude test is no better placed to predict the time to closest approach than is the range test. This means that, if no other conditions are applied, the range test determines the time of the alert. However, an additional feature of the altitude test of the ACAS implementation described in Section 4 attempts to guard against the eventuality that one of the aircraft levels off above or below the other, thus avoiding a close encounter. Two types of encounter are recognized: the first in which the current altitude separation is less than Zt (see 4.3.4.2); and a second, in which the current altitude separation is greater than Zt and the aircraft are converging in altitude. For the first type, the altitude test requires only that the critical area is projected to be penetrated. For the second an additional condition is that the time to reach co-altitude is to be less than or equal to a time threshold that is sometimes less than T, the nominal warning time. The effect is that warning time is controlled by the range test for intruders that are projected to cross in altitude before closest approach while later warnings are given for altitude crossings beyond closest approach. 3.5.6 ESTABLISHED THREATS 3.5.6.1 An established threat is an intruder that has been declared a threat and still merits a resolution advisory. 3.5.6.2 The need to give a positive result for both the range test and the altitude test on the same cycle of operation Figure A-6. Critical miss distance 1 000 1 200 m = (D + s T /4) c m ½ S = 7 o S = 6 o S = 5 o S = 4 o S = 3 o Instantaneous relative speed s (kt) m (NM) c 2007/70/II. szám Annex 10 — Aeronautical Telecommunications Volume IV 28/11/02 ATT-28 before declaring an intruder to be a threat (3.5.2.1) applies only for new threats. Subsequently, only the range test is applied and a positive result has the effect of maintaining threat status. The reason for omitting the altitude test is that a rapid pilot response, or the fact that the intruder initially only just satisfied the altitude criteria, may result in cancellation of threat status before reaching closest approach. 3.5.7 ALERT RATE 3.5.7.1 The principal variables controlling alert rate are relative velocity, miss distance and the ambient aircraft density. The principal parameters affecting alert rate are T, Dm and Zm. Alert rates can be calculated for constant velocity random traffic but the influences of see-and-avoid and ATC make such calculations for real traffic very difficult. Figure A-6 gives some guidance on some features of an encounter that might give rise to an alert although it gives no assistance concerning the result of the altitude test. For example, it can be seen that, for sensitivity level 5 (altitudes between FL 50 and FL 100) there can be no alert if the horizontal separation is greater than 5.5 km (3 NM) and the relative speed is less than about 440 m/s (850 kt). 3.5.7.2 Simulations using ground-based radar surveillance data and initial experience with ACAS equipments have indicated that the overall alert rate ranges from about 1 in 30 flight hours to 1 in 50 flight hours in typical busy airspaces. 3.5.8 THREAT RESOLUTION 3.5.8.1 COORDINATION If the threat aircraft is equipped with ACAS II or ACAS III, own ACAS is required to coordinate with the threat aircraft’s ACAS via the Mode S data link to ensure that compatible RAs are selected. The nature of the advisory selected can also be influenced by the fact that the threat is ACAS-equipped. 3.5.8.2 CLASSIFICATION OF RESOLUTION 3.5.8.2 ADVISORIES 3.5.8.2.1 ACAS escape manoeuvres are confined to the vertical plane and can be characterized by a sense (up or down) and a strength. The objective of an RA with an upward sense is to ensure that own aircraft will safely pass above the threat. The objective of an RA with a downward sense is to Figure A-7. Critical area for ideal altitude test A B O ACAS Zm Zm (a) Z cos ø m ACAS mc Dm (b) ø s Intruder m m m m c 2007/70/II. szám Attachment Annex 10 — Aeronautical Telecommunications ATT-29 28/11/02 ensure that own aircraft will safely pass below the threat. Examples of RA strengths with the upward sense are “limit vertical speed” (to a specified target descent speed), “do not descend”, or “climb”. Examples of equivalent RA strengths with the downward sense are “limit vertical speed” (to a specified target climb speed), “do not climb”, or “descend”. RAs are of two types: “positive”, meaning a requirement to climb or descend at a particular rate; and “vertical speed limit”, meaning that a prescribed range of vertical speed must be avoided. Any advisory may be “corrective” or “preventive”. A corrective advisory requires a change in own aircraft’s current vertical rate whereas a preventive advisory does not. 3.5.8.2.2 It is expected that the RA generated be consistent with flight path limitations in some regimes of flight, due to flight envelope restrictions and aircraft configurations that reduce climb capability. It is expected that the aircraft’s manoeuvre limitation indications available to ACAS will offer a conservative assessment of the actual aircraft performance capabilities. This is particularly true of climb inhibit. In the rare and urgent case of a high altitude downward sense RA being reversed to a climb, it is expected that, very often, the aircraft performance capabilities needed to comply with the RA will be available despite the climb inhibit. When such capabilities are not available, it is expected that the pilot will always be able to comply with the reversal at least partially by promptly levelling-off. 3.5.8.3 ALTITUDE SEPARATION GOAL 3.5.8.3.1 To be certain of avoiding a collision, ACAS must provide a true altitude separation at closest approach which is commensurate with aircraft dimensions and worst-case orientation of the aircraft. Since only measured altitude data are available, due allowance must be made for altimetry errors in both aircraft. Furthermore, the avoiding action must be commenced before closest approach so it is possible that this action will be based on predicted altitude separation at closest approach, which introduces a further source of error. These factors lead to a requirement that the RA provided to the pilot should be such that the desired altitude separation at closest approach can be achieved in the time available. This altitude separation goal, Al, must vary as a function of altitude in order to adequately compensate for altimetry errors. In the ACAS implementation described in Section 4, Al varies from 90 m (300 ft) to 210 m (700 ft). 3.5.8.3.2 The time to closest approach cannot be estimated accurately because the miss distance is not known, the threat could manoeuvre and the range observations are imperfect. However, limits that have been found useful and acceptable are the times to closest approach assuming the miss distance to take the largest value of concern (Dm) and the value zero, and that all other sources of error have been neglected. This interval is critical for encounters in which the range rate takes on very small values. By maintaining the altitude separation over the entire interval, the selection of the RA is made immune to potentially large errors in estimating the time of minimum range. Such errors can result from small absolute errors in estimating range rate. For preventive RAs, the assumption of an immediate change of rate to the limit recommended by the RA will cause the calculation to deliver a bound (upper for downward RAs, lower for upward RAs) on the altitude of own aircraft at closest approach. 3.5.8.4 MINIMUM DISRUPTION 3.5.8.4.1 In principle, the larger altitude separations goals could be achieved by a more vigorous escape manoeuvre but constraints are passenger comfort, aircraft capability and deviation from ATC clearance. The ACAS parameters described in Section 4 below are based on an anticipation that the typical altitude rate needed to avoid a collision is 1 500 ft/min. 3.5.8.4.2 The initial choice of the sense and strength of the RA is intended, subject to the exceptions described below, to require the smallest possible change in the vertical trajectory of the ACAS aircraft. And the advisory is expected to be appropriately weakened, if possible, at later stages of the encounter, and removed altogether when the desired separation has been achieved at closest approach. A prime consideration is the minimization of any departure from an ATC clearance. 3.5.8.5 PILOT RESPONSE Since the pilot exercises such a major influence on the effectiveness of the system, it is necessary for any ACAS design to make certain assumptions concerning the response of the pilot. The ACAS implementation described in Section 4 uses a response delay of 5 seconds for a new advisory and a vertical acceleration of g/4 to establish the escape velocity. The response time reduces to 2.5 seconds for subsequent advisory changes. ACAS may not provide adequate vertical separation if the pilot response delay exceeds the expected pilot response delay assumed by the design. 3.5.8.6 INTRUDERS IN LEVEL FLIGHT 3.5.8.6.1 Intruders that are flying level at the time of the alert and continue thereafter in level flight present few problems for ACAS. If own aircraft is also in level flight, the altitude prediction problem does not exist. All the ACAS aircraft has to do is to move in the direction which increases the current altitude separation to the target value. Possible obstacles to this simple logic are that the ACAS aircraft may be unable to climb, or may be too close to the ground to descend safely. 3.5.8.6.2 The manoeuvre limitation problems largely disappear when the ACAS aircraft is in climb or descent since separation can then often be obtained simply by levelling-off. And the prediction problem is likely to be a minor one if ACAS is fed with high resolution data for own altitude. 2007/70/II. szám Annex 10 — Aeronautical Telecommunications Volume IV 28/11/02 ATT-30 3.5.8.7 INTRUDERS IN CLIMB/DESCENT Intruders in climb or descent provide more difficulty than intruders in level flight. It is often a problem to determine their altitude rates. There is also evidence that a climbing or descending threat that is projected to pass close to own aircraft is more likely to level-off than to maintain its observed altitude rate thus avoiding the close encounter. Therefore the selection of RAs by ACAS should be biased by an expectation that threats might level-off, e.g. in response to ATC. A low confidence in the threat’s tracked altitude rate may cause RA generation to be delayed pending a better estimate of this rate. 3.5.8.8 ALTITUDE CROSSING RAS 3.5.8.8.1 Intruders that are projected to cross the altitude of an ACAS aircraft make the design of a totally effective ACAS extremely difficult because such intruders might level-off. Some of the altitude crossing RAs occasionally generated have been found counter-intuitive by pilots. Indeed, such RAs require the pilot to initially manoeuvre toward the intruder, temporarily losing vertical separation. Nevertheless, encounters for which altitude crossing RAs are clearly appropriate have been observed, and it is not yet demonstrated that it is desirable or possible to avoid them entirely. The frequency of altitude crossing RAs is likely to depend on the management and behaviour of aircraft. It is known that aircraft climbing and descending at high rates more frequently give rise to RAs, including crossing RAs, than other aircraft. The potential effect of approaching a cleared flight level at high speed and then levelling-off in close horizontal and vertical proximity to another aircraft is described below. Measures to mitigate these effects are described in 3.5.8.9. 3.5.8.8.2 For the scenario illustrated in Figure A-8, suppose that the alert occurs while the intruder is climbing towards the level ACAS aircraft. Given that the climb continues, the best escape strategy would be for own aircraft to descend towards the threat, in so doing crossing through the threat’s altitude. A climb away could possibly provide enough vertical clearance but, for the same escape velocity, a descent will give greater clearance. If own aircraft does descend it can be seen that a hazardous situation arises if the threat levels off at the cardinal flight level below own aircraft. Such Figure A-8. Induced close encounter ACAS A A A Alert Closest approach 5 seconds Intruder 1 000 ft 2007/70/II. szám Attachment Annex 10 — Aeronautical Telecommunications ATT-31 28/11/02 manoeuvres are common-place in some controlled airspaces since they are used by controllers to cross aircraft safely with the required altitude separation in situations where the horizontal separation is small. An ACAS design based on the choice of sense likely to give the greatest altitude separation could induce a close encounter where one would not otherwise occur. An ACAS design must include provisions to make it as immune as possible to such an eventuality. 3.5.8.9 Provisions for avoiding induced close encounters. In the absence of any knowledge concerning the intent of the threat, it appears reasonable to assume that the threat will continue with its current altitude rate but chooses the RA in an attempt to mitigate the effect of a likely threat manoeuvre. Other features must provide for the contingency that a subsequent threat manoeuvre is detected. For example, the implementation described in Section 4 uses the logic described below. 3.5.8.9.1 Biasing the choice of sense. If a positive non-altitude crossing advisory is predicted to give at least adequate altitude separation at closest approach (Al), then preference is given to the sense that prevents the aircraft from crossing in altitude before closest approach if the threat does not level-off. There is evidence that, in some circumstances, altitude crossing RAs are more disruptive than nonaltitude-crossing RAs. 3.5.8.9.2 Increased rate resolution advisory. If the sense chosen as a result of the process described in 3.5.8.9.1 results in own aircraft moving away from the threat the encounter may still not be resolved if the threat increases its altitude rate. In such a case the pilot of the ACAS aircraft can be invited to increase own altitude rate in an attempt to outrun the threat. 3.5.8.9.3 Altitude separation test. Sense choice biasing will not always result in an RA to move away from the threat and the altitude separation test is provided further to decrease the chance of an induced close encounter due to a threat levelling off or reducing its altitude rate. The test involves delaying the issue of the RA until the intent of the threat can be deduced with greater confidence. It is therefore not without risk of causing ACAS to be unable to resolve the encounter. The ACAS implementation described in Section 4 balances these conflicting risks with the logic described below. 3.5.8.9.3.1 For a scenario of the type shown in Figure A-8 illustrating a threat with a significant altitude rate, the alert, without this delay, would be given when the aircraft were still well separated in altitude. For example, when the warning time is 25 seconds and the altitude rate is 900 m/min (3 000 ft/min), the initial separation is 380 m (1 250 ft). If the situation is such that an altitude crossing RA would be required, i.e. biased sense choice is ineffective, ACAS delays the issue of an advisory until the current altitude separation falls below a threshold (Ac) that is smaller than the standard IFR separation. If the threat actually levels off at any altitude before crossing that threshold, as is most likely, the alert state will either be cancelled (for level-offs outside Zm), or a non-altitude crossing advisory will be generated. Otherwise, apart from the possibility that the threat has just overshot its cleared altitude, there is every indication that it is carrying on to, or through, own aircraft’s level and the altitude crossing advisory can be issued with more confidence. If the situation is such that non-altitude crossing advisory would be required, a reduced time threshold (Tv) is used for the altitude test. This vertical threshold test (VTT) is designed to hold off the RA just long enough so that a level-off manoeuvre initiated by the intruder might be detected. 3.5.8.9.3.2 The altitude separation test was intended principally to alleviate problems experienced in an IFR trafficonly environment. It may appear to be desirable to select the value for Ac such that altitude overshoots or even non-IFR separations are covered. However, the risk of ACAS to be unable to resolve the encounters is to be taken into careful consideration. 3.5.8.9.3.3 The test takes advantage of the cooperation between two equipped aircraft by causing the ACAS in the level aircraft to delay the choice of an RA until it has received a resolution message from the equipped intruder. The ACAS in the latter must almost certainly choose a reduction in its own altitude rate and the coordination process would then result in the level aircraft being able to maintain its level status. In practice the delay in starting to resolve the encounter will be small, but the risk of failure to resolve is less sensitive to delay because both aircraft are taking avoiding action. The delay is limited to 3.0 s, which is normally sufficient for the threat to have initiated coordination. 3.5.8.9.4 Sense reversal. In spite of the precautions taken to avoid induced close encounters described above, there are still situations which are not covered. For example, in airspace containing VFR traffic, threat levelling-off can occur with a nominal separation of 150 m (500 ft). The altitude separation test could be less effective in such circumstances. When ACAS determines that a threat manoeuvre has defeated its initial choice of RA, the advisory sense can be reversed. The requirement to achieve the target altitude separation at closest approach may be relaxed when this course of action is taken. 3.5.8.10 OTHER CAUSES OF INDUCED 3.5.8.10 CLOSE ENCOUNTERS 3.5.8.10.1 Altimetry errors. The altitude separation parameter representing the separation goal (Al) must include an allowance for altimetry error that is sufficient to give a high probability of not causing an ACAS-equipped aircraft to provoke a close encounter where none really existed. For gross altimetry errors, however, there remains a low probability that a close encounter will be induced when the original separation is adequate. Similarly, there is a low probability that ACAS will be unable to resolve a close encounter due to altimetry error. 2007/70/II. szám Annex 10 — Aeronautical Telecommunications Volume IV 28/11/02 ATT-32 3.5.8.10.1.1 The use of Gilham encoded data for either aircraft is a particular cause of altitude report errors, and induced close encounters have resulted. In the case of own aircraft, such errors can be prevented by using an altitude source that has not been Gilham encoded. 3.5.8.10.2 Mode C errors 3.5.8.10.2.1 Errors in encoding the threat’s altitude to provide Mode C data can, when sufficiently large, induce close encounters in much the same way as gross altimetry error. The incidence of such encounters will be very low in airspaces where ATC takes steps to advise the pilot that an aircraft’s reported altitude is incorrect. 3.5.8.10.2.2 A more severe form of Mode C error occurs when the error is confined to the C-bits. These are unchecked by ATC, which is normally content to find that an aircraft is within the specified tolerance value of its reported altitude. A stuck or missing C-bit can produce an error of only 30 m (100 ft). However, such a fault can have a more serious effect on the intruder’s altitude rate as perceived by ACAS and in this way can cause an induced close encounter or result in failure to resolve a close encounter. 3.5.8.10.3 Contrary pilot response. Manoeuvres opposite to the sense of an RA may result in a reduction in vertical separation with the threat aircraft and therefore must be avoided. This is particularly true in the case of an ACAS- ACAS coordinated encounter. 3.5.8.11 MULTI-AIRCRAFT ENCOUNTERS 3.5.8.11.1 ACAS takes account of the possibility of three or more aircraft being in close proximity and it is required to produce an overall RA that is consistent with each of the advisories that it would give against each threat treated on an individual basis. In such circumstances it cannot always be expected that the ACAS aircraft will achieve an altitude separation of Al with respect to all threats. 3.5.8.11.2 Simulations based on recorded ground-based radar surveillance data and initial experience with ACAS equipment have indicated that multi-aircraft conflicts are rare. Also, there is no evidence of a “domino” effect whereby the ACAS aircraft’s manoeuvre to avoid a threat brings it into an encounter with a third aircraft which is equipped and so on. Such an event might be expected to take place in a holding pattern, but the available evidence does not confirm this. 3.5.9 VERTICAL RATE ESTIMATION 3.5.9.1 The vertical tracking algorithm must be capable of using altitude information quantized in either 25 or 100 ft increments to produce estimates of aircraft vertical rates. This tracker must avoid overestimating vertical rate when a jump in reported altitude occurs because an aircraft with a small vertical rate moves from one quantized altitude level to another. But response limitation cannot be achieved by merely increasing tracker smoothing, since the tracker would then be slow to respond to actual rate changes. For altitude reports quantized to 100 ft, the altitude tracker (in Section 4) uses special track update procedures that suppress the response to an isolated altitude transition (altitude report that differs from the preceding altitude report) without sacrificing response to acceleration. The tracker also includes several features that contribute to reliability. 3.5.9.2 Some key features of the vertical tracking algorithm are as follows:

Source: https://magyarkozlony.hu/hivatalos-lapok/7e70cec03f34e3c2efd8610b865b65591eafd701/dokumentumok/a55dc160549d57fa4db0035e37c6a6a98dd1a0b9/letoltes