Publication: Magyar Közlöny
Issue: MK-2007-70 (Year: 2007, Number: 70)
Era: 2004-2010
Section: Melléklet a 2007. évi XLVI. törvényhez
Paragraph Index: 4771

b) use a Mode S reply processor that will re-trigger if it detects a Mode S preamble that is 2 to 3 dB stronger than the reply that is currently being processed. Note.— Care must be taken to ensure that low-level squitters (i.e. those below the conventional MTL) do not interfere with the processing of acquisition squitters for ACAS. This could happen if the low-level squitter is allowed to capture the reply processor. This can be prevented by using a separate reply processor for each function, or by requiring the reply processor to be re-triggered by a higher level squitter. a· r· a· r· 2007/70/II. szám ANNEX 10 — VOLUME IV ATT-1 28/11/02 ATTACHMENT TO VOLUME IV Guidance material related to airborne collision avoidance system (ACAS) Note 1.— The following material is intended to provide guidance concerning the technical characteristics of the airborne collision avoidance system (ACAS) having vertical resolution capability (ACAS II, unless stated otherwise). ACAS SARPs are contained in Chapter 4. Note 2.— Non-SI alternative units are used as permitted by Annex 5, Chapter 3, Section 3.2.2. In limited cases, to ensure consistency at the level of the logic calculations, units such as ft/s, NM/s and kt/s are used. 1. EQUIPMENT, FUNCTIONS AND CAPABILITIES 1.1 ACAS equipment characteristics 1.1.1 ACAS equipment includes an ACAS processing unit, Mode S transponder, control unit, appropriate antennas and means of providing advisories. 1.1.2 ACAS equipment in the aircraft interrogates SSR transponders on other aircraft in its vicinity and listens for the transponder replies. By computer analysis of these replies, the ACAS equipment determines which aircraft represent potential collision threats and provides appropriate indications (advisories) to the flight crew to avoid collisions. 1.1.3 ACAS equipment is capable of providing two classes of advisories. Traffic advisories (TAs) indicate the approximate positions of intruding aircraft that may later cause resolution advisories. Resolution advisories (RAs) propose vertical manoeuvres that are predicted to increase or maintain separation from threatening aircraft. 1.2 Advisories provided 1.2.1 TRAFFIC ADVISORIES TAs may indicate the range, range rate, altitude, altitude rate, and bearing of the intruding aircraft relative to own aircraft. TAs without altitude information may also be provided on Mode C or Mode S-equipped aircraft that do not have an automatic altitude reporting capability. The information conveyed in ACAS TAs is intended to assist the flight crew in sighting nearby traffic. 1.2.2 RESOLUTION ADVISORIES 1.2.2.1 If the threat detection logic in the ACAS computer determines that an encounter with a nearby aircraft could lead to a near-collision or collision, the computer threat resolution logic determines an appropriate vertical manoeuvre that will ensure the safe vertical separation of the ACAS aircraft. The selected manoeuvre ensures adequate vertical separation within constraints imposed by the climb rate capability and proximity to the ground of the ACAS aircraft. 1.2.2.2 The RAs provided to the pilot can be divided into two categories: corrective advisories, which instruct the pilot to deviate from the current flight path (e.g. “CLIMB” when the aircraft is in level flight); and preventive advisories, which advise the pilot to maintain or avoid certain vertical speeds (e.g. “DON’T CLIMB” when the aircraft is in level flight). 1.2.2.3 Under normal circumstances, ACAS issues only one RA during an encounter with one or multiple intruders. The RA is issued when, or shortly after, the (first) intruder becomes a threat, is maintained as long as the (any) intruder remains a threat, and is cancelled when the (last) intruder ceases to be a threat. However, the indication given to the flight crew as part of that RA may be modified. It may be strengthened or even reversed when a threat modifies its altitude profile or when the detection of a second or third threat changes the initial assessment of the encounter. It may also be weakened when adequate separation has been achieved but the (any) intruder temporarily remains a threat. 1.2.3 WARNING TIMES If a threat is detected, the ACAS equipment generates an RA some time before the closest approach of the aircraft. The amount of warning time depends on the protected volume selected for ACAS system use. The nominal resolution advisory time before closest approach used by ACAS varies from 15 to 35 seconds. A TA will nominally be issued between 5 and 20 seconds in advance of an RA. Warning times depend on sensitivity level as described in 3.5.12. 2007/70/II. szám Annex 10 — Aeronautical Telecommunications Volume IV 28/11/02 ATT-2 1.2.4 AIR-AIR COORDINATION OF RESOLUTION ADVISORIES 1.2.4.1 If the aircraft detected by the ACAS equipment has only a Mode A/C transponder and automatic pressure altitude reporting equipment, its pilot will not be aware that it is being tracked by the ACAS-equipped aircraft. When the pilot of the ACAS aircraft receives an RA in an encounter with such an aircraft and manoeuvres as advised, the ACAS aircraft will be able to avoid the intruding aircraft provided the intruder does not accelerate so as to defeat the manoeuvre of the ACAS aircraft. 1.2.4.2 If the intruding aircraft is equipped with ACAS, a coordination procedure is performed via the air-to-air Mode S data link in order to ensure that the ACAS RAs are compatible. 1.2.5 AIR-GROUND COMMUNICATION 1.2.5.1 ACAS may communicate with ground stations using the Mode S air-ground data link. The transmission of sensitivity level control commands to ACAS equipment by Mode S ground stations is one aspect of communication. This feature permits a Mode S ground station to adapt the RA warning time to the local traffic environment as an ACAS aircraft moves through the region of coverage of the station. An effective trade-off between collision warning time and alert rate is thereby ensured. 1.2.5.2 The Mode S air-ground data link may also be used to transmit ACAS RAs to Mode S ground stations. This information can then be used by air traffic services to monitor ACAS RAs within an airspace of interest. 1.2.6 FUNCTIONS PERFORMED BY ACAS 1.2.6.1 The functions executed by ACAS are illustrated in Figure A-1. To keep the illustration simple, the functions “own aircraft tracking” and “intruder aircraft tracking” have been represented once in Figure A-1, under “surveillance”. However, the trackers that are intended to support the collision avoidance function may not be suitable to support the surveillance function. Separate tracking functions may be required to adequately support both the collision avoidance and the surveillance functions. 1.2.6.2 Surveillance is normally executed once per cycle; however, it may be executed more frequently or less frequently for some intruders. For example, surveillance may be executed less frequently for some non-threatening intruders to respect interference limiting inequalities or it may be executed more frequently for some intruders to improve the azimuth estimate. 1.2.6.3 Parameters used in the implementation of the ACAS functions are adjusted automatically or manually to maintain collision avoidance protection with minimal interference to normal air traffic control (ATC) operations. 1.3 Intruder characteristics 1.3.1 TRANSPONDER EQUIPAGE OF INTRUDER ACAS provides RAs on aircraft equipped with altitude reporting Mode A/C or Mode S transponders. Some aircraft are equipped with SSR transponders but do not have altitude encoders. ACAS cannot generate RAs in conflicts with such aircraft because, without altitude information, a collision threat assessment cannot be made. ACAS equipment can generate only TAs on such aircraft, describing their ranges, range rates and bearings. Aircraft equipped with Mode A only transponders and those not equipped with or not operating Mode A/C or Mode S transponders cannot be tracked by ACAS. 1.3.2 INTRUDER CLOSING SPEEDS AND TRAFFIC DENSITIES 1.3.2.1 ACAS equipment designed for operation in high density airspace is capable of providing overall surveillance performance on intruders as defined in Chapter 4, 4.3.2 and Table 4-1. 1.3.2.2 The conditions enumerated in Table 4-1, which define two distinct density regions in the multi-dimensional condition space that affects ACAS performance, were extrapolated from airborne measurements of the performance of a typical ACAS. The airborne measurement data indicated that the track establishment probability will not drop abruptly when any of the condition bounds is exceeded. 1.3.2.3 The performance is stated in terms of probability of tracking a target of interest at a maximum closing speed in a given traffic density at least 30 seconds before the point of closest approach. The maximum traffic density associated with each of the two density regions is defined as: ρ = n(r)/πr2 where n(r) is the maximum 30-second time average of the count of SSR transponder-equipped aircraft (not counting own aircraft) above a circular area of radius r about the ACAS aircraft ground position. In the airborne measurements, the radii were different for the two density regions. In the highdensity measurements the radius was 9.3 km (5 NM). In the low-density measurements the radius was 19 km (10 NM). Traffic density outside the limits of the circular area of constant density may be assumed to decrease inversely proportional to range so that the number of aircraft is given by: 2007/70/II. szám Attachment Annex 10 — Aeronautical Telecommunications ATT-3 28/11/02 n(r) = n(ro)r/ro where ro is the radius of the constant density region. 1.3.2.4 When the density is greater than 0.017 aircraft/km2 (0.06 aircraft/NM2), the nominal radius of uniform density ro is taken to be 9.3 km (5 NM). When the density is equal to or less than indicated above, ro is nominally 18.5 km (10 NM). 1.3.2.5 The table is based on an additional assumption that at least 25 per cent of the total transponder-equipped aircraft in the highest density 0.087 aircraft/km2 (0.3 aircraft/NM2) airspace are Mode S equipped. If fewer than 25 per cent are Mode S equipped, the track probability for Mode A/C aircraft may be less than 0.90 because of increased synchronous garble. If the traffic density within ro exceeds the limits given in the table or if the traffic count outside of ro continues increasing faster than r, the actual track establishment probability for Mode A/C aircraft may also be less than 0.90 because of increased synchronous garble. If the closing speed exceeds the given limits, the tracks for Mode A/C and Mode S aircraft may be established late. If the number of other ACAS in the area exceeds the limits given in the table, the interference limiting requirements of Chapter 4, 4.3.2.2 require that the ACAS transmitter power and receiver sensitivity be further reduced, thereby resulting in a later establishment time. However, the track probability is expected to degrade gradually as any of these limits is exceeded. 1.3.2.6 The table reflects the fact that the ACAS tracking performance involves a compromise between closing speed and traffic density. Although it may not be possible to maintain a high probability of track when the traffic density and the intruder closing speed are both simultaneously large, the ACAS design is capable of reliable track establishment on high-speed intruders when operating in relatively low-density en-route airspace (typically characterized by densities of less than 0.017 aircraft/km2, i.e. 0.06 aircraft/NM2) or when operating in higher density, low-altitude terminal airspace where the closing speeds are typically below 260 m/s (500 kt) for operational reasons. 1.3.2.7 The table also accounts for the fact that higher closing speeds are associated with the forward direction than with the side or back directions so that the ACAS surveillance design is not required to provide reliable detection for the highest closing speeds in the side or back directions. 1.3.3 SYSTEM RANGE LIMITATIONS The required nominal tracking range of the ACAS is 26 km (14 NM). However, when operating in high density, the interference limiting feature may reduce system range to approximately 9.3 km (5 NM). A 9.3 km (5 NM) range is adequate to provide protection for a 260 m/s (500 kt) encounter. Figure A-1. Illustration of ACAS functions Other aircraft tracking Altitude test Altitude test Ground stations Evaluation and selection of advisory Own aircraft tracking Range test Range test Other ACAS aircraft Surveillance Traffic advisory Threat detection Resolution advisory Coordination and communication 2007/70/II. szám Annex 10 — Aeronautical Telecommunications Volume IV 28/11/02 ATT-4 1.4 Control of interference to the electromagnetic environment 1.4.1 The ACAS equipment is capable of operating in all traffic densities without degrading the electromagnetic environment. Each ACAS equipment knows the number of other ACAS units operating in the local airspace. This knowledge is used to ensure that no transponder is suppressed by ACAS activity for more than 2 per cent of the time and to ensure that ACAS does not contribute to an unacceptably high fruit rate that would degrade ground SSR surveillance performance. Multiple ACAS units in the vicinity cooperatively limit their own transmissions. As the number of such ACAS units increases, the interrogation allocation for each of them decreases. Thus, every ACAS unit monitors the number of other ACAS units within detection range. This information is then used to limit its own interrogation rate and power as necessary. When this limiting is in full effect, the effective range of the ACAS units may not be adequate to provide acceptable warning times in encounters in excess of 260 m/s (500 kt). This condition is normally encountered at low altitude where this closing speed capability is sufficient. Whenever the ACAS aircraft is on the ground, ACAS automatically limits the power of its interrogations. This limiting is done by setting the ACAS count (na) in the interference limiting inequalities to a value three times the measured value. This value is selected to ensure that an ACAS unit on the ground does not contribute any more interference to the electromagnetic environment than is unavoidable. This value will provide an approximate surveillance range of 5.6 km (3 NM) in the highest density terminal areas to support reliable ground ACAS surveillance of local airborne traffic and a 26 km (14 NM) range in very low density airspace to provide wide area surveillance in the absence of an SSR. 1.4.2 The presence of an ACAS unit is announced to other ACAS units by the periodic transmission of an ACAS interrogation containing a message that gives the address of the ACAS aircraft. This transmission is sent nominally every 8 to 10 seconds using a Mode S broadcast address. Mode S transponders are designed to accept message data from a broadcast interrogation without replying. The announcement messages received by the ACAS aircraft’s Mode S transponder are monitored by the interference limiting algorithms to develop an estimate of the number of ACAS units in the vicinity. 2. FACTORS AFFECTING SYSTEM PERFORMANCE 2.1 Synchronous garble When a Mode C interrogation is transmitted, all the transponders that detect it reply. Since the reply duration is 21 microseconds, aircraft whose ranges from ACAS are within about 2.8 km (1.5 NM) of each other generate replies that persistently and synchronously overlap each other at the interrogating aircraft. The number of overlapping replies is proportional to the density of aircraft and their range from ACAS. Ten or more overlapping replies might be received in moderate density terminal areas. It is possible to decode reliably only about three overlapping replies. Hence, there is a need to reduce the number of transponders that reply to each interrogation. Whisper-shout and directional transmit techniques are available for controlling such synchronous garble (see 3.2 and 3.3). They are both needed in ACAS equipment operating in the highest traffic densities. 2.2 Multipath from terrain reflections 2.2.1 SSR transponders use quarter-wave monopole antennas mounted on the bottom of the aircraft. A stub antenna of this sort has a peak elevation gain at an angle of 20 to 30 degrees below the horizontal plane. This is suitable for groundair surveillance, but the direct air-air surveillance path may operate at a disadvantage relative to the ground reflection path, particularly over water. 2.2.2 If the ACAS unit uses a bottom-mounted antenna, there are geometries for which the reflected signal is consistently stronger than the direct signal. However, when a top-mounted antenna is used for interrogation, its peak gain occurs at a positive elevation angle and the signal-to-multipath ratio is improved. Thus, when ACAS transmits from the topmounted antenna, the effects of multipath are reduced significantly. Even when a top-mounted antenna is used, the multipath will still occasionally exceed the receiver threshold. Thus, there is need to reject low-level multipath. ACAS can achieve this rejection through the use of variable receiver thresholds (see 3.4). 2.3 Altimetry data quality 2.3.1 MEASUREMENT ERRORS 2.3.1.1 The vertical separation between two conflicting aircraft is measured as the difference between own altitude and the intruder’s altitude as reported in its Mode C or Mode S reply. If the ACAS aircraft is an air carrier, it will normally have accurate altimetry; an intruding aircraft might have less accurate altimetry. 2.3.1.2 Errors in altimetry can cause two types of effects: first, if the aircraft are on a near collision course, errors could indicate safe passage, and the impending near mid-air collision might not be resolved by ACAS; second, if the aircraft are on a near collision course, but are separated in altitude, errors could lead to an ACAS manoeuvre in the wrong direction which could induce an even closer encounter. 2007/70/II. szám Attachment Annex 10 — Aeronautical Telecommunications ATT-5 28/11/02 2.3.1.3 ACAS attempts to achieve a difference of at least 90 m (300 ft) between aircraft at closest approach based on reported altitude. Thus, if the combination of intruder and ACAS altimetry errors approached 90 m (300 ft), there would be finite risk of inadequate vertical separation despite the presence of ACAS. Studies of the expected altimetry errors of both ACAS and non-ACAS aircraft at altitudes from sea level to FL 400 have concluded that the risk is essentially negligible if both aircraft are equipped with high accuracy altimetry systems that can achieve root-sum-square (RSS) errors of approximately 15 m (50 ft). It was further concluded that if an ACAS with high accuracy altimetry operates in a traffic environment consisting of typical general aviation aircraft (with RSS errors of approximately 30 m (100 ft), normally distributed), then altimetry errors will occasionally lead to inadequate ACAS RAs. However, this will not occur often enough to seriously interfere with the effectiveness of the system. Performance was considered to be inadequate if both aircraft in an encounter had a low accuracy altimetry system. This led to the requirement that ACAS possess a high accuracy system. 2.3.2 ALTITUDE BIT FAILURE If the Mode C or Mode S altitude reports from the intruding aircraft or the altitude data for own aircraft contain bit errors, ACAS may develop erroneous estimates of the corresponding vertical position or rate. These errors can have effects similar to the effects of measurement errors. Such errors are most likely to occur when the altitude data source is a Gilham encoder, and the use of Gilham encoded data for own aircraft altitude can have serious adverse consequences. When there is no alternative source than Gilham encoded data, two encoders must be used and a comparison function in the Mode S transponder used to detect errors in the altitude data before they are provided to ACAS. 2.3.3 CREDIBILITY OF OWN AIRCRAFT ALTITUDE All sources of own altitude data are required to be checked for credibility, including fine altitude data (which can come from various sources: gyro, air data computer, etc.) and radar altitude data. 2.4 Potential for ground-based SSR site monitors (PARROTs) to cause spurious traffic and resolution advisories An ACAS interrogates all SSR transponders within range, including ground-based transponder installations used to monitor the operation of ground radar systems, or test transponders. If these ground-based transponders reply with false altitude data, the potential exists for an ACAS to generate spurious TAs and RAs. To prevent this problem, information on the operation of position adjustable range reference orientation transponders (PARROTs) and transponder test facilities is provided in the Manual of Secondary Surveillance Radar (SSR) Systems (Doc 9684). 2.5 Allocation and assignment of SSR Mode S addresses To ensure safe operation, the system requires that all Mode Sequipped aircraft have unique addresses. Multiple aircraft with the same address or aircraft with addresses not compliant with Annex 10, Volume III, Part I, Chapter 9, can adversely affect the surveillance and coordination functions. 2.6 Potential for TCAS I systems to affect ACAS II performance Note.— For the purpose of this material, TCAS I is defined as a system that uses SSR interrogations to provide aircrew with traffic alert warning information as an aid to the “see and avoid” principle. Some TCAS I systems employ ACAS II interference limiting techniques with resolution advisories suppressed. These systems do not comply with ACAS I SARPs. Because ACAS II interference limiting relies on direct interaction with other ACAS II aircraft (using the ACAS broadcast and Mode S transponder replies), the presence of such TCAS I aircraft can directly influence the surveillance performance of nearby ACAS II aircraft. If such TCAS I systems are fitted to aircraft that are known to operate in close proximity to each other (e.g. rotorcraft or gliders) then the effect may reduce the surveillance range of other ACAS II aircraft and delay the provision of collision avoidance warnings. In light of these concerns, TCAS I systems (which employ ACAS II interference limiting techniques) must not be used for aircraft which are known to operate in close proximity to each other for sustained periods of time. Care must be taken to ensure that the effect on the SSR electromagnetic environment is acceptable, since these TCAS I units may be fitted in very large numbers. 3. CONSIDERATIONS ON TECHNICAL IMPLEMENTATION 3.1 System operation 3.1.1 SURVEILLANCE OF INTRUDERS 3.1.1.1 The main purposes of the surveillance processes described below are to obtain position reports and to correlate these to form tracks. This involves the use of trackers and requires the estimation of rates. 2007/70/II. szám Annex 10 — Aeronautical Telecommunications Volume IV 28/11/02 ATT-6 3.1.1.2 The ACAS unit transmits an interrogation sequence nominally once per second. The interrogations are transmitted at a nominal effective radiated power level of +54 ±2 dBm as measured at zero degree elevation relative to the longitudinal axis of the aircraft. When these interrogations are received by Mode A/C and Mode S altitude reporting transponders, the transponders transmit replies that report their altitude. The ACAS unit computes the range of each intruding aircraft by using the round-trip time between the transmission of the interrogation and the receipt of the reply. Altitude rate and range rate are determined by tracking the reply information. 3.1.1.3 In the absence of interference, overload, interference-limiting conditions, or other degrading effects, the equipment will nominally be capable of providing surveillance for Mode A/C and Mode S targets out to a range of 26 km (14 NM). However, because the surveillance reliability degrades as the range increases, the equipment should assess as possible collision threats only those targets within a maximum range of 22 km (12 NM). No target outside of this range should be eligible to generate an RA. However, ACAS is able to detect ACAS broadcast interrogations from ACASequipped aircraft out to a nominal range of 56 km (30 NM). 3.1.1.4 The equipment should have the capacity for surveillance of any mix of Mode A/C or Mode S targets up to a total peak target capacity of 30 aircraft. ACAS equipment is nominally capable of reliable surveillance of high-closingspeed targets in a peak traffic density of up to 0.017 aircraft per square km (0.06 aircraft per square NM) or approximately 27 aircraft in a 26 km (14 NM) radius. 3.1.1.5 When the average traffic density exceeds the above value, the reliable surveillance range decreases. ACAS equipment is capable of providing reliable surveillance of targets closing only up to 260 m/s (500 kt) in an average traffic density of 0.087 aircraft per square km (0.3 aircraft per square NM). The surveillance range required for 260 m/s (500 kt) targets is about 9.3 km (5 NM). It is possible to provide 9.3 km (5 NM) surveillance in a short-term peak traffic density of 0.087 aircraft/km2 (0.3 aircraft/NM2) or more without exceeding a total target capacity of 30. If the overall target count ever exceeds 30 at any range up to 26 km (14 NM), the long-range targets may always be dropped without compromising the ability to provide reliable surveillance of lower-speed targets. Thus a peak capability of 30 targets (any mix of Mode A/C or Mode S) is adequate for ACAS and if the number of Mode A/C plus Mode S targets under surveillance exceeds 30, excess targets are to be deleted in order of decreasing range without regard to target type. 3.1.2 SURVEILLANCE OF INTRUDERS WITH MODE A/C TRANSPONDERS 3.1.2.1 Surveillance of Mode A/C transponders is accomplished by the periodic transmission of a Mode C-only all-call (intermode) interrogation (Chapter 3, 3.1.2.1.5.1.2). This elicits replies from Mode A/C transponders, but not from Mode S transponders, thus preventing the replies of Mode S transponders from synchronously garbling the replies of Mode A/C transponders. Other techniques for reducing synchronous garble are (1) the use of directional antennas to interrogate only those aircraft in an azimuth wedge, and (2) the use of a sequence of variable power suppressions and interrogations (known as “whisper-shout”) that interrogates only aircraft that have similar link margins (see 3.2.2). The use of both of these techniques together provides a powerful tool for overcoming the effects of synchronous garble. 3.1.2.2 Whisper-shout employs a sequence of interrogations at different power levels transmitted during each surveillance update period. Each of the interrogations in the sequence, other than the one at lowest power, is preceded by a suppression transmission, where the first pulse of the interrogation serves as the second pulse of the suppression transmission. The suppression transmission pulse begins at a time 2 microseconds before the first pulse of the interrogation. The suppression pulse is transmitted at a power level lower than the accompanying interrogation so that the transponders that reply are only those that detect the interrogation and do not detect the suppression. To guard against the possibility that some transponders do not reply to any interrogation in the sequence, the suppression pulse is transmitted at a power level somewhat lower than that of the next lower interrogation. The time interval between successive interrogations should be at least 1 millisecond. This ensures that replies from transponders at long range are not mistaken for replies to the subsequent interrogation. All interrogations in the sequence are transmitted within a single surveillance update interval. 3.1.2.3 Responses to each Mode C-only all-call interrogation are processed to determine the range and altitude code of each reply. It is possible to determine the altitude codes for up to three overlapping replies if care is taken to identify the location of each of the received pulses. 3.1.2.4 After all of the replies are received in response to the whisper/shout sequence, duplicate replies should be merged so that only one “report” is produced for each detected aircraft. Reports may be correlated in range and altitude with the predicted positions of known intruders (i.e., with existing tracks). Since intruding aircraft are interrogated at a high rate (nominally once per second), good correlation performance is achieved using range and altitude. Mode A code is not needed for correlation. Reports that correlate are used to extend the associated tracks. Reports that do not correlate with existing tracks may be compared to previously uncorrelated reports to start new tracks. Before a new track is started, the replies that lead to its initiation may be tested to ensure that they agree in all of the most significant altitude code bits. A geometric calculation may be performed to identify and suppress specular false targets caused by multipath reflections from the terrain. 3.1.2.5 Tracks being initiated may be tested against track validity criteria prior to being passed to the collision avoidance 2007/70/II. szám Attachment Annex 10 — Aeronautical Telecommunications ATT-7 28/11/02 algorithms. The purpose of these tests is to reject spurious tracks caused by garble and multipath. Spurious tracks are generally characterized by short track life. 3.1.2.6 Aircraft not reporting altitude in Mode C replies are detected using the Mode C reply framing pulses. These aircraft are tracked using range as the correlation criterion. The additional use of bearing for correlation will help to reduce the number of false non-Mode C tracks. 3.1.2.7 Reply merging. Multiple replies may be generated by a Mode A/C target that responds to more than one whispershout interrogation during each whisper-shout sequence or by a target that responds to interrogations from both the top and bottom antennas. The equipment is expected to generate no more than one position report for any target even though that target may respond to more than one interrogation during each surveillance update interval. 3.1.2.8 Mode A/C surveillance initiation. The equipment will pass the initial position reports to the collision avoidance algorithms only if the conditions in a) and b) below are satisfied:

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