Publication: Magyar Közlöny
Issue: MK-2009-104 (Year: 2009, Number: 104)
Era: 2004-2010
Section: 
Paragraph Index: 2459

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 ground-air 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 top-mounted 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. 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 Annex 10 — Aeronautical Communications Volume IV 22/11/07 ATT-6 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 S-equipped 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 Attachment Annex 10 — Aeronautical Communications ATT-7 22/11/07 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.

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