ICAO Annex 10 Volume 1 Chapter 3: Localizer Coverage Requirements 

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... localizer intercept, even in worst-case scenarios. Outside of 15 degrees, however, aircraft operators still expect the ILS signal to be free from low clearance or false course indications – out to the conventional 35 degree limits. Consequently, a “reduced coverage localizer” would need to maintain a clearance signal strong enough to cover any course sidelobes. This requirement had the secondary effect that IDENT coverage would also be ensured in line with operator’s expectations. In fact, the IDENT turned out to be the most constraining function in relation to coverage requirements, as pilots have become used to completing the IDENT check as part of the check for approach, around flight level 100 – typically significantly outside of conventional coverage. Despite this diverging operational reality, it was demonstrated through a task-load study that pilots would still be in a position to complete the IDENT check during a worst-case, high-workload intercept of a reduced coverage system [1]. As a consequence of these operationally derived requirements, a new localizer design, as explained above, was undertaken. Simulations confirmed that a key benefit of the design was to increase signal quality by shifting the clearance peak away from 12 to 15 degrees to 7 to 8 degrees from the centerline. This has been evaluated at a major airport with a building reflector at 12 plus degrees through a dedicated site survey and simulation in order to address the clearance–course interference case. The ability of the new localizer to restore multipath margins in comparison to an existing solution is shown in Figure 7. The validation work of the implementation in Zurich included a review of operational data. This included feedback from flight crews, flight data analysis, as well as a detailed safety monitoring by the service provider operational staff. This confirmed that the intended goals were fully achieved, e.g., that the change in localizer design was completely unnoticeable by flight crews and ATC. In order to encourage global implementation of this solution which increases the achievable safety margins, the work was presented both to the ICAO Navigations Systems Panel and the Operations Panel [2], [3], [4], [5]. Despite coverage requirements having been successively reduced over the many years of ILS operation (first from omni-directional to ±90 degrees and then to the current ±3 5 degrees), it proved difficult to implement a relaxation of field strength requirements down to ±15 degrees. This was primarily due to concerns over FM broadcast compatibility and possible further building development. Consequently, an alternate requirements formulation was pursued [6]. The proposal currently being discussed foresees to raise the lower boundary of coverage in line with operational requirements, up to a maximum lower boundary. Thus, where operationally compatible, coverage could be raised at the limits of coverage (17NM), starting from 15 degrees and 2000ft HAT (Height Above Threshold), up to 4500ft HAT at 35 degrees, as illustrated by Figure 8. The site of the localizer 16 in Zurich has been identified as a potential site, where the coverage reduction may solve the signal in space problems. This section presents the realization of this replacement project and its technical outcome. This specific case Zurich 16 represents also a detailed supporting technical validation. Since the fifth building phase of Zurich airport, the signal in space of the localizer 16 has suffered from serious signal reflections from new buildings, especially the parking garage number 3. These signal reflections have produced the so-called "Clearance / Clearance Interference": the clearance signal towards the parking garage number 3 (which carries the information "full scale deflection right"), after a reflection on its facade, interferes with the direct and correct clearance signal which carries the information "full scale deflection left". The result of such an interference has produced in Zurich a very serious degradation of the signal in space: in the "150 Hz Dominant" domain, where the signal normally carries the information "full scale deflection left", the DDM (Difference in Depth of Modulation) is so affected that the information "full scale deflection left" was not guaranteed any more. As illustrated below by Figure 9, some false courses (where the DDM is equal to zero and where the Course Deviation Indicator is really centred) have been measured by the flight check for azimuth angles between 10° and 20° from the centreline. In parallel with the building of the parking, the quality of the signal has been continuously degraded between 2001 and 2003. Finally, in November 2003, the operational coverage of the localizer has been restricted to a beam of ±5° around the centreline. Such a restriction of ±5° of the coverage, illustrated by Figure 10, has represented a significant deviation from ICAO Annex 10 standards, which are illustrated below by Figure 11. Because of this deviation from ICAO standards, it has been decided to launch the replacement project of the ILS 16 in Zurich. The first step of the replacement study has consisted in confirming the source of the interference and the type of interference: a Clearance / Clearance interference. The calculations and simulations have clearly shown and confirmed that the reflector is the parking garage number 3 and that it is located in the Clearance domain. In order to reduce the perturbation, the solution consists in reducing the incident signal on the parking, thus consequently reducing the reflected signal, and finally the amplitude of the Clearance / Clearance interference. Reducing the Clearance incident signal means for the localizer modifying the Clearance radiating antenna diagram. Radiating less signal in the Clearance domain means also consequently reducing the lateral coverage and the operational service volume. The feasibility study has considered a new design of localizer from the ILS supplier Park Air Systems: the model NM 7220 B. This new type of localizer, which is illustrated by Figure 12 below, has integrated the so- called "Reduced Coverage" function. From a mechanical point of view, this equipment is quite similar to the previous ones. Some mechanical improvements have been implemented: frangibility, protection and integration of the cable ducts, antenna distribution unit and monitor combining unit. However, there are no mechanical differences between a full coverage NM 7720 A and the reduced coverage system NM 7720 B. From a hardware point of view, the main change consists in modifying the Clearance feeds: the distribution (amplitudes / phases) of the Clearance CSB and SBO signals to the antennas has been modified and optimized in order to get the wished and required reduced coverage. From a monitoring point of view, the philosophy and the theoretical principles remain the same. Thus, the only change in this design deals with the feeds of the Clearance signals. Based on the described distributions and antenna patterns, the resulting simulated Difference in Depth of Modulation (DDM) in free space (without any reflector), which controls the Course Deviation Indicator (CDI), presents the two "normal" characteristic domains of a ...