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13.4.4.2 Possible restriction in position determination. Depending on ground equipment geometry a region of possible
multiple solutions to the position determination algorithm may exist. This region of multiple solutions is dependent on the
locations of the elevation antenna and DME transponder relative to the runway and computed approach path. The most
pronounced effect occurs when the DME transponder lies in the region between the approach path DH point and the elevation
antenna. The position ambiguities can be resolved when the DME transponder is located behind the elevation antenna when
viewed from the approach direction. When the DME transponder is located in front of the elevation antenna it may not be
possible to resolve the position ambiguity.
13.4.5 Ground equipment geometry
13.4.5.1 The nominal ground equipment geometry in terms of the relative position of the ground components is
depicted in Figure G-29. The DME/P transponder is assumed to be collocated with the approach azimuth antenna. When
DME/P ground equipment is not available, the DME/N transponder is assumed to be located between the MLS approach
azimuth and elevation antennas.
13.4.5.2 Because of the relatively large error induced by the DME/N, the location of the DME/N transponder has no
significant influence on the calculated permissible azimuth antenna offset. This permits DME/N siting over a large area
between the azimuth and elevation antennas. Similarly, the offset of the elevation antenna will have little effect.
23/11/06 ATT G-38
Attachment G Annex 10 — Aeronautical Communications
13.5 Permissible approach azimuth antenna offset positions for
computed centre line approaches to the primary runway
13.5.1 DME results
13.5.1.1 The maximum azimuth offset represents, for a given set of conditions, the largest offset that does not exceed
the computed centre line approach error budget identified in 13.2. DME/P results are presented as a function of the azimuth
to elevation distance. The permissible azimuth antenna offsets with DME/P are presented in Figure G-30.
13.5.1.2 For a given azimuth to elevation distance, the azimuth antenna can be sited any place in the shaded area and
the resulting computed centre line approach meet requirements of 13.2.
13.5.1.3 Results were obtained when DME/N ranging accuracies are used. These results are presented in Figure G-31.
13.6 Low visibility approaches
13.6.1 Possible applications
13.6.1.1 The possibility of low visibility computed centre line applications may be limited to operations on the primary
instrument runway because of the geometry considerations involved in achieving adequate accuracy. Primary instrument
runway applications where computed centre line capability would be useful are those where the azimuth is offset from the
runway centre line due to a severe siting restriction. There may be such azimuth offset applications where low visibility
operations would be considered beneficial.
13.6.1.2 The expected airborne implementation for such low visibility computed centre line approaches would use noncomputed
elevation guidance (assuming the elevation ground antenna is sited normally) and lateral guidance derived from a
combination of azimuth (including MLS siting data contained in the basic and auxiliary data functions) and range from the
DME/P transponder.
13.6.2 Airborne system performance
13.6.2.1 Safety-critical software associated with the guidance function for non-computed low visibility approaches
mainly involves the MLS receiver. For computed centre line approaches, the DME interrogator and the navigation computations
must also be considered. The safety-critical software for these functions will have to be designed, developed,
documented and evaluated.
13.6.2.2 The necessary algorithms are relatively simple and do not pose any certification difficulty. However, experience
with flight management system (FMS) computers indicates that it would be difficult to certify a safety-critical function
implemented within an existing FMS. Current FMS architectures are not partitioned to allow separate certification of different
functions to different levels of criticality and the size and complexity of an FMS precludes safety-critical certification of the
entire FMS computer. Consequently, alternatives to FMS implementation can be considered for computed centre line capability
intended for low visibility applications (e.g. incorporation within the autopilot or within the MLS receiver). These alternatives
would provide output guidance with the same output characteristics as a normal straight-in approach.
13.6.3 Ground system performance
13.6.3.1 Based on the implementation assumed in 13.3.5, elevation guidance would be used in exactly the same
manner as for basic MLS approaches. Consequently, the elevation ground equipment integrity and continuity of service
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