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Note.— In relation to specific MLS operations it is intended that the level of integrity and continuity of service would
typically be associated as follows:
1) Level 2 is the performance objective for MLS equipment used to support low visibility operations when guidance for
position information in the landing phase is supplemented by visual cues. This level is a recommended objective for
equipment supporting Category I operations;
2) Level 3 is the performance objective for MLS equipment used to support operations which place a high degree of
reliance on MLS guidance for positioning through touchdown. This level is a required objective for equipment
supporting Category II and IIIA operations; and
3) Level 4 is the performance objective for MLS equipment used to support operations which place a high degree of
reliance on MLS guidance throughout touchdown and rollout. This level basically relates to the needs of the full
range of Category III operations.
12.5 The following example of MLS facility classification:
40:30/50 ft/E/4A
ATT G-35 23/11/06
Annex 10 — Aeronautical Communications Volume I
denotes a system with:
a) a proportional guidance sector of 40 degrees left and 30 degrees right of the zero-degree azimuth;
b) vertical guidance down to 50 ft above threshold;
c) roll-out guidance to MLS point E; and
d) integrity and continuity of service Level 4 with elevation and DME/P objectives equivalent to azimuth.
13. Computed centre line approaches
13.1 General
13.1.1 Computed centre line approaches considered below are based on a computed path along a runway centre line
where the azimuth antenna is not sited on the extended runway centre line. The simplest form of a computed centre line
approach is one in which the nominal track is parallel to the zero-degree azimuth. In order to conduct MLS/RNAV operation,
a greater capability than that available in the basic MLS receiver is required.
13.1.2 Computed centre line approaches to the MLS primary runway are conducted to the runway whose relationship
to the MLS ground equipment is identified in the auxiliary data words.
13.1.3 When the final segment is contained in the MLS coverage volume, computed centre line approaches can be
conducted along a straight final segment on a descent gradient down to the decision height (DH). Computed centre line
approaches may result in decision heights that are above decision heights achievable with aligned MLS approaches.
13.2 Computed centre line approach error budget
13.2.1 RTCA (RTCA/DO-198) has described a total system error budget for MLS area navigation (RNAV) equipment.
This error budget includes contributions due to:
a) ground system performance;
b) airborne sensor performance;
c) ground system geometry effects;
d) MLS/RNAV computer computational error; and
e) flight technical error (FTE).
13.2.2 The composite of the above errors with the exclusion of FTE is referred to as total position error. Within 3.7 km
(2 NM) of the MLS approach reference datum the permissible total lateral position error for MLS/RNAV equipment at a
position 60 m (200 ft) above the MLS datum point on a 3-degree elevation angle and a runway length of 3 000 m (10 000 ft), is
15 m (50 ft) (see the note below). Similarly, the permissible total vertical position error is 3.7 m (12 ft) at the same position. A
portion of the total position error budget has been reserved for the MLS/RNAV computer performance (computational error).
Within 3.7 km (2 NM) of the MLS approach reference datum, the portion of the error budget reserved for computational error is
±0.6 m (2 ft) both laterally and vertically. The results presented in 13.5 are dependent on meeting this computational accuracy
requirement.
23/11/06 ATT G-36
Attachment G Annex 10 — Aeronautical Communications
ATT G-37 23/11/06
13.2.3 Using root sum square methodology the permissible total lateral position error, exclusive of MLS/RNAV
computer performance, is slightly less than ±15 m (50 ft). Similarly, the permissible total vertical position error, exclusive of
computational error is slightly less than ±3.7 m (12 ft). Hence, the combined error due to ground system performance,
airborne sensor performance and ground system geometry effects is not expected to exceed ±15 m (50 ft) laterally and 3.7 m
(12 ft) vertically at the described location. Using this information and assumptions about ground and airborne sensor
performance, the maximum permissible azimuth and elevation antenna offsets (geometry effects) from the runway centre line
can be obtained.
13.2.4 The CMN does not exceed ±7.3 m (24 ft) laterally and ±1.9 m (6.3 ft) vertically, or the linear equivalent of
± 0.1 degree, whichever is less. The linear values are based on nominal antenna sitings (azimuth antenna to threshold distance
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