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expected to occur until the aircraft receives a validated back azimuth signal, but it is intended to occur before the approach
azimuth guidance becomes too sensitive to fly. Switching based on loss of approach azimuth may not occur until the aircraft
is very close to the approach azimuth antenna resulting in unflyable guidance. Switching based only on loss of elevation
guidance may occur prior to the aircraft receiving a valid back azimuth signal. However, switching might be based on loss of
elevation guidance once the back azimuth signal has been validated. Automatic switching at or near the mid-point between
azimuth antennas will provide a method which results in continuous guidance during the transition. The mid-point switching
methodology may require the use of DME information by the MLS receiver. Precautions are to be taken so that approach to
back azimuth switching does not automatically occur unless a missed approach has been initiated.
8. Operations at the limits of and outside the promulgated MLS coverage sectors
8.1 The limits of the azimuth proportional guidance sectors are transmitted in basic data words 1 and 5. These limits do
not indicate the maximum flyable MLS approach and back azimuth angles which will normally be at some angle inside these
limits. For example, for an approach azimuth providing a proportional guidance sector of ±40 degrees, flyable MLS approach
azimuth angles with a full course width of ±3 degrees will exist to approximately ±37 degrees. For a back azimuth, flyable
back azimuth angles with full course width will exist to within 6 degrees of the proportional guidance sector limits.
8.2 The basic MLS antenna designs should preclude the generation of unwanted signals outside coverage. Under some
unusual siting conditions, MLS signals might be reflected into regions outside the promulgated coverage with sufficient
strength to cause erroneous guidance information to be presented by the receiver. As in current procedure the implementing
authority would specify operational procedures based on the use of other navaids to bring the aircraft into landing system
coverage without transiting the area of concern or may publish advisories which alert pilots to the condition. In addition, the
MLS signal format permits the use of two techniques to further reduce the probability of encountering erratic flag activity.
8.2.1 If the undesired MLS signals are reflections and if operational conditions permit, the coverage sector can be
adjusted (increased or decreased) such that, at the receiver, either the direct signal is greater than any reflection or the
reflector is not illuminated. This technique is referred to as coverage control.
8.2.2 Out-of-coverage indication signals can be transmitted into the out-of-coverage sectors for use in the receiver to
ensure a flag whenever an undesired angle guidance signal is present. This is accomplished by transmitting an out-ofcoverage
indication signal into the region which is greater in magnitude than the undesired guidance signal.
8.3 If it is operationally required to confirm the selected MLS channel outside the promulgated coverage sectors of the
MLS, it is intended that this confirmation be derived from the identification of the associated DME. MLS status information
is not available outside the promulgated MLS coverage sectors.
9. Separation criteria in terms of signal ratios and propagation losses
9.1 Geographical separation
9.1.1 The separation criteria are provided in 9.2 and 9.3 as desired signal-to-noise ratios and when combined with
appropriate propagation losses allow evaluation of MLS C-Band frequency assignments as regards on-channel and adjacent
23/11/06 ATT G-28
Attachment G Annex 10 — Aeronautical Communications
channel interference. When selecting frequencies for MLS facilities, a similar criteria for the DME/P element or an associated
DME/N as provided in Attachment C to this Part need to be considered.
9.2 Co-frequency requirements
9.2.1 Co-frequency MLS channel assignments should be made to preclude the acquisition of DPSK preambles of an
undesired co-channel facility. The required level of the undesired signal is less than minus 120 dBm, which is 2 dB below
a sensitive MLS airborne system, as shown below:
— receiver sensitivity = –112 dBm
— margin for aircraft antenna
gain above minimum
=
–6 dBm
–118 dBm
Considering the system power budget in Table G-1, which shows the minimum signal level at the aircraft is required to be
at least minus 95 dBm, the minus 120 dBm requirement is achieved by placing the undesired co-channel at a geographic
separation which exceeds the radio horizon distance at any point in the promulgated coverage sector of the desired facility.
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