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basic word B6, and auxiliary words B40-B41.
2.8 Adjacent channel interference considerations
2.8.1 The standard has been structured such that there is at least a 5-dB margin to account for variations in the effective
radiated power above the minimum power density specification. The interference specification is based upon worst-case
antenna beamwidth combinations, data rate, and undesired interference synchronization.
3. Ground equipment
3.1 Scanning beam shape
3.1.1 The azimuth scanning beam envelope on the antenna boresight and the elevation scanning beam envelope at the
preferred elevation angle, as detected by a standard receiver, has to conform to the limits specified in Figure G-16 under
ATT G-11 23/11/06
Annex 10 — Aeronautical Communications Volume I
conditions of high SNR and negligible multipath (e.g. during a trial on an antenna range). The –10 dB symmetry relative to
accuracy performance is not necessarily expected in the equipment design.
3.2 Scanning beam side lobes
3.2.1 Performance specification. The antenna side-lobe design has to satisfy two conditions: 1) the dynamic side-lobe
level does not prevent the airborne receiver from acquiring and tracking the main beam. Satisfactory performance cannot be
assured if dynamic side lobes persist at levels above –10 dB; 2) the effective side-lobe level is compatible with the system
error budget.
3.2.2 The effective side-lobe level (PESL) is related to the dynamic side-lobe level (PDYN) by:
PESL = K × PDYN
where K is a reduction factor which depends upon the antenna implementation. The reduction factor may be dependent upon:
a) a directive antenna element pattern which reduces the multipath signal level relative to the coverage volume;
b) the degree of randomness in the dynamic side lobes.
Note.— The dynamic side lobes are of least concern, if the measured dynamic side-lobe levels are less than the specified
effective side-lobe levels.
3.2.3 Lateral multipath reflections from the azimuth antenna side lobes and ground multipath reflections from elevation
antenna side lobes can perturb the main beam and induce angular errors. To ensure that the error dv generated by the antenna
side lobes is within the propagation error budgets, the required effective side-lobe level ESL can be estimated using:
BW R MA
d
PESL
P P
θ
=
θ
where PR is the multipath obstacle reflection coefficient, θBW is the ground antenna beamwidth and PMA is the motion
averaging factor.
3.2.4 The motion averaging factor depends on the specific multipath geometry, the aircraft velocity, the function data
rate and the output filter bandwidth. For combinations of multipath geometry and aircraft velocity such that the multipath
scalloping frequency is greater than 1.6 Hz, the motion factor is:
MA
2 (output filter noise bandwidth)
Function data rate
P =
3.2.5 This factor can be further reduced at higher multipath scalloping frequencies where the multipath-induced beam
distortions are uncorrelated within the time interval between the TO and FRO scans.
3.3 Approach elevation antenna pattern
3.3.1 If required to limit multipath effects, the horizontal radiation pattern of the approach elevation antenna gradually
de-emphasizes the signal away from the antenna boresight. Typically the horizontal pattern of the approach elevation antenna
is to be reduced by 3 dB at 20 degrees off the boresight and by 6 dB at 40 degrees. Depending on the actual multipath
conditions, the horizontal radiation pattern may require more or less de-emphasis.
23/11/06 ATT G-12
Attachment G Annex 10 — Aeronautical Communications
3.4 Approach/back azimuth channels
3.4.1 When a runway has MLS installed for both approach directions, the equipment not in use for the approach may
be operated as a back azimuth. If it is desired to assign different channels to each runway direction, necessarily the azimuth
units will be operated on different frequencies depending on the mode of operation — approach or back azimuth. Care must
be taken in the channel assignments so that the two frequencies are close enough to avoid any mechanical adjustment of the
azimuth antenna vertical pattern when the approach direction is reversed.
3.4.2 The frequency separation should be limited such that the loss in pattern gain for back azimuth (from the optimum
approach value) can be accommodated by the transmitter power margins shown in Table G-1 for the back azimuth function.
4. Siting considerations
4.1 MLS/ILS collocation
4.1.1 MLS elevation antenna
4.1.1.1 Introduction
4.1.1.1.1 When collocating an MLS elevation antenna with an ILS glide path, a series of decisions will have to be
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