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Numerical
installation procedure of a glidepath on a difficult site
If the reflection plane in front
of the glidepath is sufficiently tridimensional the classical simplified
installation procedure on the base of the well known formulas, is no longer
applicable. These formulas use the slopes of the reflection plane (forward
slope, sideward slope).
Simple approximate methods cannot
be applied directly also. These are mostly based on closed formulas for
the scattering pattern of regular metall sheets drived from Kirchhoffs-Integration.
Other two-dimensional GTD/UTD-methods or linearized sliding 3D /2,3/ are
not applicable also due to the generally non-adapted method.
The general tridimensional UTD-approach
/1/ for the analysis and optimization of the glidepath consists of the
following steps :
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theoretical engineering pre-analysis;
site survey and identification of relevant objects, analysis of maps ,
selection and determination of the relevant objects, provisional pre-selection
of the system type (0,B,M)
-
modelling of the ground in the relevant
reflection region by convex patches (first model without objects)
-
modelling of the relevant objects (buildings,
fences, dunes, power lines etc.)
-
modelling of the antenna(s) for the
preselected system type
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theoretical approximate determination
of initial values for the antenna position and the radiator geometry on
the base of the approximations using the forward and sideward slopes (Fig.
2)
-
iterative optimization of the antenna
data with respect to the main glidepath parameters (glideslope angle, crossing
height, (vertical) width)
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systematic UTD-calculation of the spatial
system characteristics (structure on the glidepath, vertical and azimuthal
coverage, window pattern, level run, off glidepath approach)
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incorporation of the relevant objects;
calculations of the effects of the objects on the system performance
-
if necessary, improvement of the system
type (e.g. 0 to M), recalculation of all relevant parameters
-
modification and optimization of the
antenna currents for minimized distortions (i.e. evaluation of the socalled
"modified M-type")
-
recalculation of the main parameters
and of the window pattern in case of the modified M-type; definition of
the resultant azimuthal coverage.
The numerical process for the Localizer
is similar, but simpler. In case of cross checks with flight check results,
the entire procedure is reduced to the numerical analysis of a given installation
and an given site.
The performance of the glideslope
is sampled spatially around the glidepath (Fig. 8) and especially for some
distance (e.g. 10km, 10nm) by the socalled window pattern. Fig. 9 shows
an example of the window pattern for the glideslope GP04R of the New Athen
International Airport (see Fig. 4). It can be clearly seen that the strong
DDM-distortions due to the cliff can be accepted if the above mentioned
definitions for the azimuthal coverage of the glideslope are applied.
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