Abstract
The development of modern tridimensional
numerical methods enables the analysis and the optimization of ILS to be
installed on difficult sites. The accuracy depends very much on the physical
validity of the numerical model. In case of the classical ILS-glideslope
the accuracy is reliable in such a way that the numerical results and the
flight check results can be used mutually for verification purposes. Flight
check errors can be detected directly by equivalent analysis of the given
installation on the given site.
The numerical analysis and the flight
check apply in principle the same specifications. However, some methods
of the numerical approach can be transfered to the flight check procedures.
Others cannot be transfered, because the analysis can use arbitrary tracks
in space and can calculate very specific but highly adequate parameters
such as the common locus for DDM=0. By this the glidepath angle and the
crossing height can be calculated essentially correct according Annex 10.
The detailed procedure for the analysis
and the optimization of a classical glideslope on difficult site is described.
Introduction
Usually today the installation of
an ILS and the flight check (commissioning and routinely checks) are independant
procedures.This is certainly true for simple cases where the installation
is straight forward. In case of difficult sites the theoretical and numerical
analysis and the flight check should be treated as complementary and adjusted
procedures.
If the ILS glidepath is installed
on a perfect ideally flat site without any obstructions its numerical analysis
and the flight check procedures are easy and straight forward. The performance
of the ILS-Localizer is to a high degree independant of the ground parameters
whereas the classical ILS-glidepath does depend inherently on the ground
due to its image type principles.
Many existing airports are equipped
today with new ILS and new airports have to be constructed on imperfect
sites due to lack of free space. Major ground movements are almost impossible
on existing airports and are very costly in any case in order to meet the
classical grading requirements. These requirements are defined mostly under
worst case assumptions and for more or less regular geometries of the glidepath
radiators. Classical parameters in that respect are the forward and sideward
slopes of the ground in front of the glidepath (Fig. 2). However, in the
general tridimensional case these figures cannot be defined in a meaningfull
way. The glidepath is positioned beside the runway (Fig. 1) which is compensated
by a lateral displacement of the radiators. These displacement figures
cannot be defined by classical means in case of relevant tridimensional
ground which includes the first part of the runway also. The socalled conical
coordinates of the glidepath radiation are not applicable also because
the DDM=0 equi-surface does not form a cone. By the conical coordinates
(Fig. 1) the DDM=0 glidepath extends to a hyperbolic asymptotic behaviour
close to the threshold.
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