Gerhard Greving
NAVCOM Consult, Bahnhofstr.
4, D-71672 Marbach
Presented on the DGON-IRS 98 (International
Radar Symposium), Munich 1998. The following extended version is published
in the DGON-magazine 3/1998
Introduction
In normal radar application the radar
is located at a position which is optimized for its operational purpose.
It is tried to install the radar e.g. as high as possible and at a location
where distortions of the radar signal are as low as possible and where
the range/coverage is reduced by environmental conditions as low as possible.
The degrees of freedom for the location
of radars on an airport are limited and the potentially distorting objects
are existing and cannot be removed. Moreover, newly built objects such
as hangars and terminals etc. may distort the radiation and the signal
evaluation of the airport radars. These radars are
-
primary surveillance radars (ASR, PSR)
-
secondary surveillance radars (SSR,
MSSR)
-
airport surveillance radars or ground
radars (ASDE) and to some extent
-
weather radars and wind profiler radar.
These radars are in addition to other
vital navaids systems on the airport, e.g.
-
instrument-landing system ILS (localizer,
glideslope)
-
distance measuring equipment system
DME
-
tactical air navigation TACAN.
The propagation effects and distorting
effects may be summarized by multipath effects. However, the system implications
of multipath for the different radar systems are very much different.
The potentially distorting effects
may be false/multiple targets, false directions, ghost echos etc. The worst
case in the actual ATC-concepts are persistant ghost echos for some time
which are processed by the surveillance radars as "false tracks" (Fig.
1 and 3). Short time ghost echos are not critical if they do not lead to
false tracks.
The surveillance radars on airports
or around the airports have very specific radar tasks which are safety
relevant to some degree for the final landing process. The aircraft have
to be guided and controlled at least up to the intercept points of the
classical landing system ILS. By its nature the classical ground based
landing system (ILS) provides a proportional guidance to the aircraft only
within a small width angular sector around the defined glidepath. When
intercepted on the glidepath the guidance quality and safety of the landing
systems do not need in principle a further surveillance of the aircraft
by radar means. However, the separation on the glidepath is controlled
by radar means. The modern MLS-landing system would not require an additional
radar surveillance in principle in the total MLS coverage zone (±
40° azimuth, +1° to 15° elevation) due to its full 3D-guidance
information. However, at high traffic airports the radars supply additional
information for the controller about the actual position of the landing
aircraft. This is especially for the SSR-radar which is identifying the
aircraft and interrogating information from the aircraft. The surveillance
role of ASR/PSR and SSR/MSSR can be devided into several sub-categories
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