Page 149 - Planning and Design of Airports

118 Airp o r t Pl anning

Air Route Surveillance Radar
A long-range radar for tracking en route aircraft has been established
throughout the continental United States and in other parts of the
world. While in the United States there is complete radar coverage in
the 48 contiguous states, this is not the case elsewhere in the world.
These radars have a range of about 250 nm. Strictly speaking radar is
not an aid to navigation. Its principal function is to provide air traffic
controllers with a visual display of the position of each aircraft so
they can monitor their spacings and intervene when necessary. How-
ever, it can be and is used by air traffic controllers to guide aircraft
whenever this is necessary. For this reason it has been included as an
aid to navigation.
The VOR and NDB, often combined with radar-based surveillance
from air traffic control, have traditionally been used in both en route
navigation and for navigation on approach to landing at an airport.
Navigation on approach to an airport using these ground-based sys-
tems is performed by following predetermined, published, approach
procedures. These procedures are often updated and published by the
FAA in the form of approach charts. Figure 3-14 provides an example
of an approach chart depicting an approach procedure using an NDB
as an aid to navigation, while Fig. 3-15 illustrates a similar approach
using a VOR as the primary aid to navigation. It is strongly recom-
mended that the airport planner understand the information provided
in these charts. Approaches based on NDB and VOR navaids are con-
sidered “nonprecision” approaches, as they provide lateral navigation
assistance but not vertical navigation. That is, these instruments may
be referenced to determine which direction to fly when approaching an
airport, but do not provide instrument-based guidance in determining
the appropriate altitude or descent rate on approach.

Instrument Landing System
Until the recent proliferation of published navigation procedures which
rely on the satellite based GPS system, the instrument landing system
(ILS) was the only ground-based system certified to provide both lat-
eral and vertical guidance to aircraft on approach to an airport, and as
of 2008 is the only navigational aid certified by the FAA to provide
“precision” navigation for aircraft, and is still the most widely used
method of approach navigation at the world’s larger airports.
An ILS system consists of two radio transmitters located on the
airport. One radio beam is called the localizer and the other the glide
slope. The localizer indicates to pilots whether they are left or right of
the correct alignment for approach to the runway. The glide slope
indicates the correct angle of descent to the runway. Glide slopes are
in the order of from 2°–3° to 7.5°.
In order to further help pilots on their ILS approach, up to three
low-power fan markers called ILS markers are usually installed so