Page 145 - Planning and Design of Airports

114 Airp o r t Pl anning

is 20 nm. Over the oceans the separation varies from 60 to 120 nm
depending on location [3].

Navigational Aids
Aids to navigation, known as NAVAIDS, can be broadly classified
into two groups, ground-based systems and satellite-based systems.
Each system is complimented by systems installed in the cockpit.

Ground-Based Systems
Nondirectional Beacon
The oldest active ground-based navigational aid is the nondirectional
beacon (NDB). The NDB emits radio frequency signals on frequen-
cies between 400 and 1020 Hz modulation. NDBs are typically
mounted on a pole approximately 35 ft tall. They may be located on
or off airport property, at least 100 ft clear of metal buildings, power
lines, or metal fences. While the NDB is quickly being phased out in
the United States, it is still a very common piece of navigational
equipment in other parts of the world, particularly in developing
nations. Figure 3-8 provides an illustration of an NDB.
Aircraft navigate using the NDB by referencing an automatic
direction finder (ADF) located on the aircraft’s panel. The ADF sim-
ply points toward the location of the NDB. Figure 3-9 illustrates an
ADF system.

FIGURE 3-8 Nondirectional beacon.