The OM normally indicates a position where an aircraft at the proper altitude will
intercept the glide path on the localizer course. The MM indicates a position
approximately 3,500 feet from the landing threshold. This is also the position where an
aircraft on glide path will be at an altitude of approximately 200 feet above the elevation
of the touchdown zone. The IM will indicate a point where an aircraft is at a designated
DH on a glide path between the MM and landing threshold.
A radio beacon used in conjunction with ILS markers is called a compass locator. This
beacon transmits nondirectional signals that are used by pilots to determine bearings.
The range of a compass locator transmitter is at least 15 miles and is often located at
the ILS, MM, and OM sites. Each compass locator transmits a two-letter identification
groups. The outer locator transmits the first two letters of the localizer identification
group, and the middle locator transmits the last two letters of the localizer identification
Global Positioning System
Global Positioning System (GPS) is a space-based navigation system that relies on
multiple satellites to provide input to aircraft systems. It is typically used with a map
overlay showing the aircraft s exact position, within feet or inches, on the surface of the
The GPS system is maintained by the United States Air Force and is made up of 24
32 satellites located within the earth s medium orbit. These satellites broadcast signals
from space with their exact location within orbit. This information is calculated into
distance and time by the aircraft s receiver, and when utilized in the tri-laterization (or
use of at least three satellites) algorithm gives the aircraft its exact location on the earth.
Due to the complexities of this system, exact timing is required for navigation. The
slightest offset in time can place an aircraft several miles from its intended flight path.
Furthermore, the GPS system is also subject to outages that occur due to satellites
failing to maintain their appropriate orbit, satellite transmission failure, or the natural
offset of orbits by multiple satellites. These outages are usually for a small amount of
time, and NOTAMS are automatically generated to inform aviators of the projected
lapse in service and associated anomalies.
GPS is changing the landscape of air navigation at a phenomenal rate. The use of
these systems is increasing dramatically and has even evolved into non-precision
instrument approaches at many smaller airfields without any maintenance costs being
incurred by those facilities. The Wide Area Augmentation System (WAAS) is being
developed to provide precision approaches with a tolerance of 25 feet, both laterally and