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Ground-Based Navigation - Instrument Landing System (ILS)


ILS Localizer The ILS has been the mainstay of landing navigation aids for well over 50 years. The modernized versions used by the FAA provide aircraft with precision vertical and horizontal navigation guidance information during approach and landing. Associated Marker Beacons and/or Low Power Distance Measuring Equipment (LPDME) identify distance to the runway. The attractiveness of ILS lies in the economy of its avionics costs and its wide international acceptance. Technology advances over the years have yielded great improvement in accuracy, dependability, and maintainability.

The GBNG supports all ground-based ILS systems in the National Airspace System (NAS) and will continue procuring and deploying new/replacement ILS for the foreseeable future. It is expected that ILS will eventually be replaced with some variant of a GPS system in the future (see WAAS and GBAS). Presently the FAA has a contract with Thales Air Traffic Management (TATM) corporation to procure the existing NAS-deployable Mark 20A ILS system on a requirements contract. A new TATM FAA ILS 420 system has been developed and is currently undergoing Operational Test. The FAA ILS 420 will be procured after rendering of the In-Service Decision in 2013.

ILS Glide Slope The Localizer generates and radiates signals to provide final approach azimuth navigation information to landing aircraft. The antenna sends a VHF carrier signal with 90-Hz and 150-Hz sideband signals that the aircraft instruments determine as left and right of the centerline. The aircraft interprets the signal and displays them on the cockpit indicator guiding the pilot until the runway is in sight.

In a similar manner as the Localizer signal (just turned 90 degrees on axis), the Glide Slope sends a UHF carrier signal with the same two 90-Hz and 150-Hz sideband frequencies that aircraft instruments determine as above or below the desired glide path. This is approximately 3 degrees to the horizon which gives the aircraft a descent rate of approximately 500 feet per minute.

An ILS precision approach and landing requires several components. For properly ILS-equipped aircraft certified for the category of service utilized, the ground-based ILS systems are the electronic processing and antenna components. The runway requires proper lights and markings along with an approach lighting system. Other components may be required such as Runway Visual Range (RVR) and Marker Beacons or LPDME. Note that the more precise the approach is (lower weather minimums/visibility) the more ancillary components may be required.

Acquisition Lead

  • FAA ILS 420 and Mk-20A - Sam Mallipudi

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