Ground-Based Navigation - Instrument Landing System (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 GBNA (Ground Based Navigation Aids) 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. Presently the FAA has a contract with Thales ATM, Inc. to procure the ILS 420 system. The ILS 420 completed In-Service Decision in June 2015.

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.

Project Lead: Max Yuan -

Last updated: Wednesday, February 08, 2023