Instrument landing system glide path


Instrument landing system glide path, commonly referred to as a glide path or glide slope, is "a system of vertical guidance embodied in the instrument landing system which indicates the vertical deviation of the aircraft from its optimum path of descent", according to Article 1.106 of the ITU Radio Regulations.

Principle of operation

A glide slope station uses an antenna array sited to one side of the runway touchdown zone. The GS signal is transmitted on a carrier signal using a technique similar to that for the localizer. The centre of the glide slope signal is arranged to define a glide path of approximately 3° above horizontal. The beam is 1.4° deep.
The pilot controls the aircraft so that the glide slope indicator remains centered on the display to ensure the aircraft is following the glide path to remain above obstructions and reach the runway at the proper touchdown point.

Carrier frequency pairings

Localizer and glide slope carrier frequencies are paired so that the navigation radio automatically tunes the G/S frequency which corresponds to the selected LOC frequency. The LOC signal is in the 110 MHz range while the G/S signal is in the 330 MHz range.
LOC carrier frequencies range between 108.10 MHz and 111.95 MHz.
Two signals are transmitted on one of 40 ILS channels. One is modulated at 90 Hz, the other at 150 Hz. These are transmitted from co-located antennas. Each antenna transmits a narrow beam.