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Keysight EXG and MXG X-Series Signal Generators User’s Guide 375

Using Avionics VOR/ILS Softkeys for N5171B/72B and N5181B/82B with Option 302

Using ILS Glide Slope Softkeys

An ILS [Instrument Landing System] is a navigation system, used by aircraft to

obtain guidance to a runway (performing a “runway approach” while

attempting to land the aircraft) and includes the following three functions

operating in tandem (working together):

— ILS Localizer - ensures that the lateral/horizontal left-right approach of an

aircraft is aligned with the center of a runway

— ILS Glide Slope - ensures that the vertical descent path of an aircraft is

aligned to the proper descent path (~ 3 degrees downwards slope) of a

runway

— ILS Marker Beacons - ensures the proper distance from a runway; the

distances are based on the Outer, Middle, and Inner ILS Marker Beacons

The purpose of the ILS Glide Slope is to provide signals that indicate whether

an aircraft is above, below, or in-line with the glide path to a runway and

enables a pilot to adjust the aircraft up or down correctly during landing. This

is the same type of information as provided by the ILS Localizer, but for the

vertical reference rather than the lateral/horizontal reference; the same

modulation and antenna techniques are used. This vertical correction is

performed using two AM signals with an AM depth of 40%, operating at a

carrier frequency range between 329.90 to 334.70 MHz.

For an aircraft on approach, using the ILS Glide Slope to land on a runway, the

upper signal is modulated at a frequency of 90 Hz AM by default, and the

lower signal is modulated at a frequency of 150 Hz AM by default.

The primary mechanism which makes it possible for a pilot to obtain guidance

to a runway is the aircraft receiver's ability to detect the “Difference in Depth of

Modulation (DDM)” between this 90 Hz AM and 150 Hz AM modulation.

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