Keysight EXG and MXG X-Series Signal Generators User’s Guide 141Avionics VOR/ILS (Option 302)Using ILS Glide Slope SoftkeysUsing ILS Glide Slope SoftkeysAn ILS [Instrument Landing System] is a navigation system, used by aircraft toobtain guidance to a runway (performing a “runway approach” whileattempting to land the aircraft) and includes the following three functionsoperating in tandem (working together):— ILS Localizer - ensures that the lateral/horizontal left-right approach of anaircraft is aligned with the center of a runway— ILS Glide Slope - ensures that the vertical descent path of an aircraft isaligned to the proper descent path (~ 3 degrees downwards slope) of arunway— ILS Marker Beacons - ensures the proper distance from a runway; thedistances are based on the Outer, Middle, and Inner ILS Marker BeaconsThe purpose of the ILS Glide Slope is to provide signals that indicate whetheran aircraft is above, below, or in-line with the glide path to a runway andenables a pilot to adjust the aircraft up or down correctly during landing. Thisis the same type of information as provided by the ILS Localizer, but for thevertical reference rather than the lateral/horizontal reference; the samemodulation and antenna techniques are used. This vertical correction isperformed using two AM signals with an AM depth of 40%, operating at acarrier frequency range between 329.90 to 334.70 MHz.For an aircraft on approach, using the ILS Glide Slope to land on a runway, theupper signal is modulated at a frequency of 90 Hz AM by default, and thelower signal is modulated at a frequency of 150 Hz AM by default.The primary mechanism which makes it possible for a pilot to obtain guidanceto a runway is the aircraft receiver's ability to detect the “Difference in Depth ofModulation (DDM)” between this 90 Hz and 150 Hz amplitude modulation.
