The Instrument Landing System is used for final approach and is based on directional beams propagated from two transmitters at the field,one transmitter is Glide Slope which provides guidance in the vertical plane having range of 10nm while other is The Localizer which guides Aircraft in Horizontal Plane.
It is a ground-based Instrument Approach system that provides precision guidance to an Aircraft approaching and landing on a Runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during Instrument Metrological Conditions(ICM), such as low Ceilings or reduced visibility due to fog, rain, or blowing snow.

ILS GROUND EQUIPMENTS:-

 There are Three main Ground equipments of ILS which are stated as under:
1)Localizer
2)Glide Slope
3)Marker Beacons

LOCALIZER:-
A localizer is a  normally located Antenna  beyond the departure end of the runway and generally consists of several pairs of directional antennas. Two signals are transmitted on one of 40 ILS channels.
It transmitts in the VHF frequency range 108-112 MHZ in 0.5 MHZ increments.

  GLIDE SLOPE :-

The Glide Slope is the signal that provides vertical guidance to the aircraft during the ILS approach. The standard glide-slope path is 3° downhill to the approach-end of the runway. Follow it faithfully and your altitude will be precisely correct when you reach the touchdown zone of the runway. 
It operates UHF frequency Band,328.6 to 335 MHZ at 150 kHZ spacing.upper and lower lobe are  modulated at 90 Hz and 150 Hz respectively.

 

 

 

MARKER BEACONS:-

Marker beacons are used to alert the pilot that an action (e.g., altitude check) is needed. This information is presented to the pilot by audio and visual cues. The ILS may contain three marker beacons: inner, middle and outer. The inner marker is used only for Category II operations. The marker beacons are located at specified intervals along the ILS approach and are identified by discrete audio and visual characteristics (see the table below). All marker beacons operate on a frequency of 75 MHz.