Instrument Landing System (ILS)
M. A. L. C. N. Perera
Hundreds of flight instruments and systems onboard and on the ground helps an aircraft to have a safe flight from take-off to landing. Those instruments can be divided in to four main categories. They are Navigation, Communication, Information and safety. Instrument landing system (ILS) is an internationally normalized system, which guides aircrafts to approach and line-up with the runway and land upon the final approach for landing. Today, because of several reasons Instrument landing system required to be updated or to find an alternative more advanced system with more accurate information to guide aircrafts, when they approaching to runway for landing.
1. Instrument Landing System (ILS)
The first landing of a scheduled US passenger airliner using an ILS was on 26 January 1938, when a Pennsylvania-Central Airlines Boeing 247-D flew from Washington, DC, to Pittsburgh and landed in a snowstorm using only the ILS system. 
ILS is based on directional beams and used for final approach. Directional beams propagate from two transmitters at the airfield. Two transmitters known as Glider (located closer to terminals) and localizer (Across the end of the runway). Marker beacons marks guides with the distance from runway to the aircraft. Marker beacons work together with glider and localiser. For example, glidepath starts to send signals when aircraft pass outermost marker. 
1.1. Marker Beacons
Generally, an equipped runway has three marker beacons. They are outer, middle and inner marker beacons. Low-power transmitters use for marker beacons. They operate at a frequency of 75MHz with 3W or less power output. Each beacon radiates a fan-shaped beam pattern of elliptical cross section upward from the ground, the axis of the fan being at right angles to the airway. At an altitude of 1000 ft, the beam dimensions are 2400 ft long and 4200 ft wide. At higher altitudes, the dimensions increase...