ILS (Instrument Landing System)
ILS is an instrument approach which is ground based that can provide lateral and vertical guidance to an aircraft that is approaching the runway. It is possible due to combination of radio signals and high intensity lighting rays. It was accepted by International Civil Aviation Organization in 1947 A.D.
ILS approach is only valid if it is used within certain boundaries either of LOC and GS beams as documented on the corresponding IAP (Instrument Approach Procedure). These limits are known as FSD (Full Scale Deflection) of the deviation indicated on ILS display in the deck. Once the deviation in respect of either the LOC or GS reaches FSD it will become impossible to find out the extent of the deviation. Due to this pilots who are navigating there aircraft onto an ILS whether from above or below the GS have been expected. When acquiring the approach, to cross check there range from touchdown against their altitude and confirm that their aircraft is on the promulgated IAP.