Instrument landing system in the context of Distance measuring equipment

Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF are denoted high frequency (HF), and the next higher frequencies are known as ultra high frequency (UHF).

VHF radio waves propagate mainly by line-of-sight, so they are blocked by hills and mountains, although due to refraction they can travel somewhat beyond the visual horizon out to about 160 km (100 miles). Common uses for radio waves in the VHF band are Digital Audio Broadcasting (DAB) and FM radio broadcasting, television broadcasting, two-way land mobile radio systems (emergency, business, private use and military), long range data communication up to several tens of kilometers with radio modems, amateur radio, and marine communications. Air traffic control communications and air navigation systems (e.g. VOR and ILS) work at distances of 100 kilometres (62 miles) or more to aircraft at cruising altitude.

In aviation, an instrument approach or instrument approach procedure (IAP) is a series of predetermined maneuvers for the orderly transfer of an aircraft operating under instrument flight rules from the beginning of the initial approach to a landing, or to a point from which a landing may be made visually. These approaches are approved in the European Union by EASA and the respective country authorities, and in the United States by the FAA or the United States Department of Defense for the military. The ICAO defines an instrument approach as "a series of predetermined maneuvers by reference to flight instruments with specific protection from obstacles from the initial approach fix, or where applicable, from the beginning of a defined arrival route to a point from which a landing can be completed and thereafter, if landing is not completed, to a position at which holding or en route obstacle clearance criteria apply."

There are three categories of instrument approach procedures: precision approach (PA), approach with vertical guidance (APV), and non-precision approach (NPA). A precision approach uses a navigation system that provides course and glidepath guidance. Examples include precision approach radar (PAR), instrument landing system (ILS), and GBAS landing system (GLS). An approach with vertical guidance also uses a navigation system for course and glidepath deviation, just not to the same standards as a PA. Examples include baro-VNAV, localizer type directional aid (LDA) with glidepath, LNAV/VNAV and LPV. A non-precision approach uses a navigation system for course deviation but does not provide glidepath information. These approaches include VOR, NDB, LP (Localizer Performance), and LNAV. PAs and APVs are flown to a decision height/altitude (DH/DA), while non-precision approaches are flown to a minimum descent altitude (MDA).

The Airports Authority of India (AAI) is a Category-1 Public Sector Enterprise under the ownership of the Ministry of Civil Aviation, Government of India. It is responsible for creating, upgrading, maintaining, and managing civil aviation infrastructure in India. It provides Communication Navigation Surveillance/Air Traffic Management (CNS/ATM) services over the Indian airspace and adjoining oceanic areas. AAI currently manages a total of 137 airports, including 34 international airports, 10 Customs Airports, 81 domestic airports, and 23 Civil enclaves at Defense airfields. AAI also has ground installations at all airports and 25 other locations to ensure the safety of aircraft operations. AAI covers all major air routes over the Indian landmass via 29 Radar installations at 11 locations along with 700 VOR/DVOR installations co-located with Distance Measuring Equipment (DME). 52 runways are provided with Instrument landing system (ILS) installations with Night Landing Facilities at most of these airports and an Automatic Message Switching System at 15 Airports.