Avionics: Microwave landing systems: As MLS technology enters its third decade, competing approaches contend for standardization at the international level

In the world of aviation, Microwave Landing Systems (MLS) have long been regarded as the preferred alternatives to the present international VHF/UHF Instrument Landing Systems (ILS) developed during World War II. The microwave system readily overcomes the major drawbacks of ILS - among others, limited number of frequency channels, difficulty in finding suitable ground sites at some airports, and susceptibility to interference by severe signal-multipath conditions. But though microwave landing sytems have found use, predominantly in military applications, they have yet to be applied to civil aviation to any great extent.