A Survey of Instrument Approach Systems in the United States

A brief history and technical description of instrument landing system (ILS) and precision approach radar (PAR) is given. Improvements in ILS localizer and glide-slope designs are mentioned which reduce adverse site effects and hasten the day when fully automatic approaches may be authorized. Localizer improvements include a highly directional slotted-waveguide antenna. The waveguide system is essentially "added to" the conventional localizer and no change is required in the existing airborne receivers. This system is now operational (a version of the system is also in military use). For the glide slope, a runway-flush antenna has been developed and demonstrated. There are now 170 civil and about 50 military ILS installations in the United States with more under way. PAR (known in military service as GCA) has only limited civilian use but rather general use by the military. In this system, the controllers on the ground "see" the aircraft and its deviation from the desired approach. Corrections in heading and rate of descent are transmitted to the pilot in voice with advice also on distance to touchdown. Based on present trend and international acceptance of ILS, it is concluded that the number of ILS installations will be expanded to cover all aviation hubs and that automatic approaches will be authorized to further increase schedule efficiency and safety. PAR will probably not be further implemented for civil use.