Instrument Landing Systems

A historical background of instrumental approach and landing equipment for aircraft is given beginning with post-World War I efforts of J. D. Doolittle and emphasizing military-civil standardization efforts that began in 1948 and are continuing. The inadequacies of the current Instrument Landing System (ILS) are detailed and the various efforts of special committees to define an eventual replacement system are described. These committees include the Radio Technical Commission for Aeronautics (RCTA), Special Committee SC-117; the NATO Industrial Advisory Group (NIAG) Subgroup (SG-1); and the International Civil Aviation Organization, All Weather Panel (ICAOAWOP). A five-year national plan for development of a Microwave Landing System (MLS) is being carried out under the leadership of the FAA. The goals of the MLS program are stated and discussed. A number of the fundamental issues on system requirements and system design that were decided by the RTCA are described with their alternates. These issues include the use of air-derived data for extracting aircraft position data; the use of precision distance measuring equipment (DME); the requirement to provide flare-out guidance; and the establishment of a two-band approach using frequencies in the and Kubands. An important issue not resolved was the selection of a technique for providing precision angular measurement of aircraft position. Two approaches are defined: scanning beam and Doppler technique. The basic configuration of a generic MLS is given to establish those functions that are common to both scanning beam and Doppler techniques. The two techniques are then compared in detail to show the advantages and disadvantages of each. The on-going MLS program has as a basic objective the resolution of this issue.