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The common civil military program for navigational aids for use on and off the airways is based on a polar co-ordinate system from which a pilot may obtain his position relative to a known ground location. This navigational information is supplied in the form of azimuth and distance measurement from any preselected ground facility. Under this program, azimuth information is supplied by the vhf omnnirange which is now being installed on a country-wide basis. Development work on the final version of the other parameter of the navigation system, namely, the distance-measuring equipment, is nearly completed and plans are being made to add that equipment to the vhf omnirange facilities in the near future. Addition of the distance-measuring equipment to the instrument landing system (ILS) to provide continuous distance information to the runway touchdown point also is contemplated. The distance-measuring equipment, commonly referred to as DME, employs pulse techniques and the interrogator-transpondor or challenge-reply principle so widely used in identification equipment during World War II. A portion of the frequency band, 960 to 1,215 mc, assigned to aids to air navigation, is being used for DME implementation. This report outlines the history and development of the present nationally adopted 100-channel DME, and describes the modifications necessary to convert earlier 50-channel equipment into equipment suitable for operating within a 100-channel system. In operation, the aircraft unit of the DME, commonly called the interrogator, continuously transmits a train of short duration rf pulses.