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Distance-measuring equipment (DME), developed for the USAF and CAA, providing automatic distance indications for aircraft navigation is described. Distance is measured with respect to a ground transpondor beacon by the length of time elapsing between the transmission of a pulse "interrogation" signal from the aircraft and the reception of a similar pulse "reply" from the beacon. Traffic to and from several beacons is channeled by a combination of frequency and pulse-pair coding wherein each signal consists of a pulse pair of distinctive spacing. Circuits are described that recognize only pulse pairs whose leading edges are separated by the proper time interval (of the order of 10 to 25 microseconds). Data is presented of the extent to which such pulse pairs can be used to permit several transpondor beacons, within overlaping service areas, to operate on common interrogation and common reply frequencies when serving a multiplicity of interrogator-responsors of the automatic searching and tracking type. While the equipment described operates with only 52 channels, the experience obtained with this equipment has been used to standardize internationally the same channeling method on 100 channels.