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Distance measuring systems using random noise as the modulating function are described. The distance measurement is accomplished by correlating the modulation on the transmitted and received signals. The spectrum of the modulating function determines the way in which this correlation, and hence system output, depends on distance to a reflecting target. Physical realizability of filters limits the output-to-distance behavior of linear, noise-modulated systems. Theoretically, either amplitude or frequency modulation can be used, but the latter has distinct advantages in overcoming incidental spurious signals generated within the system. Actual multiplication of signals is avoided through use of a conventional mixer. The resulting system is similar to existing altimeters but is free of the ambiguities inherent in periodically modulated systems, avoids the "fixed error," and is capable of measuring distances down to a few feet. This makes it particularly suited for use as an altimeter in blind landing systems.