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This paper outlines the steady progress that has been made in radar ranging technique, and shows how fundamental physical principles have been embodied in practical equipment for Service use. The development of a precision ranging system suitable for Naval main armament fire-control is described in detail, together with the practical difficulties encountered. The system employs a crystal-controlled oscillator as the time standard, one period of the crystal frequency being equal to the time taken by an electro-magnetic wave to travel a distance of 2 000 yd; this is equivalent to a radar range of 1,000 yd. Continuous subdivision of the standard timing intervals is effected by a phase-shifting transformer in the output of the crystal oscillator, and since one mechanical revolution of the transformer gives a linear phase-change of 360Â°, it can be calibrated in terms of radar range, one degree representing 2.77 yd. The radar and timing signals are displayed on a cathode-ray tube in a form that enables measurements to be made with great precision. In the conclusion, the most important advances made in recent applications of the system are described. The Appendix discusses the factors governing the accuracy of the phase-shifting transformer.