Abstract: A distance measuring system, particularly suitable for targets which are moving slowly relative to a fixed reference point, such as the upper surface of the burden in a blast furnace. A doubly modulated microwave beam is directed at the target, and the reflected beam is mixed with part of the original beam, which part has its phase varied sinusoidally at a frequency lower than the modulating frequencies. The difference frequency detected between the phase-varied signal and the reflected signal is directly proportional to the distance to the target.