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Abstract

A method for measuring the distance between a first object, which incorporates a transmitter-receiver unit, and a second object, which incorporates a transponder. In accordance with the invention there is transmitted from the transmitter-receiver unit (19) a first microwave signal (S1), which is received in the transponder (18). A second signal (S3) of much lower frequency F.sub.m is generated in the transponder and is modulated on a signal (S6) of microwave frequency in the transponder to form a modulated signal (S7). The modulated signal (S7) is transmitted from the transponder to the transmitter-receiver unit (19), where it is received and mixed down with the first signal (S1) and thereafter is (a) low-pass filtered to form a first measuring signal (S10) having the frequency F.sub.m, the phase of which corresponds to the phase difference between the first microwave signal (S1) and the microwave signal (S8) received in the transmitter-receiver unit (19) and (b) de-modulated, thereby to form a second measuring signal (S12) having the frequency F.sub.m, the phase of which corresponds to the phase of the second signal (S3) generated in the transponder (18). The phases of the two measuring signals (S10, S12) are compared in a phase comparison circuit (27) to form a difference therebetween, this difference being proportional to the distance between the objects.

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First Claim