Abstract: Devices for registration of an object and its movements in relation to another object by means of a high frequency signal transmitted from one of the objects and reflected from the other object, the reflected signal being modulated with an identification code and received back at the first mentioned object where the registration of the code takes place. To separate the reflected signal, from the first transmitted signal the frequency of the reflected signal is changed at the same time as it is reflected, the frequency change of the reflected signal being made in a way which allows the frequency of the reflected signal to be chosen according to wish, and that the frequency of the reflected signal is not equal to any of the stronger harmonic of the fundamental frequency of the first, originally transmitted signal. Thereby the reflected signal can be properly received and detected even though much weaker than the transmitted signal or any of its harmonics.