a radio distance measuring equipment emits a frequency modulated carrier wave signal , having a periodic frequency deviation δf and a mean frequency f o , toward an object , and receives a portion of the carrier wave signal reflected by the object . a beat signal produced by mixing the emitted and received signals is applied to a control loop which determines the distance . operation of the equipment is periodically interrupted , under the control of a microprocessor , to effect calibration of the frequency deviation and the mean frequency .