A localization system to localize an interventional instrument in the body of a patient. The localization system includes an electromagnetic wave source which splits an electromagnetic signal into components propagating along a probe path and a reference path, respectively. The probe path includes a signal outlet for emitting the signal at the point to be located and at least one detector for picking up the emitted signal. A correlator is used to determine the correlation between the signal components that propagated along the probe path and the reference path respectively. Knowing the length of the reference path, the unknown distance between the signal outlet and the detector in the probe path can be estimated based on the correlation information.
