Abstract: A ground station of a radionavigation system, responding to interrogation pulses from aircraft passing within range, emits message pulses whose timing enables the on-board equipment of the interrogating aircraft to determine its distance from the ground station and/or its azimuthal position relative thereto. To prevent such information from being utilized by aircraft not entitled thereto, the emission of the message pulses is subjected to an artificial delay whose randomly varying magnitude is conveyed by a multibit code word subjected in a distorter to a predetermined series of transformations based upon selected bits thereof. The on-board equipment of privileged aircraft includes a decipherer complementary to the distorter which restores the original code word and derives from it an indication of the artificial delay to be deducted from the measured time interval.