Abstract: A ground station system for automatically monitoring a predetermined plurality of carrier frequencies for radio communication calls from aircraft which transmit and receive traffic over one of said carrier frequencies, wherein the aircraft transmits a calling code signal for a selected ground station, and each of the ground stations has a separate receiver for each of said carrier frequencies. Each receiver is connected to a decoding device which detects only complete and correct calling code signals for that particular ground station. In addition to each of the calling code signals having a first calling part for the pre-selected ground station, each calling code signal also may have a second or an information part indicating the direction sector of the aircraft with respect to the called ground station, the way of modulation or modulation scheme, e.g. A-M or S.S.B., and the company or party being called at that station.