In an apparatus for maintaining the orientation of an antenna system with respect to a beacon, radio signals are received in four quadrants and converted into sum and difference signals, which are transformed in frequency, amplified in an amplifier (6) and detected in a first- and second-phase detector (9, 15) present in a corresponding channel (7, 8) to derive error voltages in first and second time intervals respectively for aligning the antenna system with respect to the beacon. During the tracking mode of the apparatus, the difference and sum signals separately pass in first, second and third, fourth time intervals, respectively. a VCO (21) supplies two orthogonal signals as reference voltages, to the phase detectors. In the tracking mode the VCO driven by a signal derived from the signals obtained in the first channel (7) in the fourth time interval and in the second channel (8) in the third time interval. With a gain control circuit (27) the gain of the amplifier (6) is adjusted in response to signals obtained in the first and the second channels (7, 8) in the said third and fourth time intervals respectively in such a way that the output voltage of the first phase detector (9) in the third and first time interval and that of the second phase detector (15) in the fourth and second time interval are kept constant.