Calibration system for calibrating a number of receive signal paths, each comprising a receiver (151, 153, 155, 251, 253, 255), of an antenna system (250, 550), the calibration system comprising: a reference signal generator (101, 201) that is configured to generate a reference signal (102, 202), a reference beamforming unit (103, 203, 303) that is coupled to the reference signal generator (101, 201) and that is configured to duplicate the reference signal (102, 202) into a calibration signal (104, 105, 106) for every one of the receivers (151, 153 155, 251, 253, 255) and to apply a beamforming with predetermined beamforming weights to the calibration signals (104, 105, 106) and to provide each one of the receivers (151, 153, 155, 251, 253, 255) with the respectively beamformed calibration signal (104, 105, 106), and an estimation unit (110, 210) that is coupled to respective output interfaces of the receivers (151, 153, 155, 251, 253, 255) and that is configured to determine amplitude deviations and/or phase deviations of the receivers (151, 153, 155, 251 253, 255) based on output signals (107, 108, 109) provided by the receivers (151, 153, 155, 251, 253, 255) via the respective output interfaces, characterised in that the reference beamforming unit (103, 203, 303) is configured to select the beamforming weights such that the calibration signals (104, 105, 106) cancel out at a receive beamformer of the antenna system (250, 550) when the receive beamformer is configured to apply beamforming weights to the output signals (107, 108, 109) of the receivers (151, 153, 155, 251, 253, 255) that are designed to maximize the signal strength at boresight, and the calibration system is configured to only perform the calibration when the beamformer uses beamforming weights in boresight or in a predefined region around boresight. a reference signal generator (101, 201) that is configured to generate a reference signal (102, 202), a reference beamforming unit (103, 203, 303) that is coupled to the reference signal generator (101, 201) and that is configured to duplicate the reference signal (102, 202) into a calibration signal (104, 105, 106) for every one of the receivers (151, 153 155, 251, 253, 255) and to apply a beamforming with predetermined beamforming weights to the calibration signals (104, 105, 106) and to provide each one of the receivers (151, 153, 155, 251, 253, 255) with the respectively beamformed calibration signal (104, 105, 106), and an estimation unit (110, 210) that is coupled to respective output interfaces of the receivers (151, 153, 155, 251, 253, 255) and that is configured to determine amplitude deviations and/or phase deviations of the receivers (151, 153, 155, 251 253, 255) based on output signals (107, 108, 109) provided by the receivers (151, 153, 155, 251, 253, 255) via the respective output interfaces, characterised in that the reference beamforming unit (103, 203, 303) is configured to select the beamforming weights such that the calibration signals (104, 105, 106) cancel out at a receive beamformer of the antenna system (250, 550) when the receive beamformer is configured to apply beamforming weights to the output signals (107, 108, 109) of the receivers (151, 153, 155, 251, 253, 255) that are designed to maximize the signal strength at boresight, and the calibration system is configured to only perform the calibration when the beamformer uses beamforming weights in boresight or in a predefined region around boresight.