There is provided an apparatus for calibrating a multi-beam satellite system comprising a beam forming network providing a plurality of signal paths, the apparatus comprising a calibration processor for determining the phase and amplitude shift of a test path relative to a reference path, the calibration processor being configured to determine the phase and amplitude shifts of the test path relative to the reference path for at least two calibration tones of at least two different frequencies to allow a phase and amplitude offset of the test path for a signal of any frequency within an operational frequency range of the multi-beam satellite system to be calculated.