A method for calibrating a rotating antenna system including an antenna element or antenna elements connected to a signal receiver unit or signal receiver units at an RF signal input port thereof, the method comprising: generating an RF calibration signal; applying the RF calibration signal to the RF signal input port of the signal receiver unit or signal receiver units without applying the calibration signal via the antenna element or antenna elements; outputting an RF analogue signal from the signal receiver unit or signal receiver units as a response thereof to the applied RF calibration signal; measuring the response of the rotating antenna system to the calibration signal; calculating a signal correction according to the measured response; and applying the signal correction to the response of the rotating antenna system to signals subsequently received at said signal receiver unit or signal receiver units via the antenna element or antenna elements of the rotating antenna system, characterised by the method further comprising: receiving the analogue RF signal at an optical modulator or optical modulators and modulating an optical signal therewith to produce a modulated analogue optical signal or analogue optical signals; optically transmitting the modulated analogue optical signal or analogue optical signals to an opto-electrical converter or opto-electrical converters via an optical rotary joint including a reversion prism; converting said modulated analogue optical signal or analogue optical signals into an analogue electrical signal or analogue electrical signals using said opto-electrical converter or opto-electrical converters; and calculating a signal correction or signal corrections according to the analogue electrical signal or analogue electrical signals. generating an RF calibration signal; applying the RF calibration signal to the RF signal input port of the signal receiver unit or signal receiver units without applying the calibration signal via the antenna element or antenna elements; outputting an RF analogue signal from the signal receiver unit or signal receiver units as a response thereof to the applied RF calibration signal; measuring the response of the rotating antenna system to the calibration signal; calculating a signal correction according to the measured response; and applying the signal correction to the response of the rotating antenna system to signals subsequently received at said signal receiver unit or signal receiver units via the antenna element or antenna elements of the rotating antenna system, receiving the analogue RF signal at an optical modulator or optical modulators and modulating an optical signal therewith to produce a modulated analogue optical signal or analogue optical signals; optically transmitting the modulated analogue optical signal or analogue optical signals to an opto-electrical converter or opto-electrical converters via an optical rotary joint including a reversion prism; converting said modulated analogue optical signal or analogue optical signals into an analogue electrical signal or analogue electrical signals using said opto-electrical converter or opto-electrical converters; and calculating a signal correction or signal corrections according to the analogue electrical signal or analogue electrical signals.