A system for protecting a power unit on a spacecraft, comprising: a ground station, the ground station comprising: an individual earth reference cell traffic loading module to receive one or more transmission signals, each transmission signal being received from a communications device of a user for forwarding by the ground station to the spacecraft to be formed into a beam for onward transmission to a specific communications device of other individual ground users associated with an earth reference cell; a ground station power limiter for each received transmission signal to regulate the power of the transmission signal to each specific earth reference cell; a ground based beamforming module to receive the transmission signal from each of the ground station power limiters; a power unit loading estimation and monitoring module for estimating a power loading level on the power unit of the spacecraft; a spacecraft loading, monitoring and model correction module to compare an estimated power loading level at a predetermined spacecraft power measurement location provided by the power unit loading estimation and monitoring module with a measured power level at the predetermined spacecraft power measurement location, wherein the predetermined spacecraft power measurement location is a predetermined location within a communications system of the spacecraft; and a further ground station power limiter configured to operate under the control of the power unit loading estimation and monitoring unit to regulate a power of a signal or signals from the ground based beamforming module and passed to a ground RF transmitter unit for transmission from the ground station to the spacecraft to prevent overloading the power unit of the spacecraft, wherein the further ground station power limiter is configured to regulate the power of the signal for transmission to the spacecraft based on the comparison of the estimated power loading level with the measured power level such that, in the event that an abnormality is detected in the measured power level or if the measured power level exceeds a predetermined level, the further ground station power limiter is configured to limit the power of the signal for transmission to the spacecraft. an individual earth reference cell traffic loading module to receive one or more transmission signals, each transmission signal being received from a communications device of a user for forwarding by the ground station to the spacecraft to be formed into a beam for onward transmission to a specific communications device of other individual ground users associated with an earth reference cell; a ground station power limiter for each received transmission signal to regulate the power of the transmission signal to each specific earth reference cell; a ground based beamforming module to receive the transmission signal from each of the ground station power limiters; a power unit loading estimation and monitoring module for estimating a power loading level on the power unit of the spacecraft; a spacecraft loading, monitoring and model correction module to compare an estimated power loading level at a predetermined spacecraft power measurement location provided by the power unit loading estimation and monitoring module with a measured power level at the predetermined spacecraft power measurement location, wherein the predetermined spacecraft power measurement location is a predetermined location within a communications system of the spacecraft; and a further ground station power limiter configured to operate under the control of the power unit loading estimation and monitoring unit to regulate a power of a signal or signals from the ground based beamforming module and passed to a ground RF transmitter unit for transmission from the ground station to the spacecraft to prevent overloading the power unit of the spacecraft, wherein the further ground station power limiter is configured to regulate the power of the signal for transmission to the spacecraft based on the comparison of the estimated power loading level with the measured power level such that, in the event that an abnormality is detected in the measured power level or if the measured power level exceeds a predetermined level, the further ground station power limiter is configured to limit the power of the signal for transmission to the spacecraft.