A system for simultaneous calibration and communication of active arrays of a satellite comprising: a base transmitter comprising a first antenna for simultaneously transmitting a calibration carrier having a first frequency and a communication carrier having a different second frequency to a satellite receiver; the satellite receiver comprising a second antenna comprising a first active array for simultaneously receiving the calibration carrier and the communication carrier transmitted from the base transmitter and for simultaneously transmitting the calibration carrier and the communication carrier to a satellite transmitter; the satellite transmitter comprising a third antenna comprising a second active array for simultaneously receiving the calibration carrier and the communication carrier transmitted from the satellite receiver and for simultaneously transmitting the calibration carrier and the communication carrier to a base receiver; and the base receiver comprising a fourth antenna for simultaneously receiving the calibration carrier and the communication carrier transmitted from the satellite transmitter; wherein the system further comprises at least one computer arranged to calibrate the elements of the first active array and elements of the second active array so that they are all in relative phase with the calibration carrier based on calibration data received from the calibration carrier, characterized in that the system further comprises, the at least one computer being arranged to: determine a correction factor based on the difference in frequency of the first frequency and the second frequency; and calibrate the elements of the first active array and elements of the second active array so that they are all in relative phase with the communication carrier, based on applying the correction factor to the calibration data received from the calibration carrier. a base transmitter comprising a first antenna for simultaneously transmitting a calibration carrier having a first frequency and a communication carrier having a different second frequency to a satellite receiver; the satellite receiver comprising a second antenna comprising a first active array for simultaneously receiving the calibration carrier and the communication carrier transmitted from the base transmitter and for simultaneously transmitting the calibration carrier and the communication carrier to a satellite transmitter; the satellite transmitter comprising a third antenna comprising a second active array for simultaneously receiving the calibration carrier and the communication carrier transmitted from the satellite receiver and for simultaneously transmitting the calibration carrier and the communication carrier to a base receiver; and the base receiver comprising a fourth antenna for simultaneously receiving the calibration carrier and the communication carrier transmitted from the satellite transmitter; wherein the system further comprises at least one computer arranged to calibrate the elements of the first active array and elements of the second active array so that they are all in relative phase with the calibration carrier based on calibration data received from the calibration carrier, characterized in that the system further comprises, the at least one computer being arranged to: determine a correction factor based on the difference in frequency of the first frequency and the second frequency; and calibrate the elements of the first active array and elements of the second active array so that they are all in relative phase with the communication carrier, based on applying the correction factor to the calibration data received from the calibration carrier. determine a correction factor based on the difference in frequency of the first frequency and the second frequency; and calibrate the elements of the first active array and elements of the second active array so that they are all in relative phase with the communication carrier, based on applying the correction factor to the calibration data received from the calibration carrier.