The present invention relates to antenna calibration for active phased array antennas. Specifically, the present invention relates to a built in apparatus for autonomous antenna calibrationAccordingly, the present invention provides a method of calibrating an antenna array comprising the steps of: (i) loading a set of correction coefficients for all or a portion of the antenna array; (ii) outputting a known test signal from all or a portion of the antenna array using said correction coefficients; (iii) measuring the performance of all or a portion of the antenna array using a plurality of calibration antennas; and (iv) generating a new set of correction coefficients to correct the performance of all or a portion of the antenna array.