Synthesis of the Inmarsat 4 multibeam mobile antenna

The Inmarsat 4 Spacecraft user link or mobile antenna system consists of a 9m deployable reflector and a feed array with 120 helical elements. Over 220 simultaneous RF beams are created by applying vector weights to the feed elements under the control of the onboard digital signal processor (DSP). The simultaneous transmission and reception requires the achievement of very low levels of passive intermodulation (PIM). The mission requires continuous coverage over fixed ground cells for orbital inclinations of up to 3/spl deg/. This is achieved by uploading modified beam weights on a daily basis. This requires a large number of beam weights to be presynthesized. In addition to the communication beams, a system of RF sensing beams are deployed in order to enhance the spacecraft pointing capability and hence overall communication performance.