Complex array signal processing algorithms like Multiple Signal Classification often require a precise knowledge of the radiation characteristics of the array antenna in use. Calibration of group antennas by determination of antenna array radiation patterns is therefore a fundamental task for improving the performance of these algorithms. From a practical point of view this requirement is challenging, since the environment surrounding the antenna may alter over time or may even be completely unknown. This environment can have a severe influence on the group antenna radiation patterns. The approach proposed in this paper uses a Microwave Imaging technique to reconstruct position and shape of scatterers surrounding the array antenna. This information can further be used to calculate the field scattered by these objects as well as the total radiation pattern of group antenna and environment. The proposed algorithm only requires S-parameters of the antenna array measured in presence of the scatterers. The theoretical concept of this method is presented in this paper. Results obtained by numerical simulation show that this algorithm provides better radiation pattern estimates than other commonly known calibration methods.