Herein, we study transmit beamforming techniques in an underlay cognitive mode for the coexistence of satellite and terrestrial networks with the satellite forward link as primary and the terrestrial downlink as secondary. Since geostationary satellite terminals have predetermined propagation characteristics so that they always point towards the geostationary satellite, the interference received by the satellite terminals from the terrestrial Base Station (BS) is confined in an angular sector. Based on this a priori knowledge, we propose transmit beamforming techniques at the BS to maximize the Signal to Interference plus Noise Ratio (SINR) towards the desired Secondary User (SU) and to mitigate the interference towards the primary satellite terminals. Different types of Linearly Constrained Minimum Variance (LCMV) techniques have been proposed for our considered scenario where the exact locations and the number of satellite terminals within a specific angular sector are not known while designing the beamformer. Furthermore, an optimization problem is formulated for maximizing the SU rate and it is shown that the worst case SU rate depends on the Primary User (PU) distance, PU interference threshold and the angular separation of the desired SU from the region of interest.