In this paper, modified power control algorithms are proposed for a satellite mobile communications system with ancillary terrestrial component (ATC). In order to increase system capacity and reduce the transmitting power of the user's equipment, we propose the modified power control scheme consisting of the modified closed-loop and open-loop power control. The modified closed-loop power control (CLPC) algorithm, combining the delay compensation algorithms and pilot diversity, is mainly applied to the ATC link in urban areas because it is more suitable to the short round-trip delay (RTD). In the case of rural areas where ATCs are not deployed or a signal is not received from ATCs, combining monitoring transmitting power equipment and open-loop power control (OLPC) algorithms using an efficient pilot diversity is mainly applied to the link between the user's equipment and the satellite. Two power control algorithms are applied equally to the boundary areas where two kinds of signals are received in order to ensure coverage continuity. The simulation results show that the modified power control scheme has good performance compared to conventional power control schemes in a geostationary earth orbit (GEO) satellite system with ATC.