In this paper, we propose an enhanced cooperative transmit diversity scheme in integrated mobile satellite service (MSS) systems. The proposed scheme is an efficient technique to adapt various power differences between a satellite signal and CGC signals, depending on the position of user equipments in integrated MSS systems. In the proposed scheme, we utilize modified space-time block coding (STBC) in order to realize the transmit diversity for the mobile satellite system with several complementary ground components (CGCs). The satellite and two groups of CGCs operate in unison the send the encoded signals, so that the receiver may realize diversity gain. The simulation results show that the proposed method can provide stable improved performance.