Efficient secondary use of satellite spectrum is studied in this paper. Terrestrial mobile communication system accesses the C band spectrum using low power small cells or device-to-device (D2D) transmission mode. Link budget analysis conducted with a realistic path loss model in an urban environment shows that protection zones are dependent on multiple parameters such as antenna patterns, elevation angles of satellite earth stations and transmission power of terrestrial users. Impact of mentioned parameters as well as e.g., shielding and aggregate interference to the protection distances is analyzed.