Communications of a mobile station with a satellite mobile communications system and a terrestrial mobile communications system are coordinated. The mobile station is registered with the terrestrial mobile communications system and, responsive to the registration of the mobile station with the terrestrial mobile communications system, the mobile station is concurrently registered with the satellite mobile communications system. The concurrent registration may include implicitly registering the mobile station with the satellite mobile communications system, e.g., by storing information identifying the mobile station may be stored in a location register of the satellite mobile communications system responsive to the registration of the mobile station with the terrestrial mobile communications system, and maintaining synchronization between the two registrations. Authentication tokens may be pre-generated for quick re-registration with a satellite mobile communications system.