Third and Fourth Generation (3G and 4G) terrestrial systems provide higher speed Internet Protocol multimedia services to end-users with differentiated QoS across applications. To facilitate this, terrestrial architectures are moving towards an end-to-end all-IP architecture that unifies all services, including Voice over IP bearer. In parallel, Mobile Satellite Systems (MSS) are being designed to complement and/or co-exist with terrestrial coverage depending on spectrum sharing rules and operator choice. The challenge for MSS designers is to address the different physical layer characteristics while maintaining interoperability and compatibility with terrestrial services and reuse 3G Partnership Project core networks.To this end, MSS have been developed that mirror terrestrial Second Generation, 2.5G and 3G systems and are already operational in different parts of the world using different orbital constellations from low Earth orbits to geostationary orbits and both proprietary and standardized air interfaces. This paper describes architectures and methods for mobile satellite systems to achieve services similar to 3G and Fourth Generation terrestrial systems, focusing on the European Telecommunications Standards Institute Geostationary-Mobile Radio GMR-1 3G satellite air interface standard. now an approved standard in European Telecommunications Standards Institute and International Telecommunication Union [4,8] and is expected to be extensible to Release 12 3GPP specifications.These systems provide IP data services with data rates up to 590 kbps. Consistent with 3G/Fourth Generation (4G) specifications, the system provides differentiated QoS both across users and applications. In this framework, a given user terminal (UT) can invoke multiple applications simultaneously, each receiving their appropriate QoS treatment. The system permits both user mobility and terminal mobility across spot-beams within the satellite system as well as between satellite system and terrestrial systems using 3GPP standard signaling for intra-Radio Access Technology and inter-Radio Access Technology handovers. The terrestrial 3G standard was enhanced to achieve better spectral efficiency and improved throughput for satellite operation that is characterized by long delays, spectral scarcity, and limited link margins. Enhancements were primarily in the radio specific layers, namely physical layer, Media Access Control/Radio Link Control (MAC/RLC) layer and Packet Data Convergence Protocol (PDCP) layer, to achieve the necessary spectral efficiency and throughput improvement. The system leverages 3GPP IP Multimedia Subsystem-based services, including Voice over IP (VoIP) with circuit-switched spectral efficiency, resource efficient multicast, IP data with delay-optimized Bandwidth on Demand, dynamic link adaptation, Link Layer Automatic Repeat-reQuest, multiple levels of QoS, Transmission Control Protocol (TCP) Performance Enhancing Proxy (PEP), policy-based resource management, load balancing and position-based admission control, billin...