The ultimate goal of communications systems is to provide pocket-sized, wireless devices that will accommodate voice and data services between any two locations on the globe. Satellite communications is not a replacement of the existing terrestrial systems but rather an extension of wireless systems. A satellite communications system can be viewed as consisting of two parts: the space and ground segments. The space segment consists of the satellites and all their on-board tracking and control system. The earth segment comprises the earth terminals, their associated equipment, and the links to terrestrial networks. Several attractive services can be offered by utilizing the small satellite technology. These include personal communication service in a global scale, digital audio broadcasting, environmental data collection and distribution, remote sensing/earth observation, and several military applications. Satellite networks already offer mobile telephone and paging service, but customers want global multimedia and broadband services.