An Overview of Major Satellite Systems N. PANAGIOTARAKIS * , I. MAGLOGIANNIS ** , G. KORMENTZAS ** * Horama Marketing & Engineering S.A, Belgian Liaison Office 284, rue Theodore De Cuyper 1200 Brussels, BELGIUM ** University of the Aegean Dept. of Information and Communication Systems, GR-83200, Karlovassi, GREECE Abstract: A satellite communication system is distinguished by its global coverage, inherent broadcast capability, ability to support mobility and bandwidth-on-demand flexibility. Taking into account this emerging communication role of the satellite systems, the paper provides an overview of major narrowband GEO, MEO and LEO satellite systems. It presents some typical commercial examples of such satellite systems and discusses their basic features consisting of used frequency bands, supported applications and terminals and critical performance issues. Key-Words: GEO, LEO, MEO, Narrowband Satellite Systems, Onboard Processing. 1 Introduction Various wired and wireless technologies are competing for the provis ion of high-bandwidth access. Flexibility, speed, cost, and time-to-market are major factors affecting the evolution of these technologies. Cable and Digital Subscriber Loops (xDSL) modems are the most promising wired broadband access systems. However, the low flexibility and high installation cost of cable modems and the great dependence of xDSL systems on distance prevent them from achieving significant acceptance and popularity in the domain of broadband services. The wireless broadband access systems are commonly classified, according to their type, as MMDS (Multichannel Multipoint Distribution System) [1], or LMDS (Local Mutlipoint Distribution System) [2]. MMDS systems operate at frequencies lower than 5GHz with coverage areas (cell radius) up to 40km. They are suitable for transmission of video and broadcast services in rural areas. LMDS systems operate at higher frequencies, where large portions of the spectrum are still free. In this case, the coverage area is realized with smaller cells (typically up to a radius of 5km). The extension of this radius usually requires repeaters to be placed in a LOS (Line Of Sight) configuration. Passing to the satellite systems, their communication role is not to compete with the land- based fixed, wireless or mobile communications systems previously mentioned but to complement them both in a “geographical” sense (where it is impossible or economically unfeasible for the terrestrial systems to provide services coverage) and in a “service complement” sense (satellite delivery is more appropriate and cost efficient for broadcast/ multicast type of services). The rest paper is organized as follows: Section 2 gives an overview of satellite communication fundamentals. The next Section 3 discusses the major narrowband GEO, MEO and LEO satellite systems. Lastly, Section 4 concludes the paper. An appendix is also included, which tabulates the basic characteristics of current commercial satellite systems. 2 Key Satellite Communication Isuues A satellite system consists of a space and a ground segment. The space segment is composed of satellites, which may be classified into geostationary earth orbit (GEO) and nongeostationary earth orbit (NGEO) satellite, including middle earth orbit (MEO) and low earth orbit (LEO) satellite, according to the orbit altitude above the earth’s surface [3]. The majority of satellites in operation nowadays are GEO. The most significant problem