信息科学与电子工程专业英语 教学课件 Unit 7 Satellite Communications.pptVIP

Technical English For Information Science and Electronic Engineering Unit 7 Satellite Communications Part I Applications of Communications satellite New Words 1 A communications satellite (sometimes abbreviated to comsat) is an artificial satellite stationed in space for the purposes of telecommunications. Modern communications satellites use a variety of orbits including geostationary orbits, elliptical orbits and low (polar and non-polar) Earth orbits. 2 For fixed (point-to-point) services, communications satellites provide a technology complementary to that of fiber optic submarine communication cables. They are also used for mobile applications such as communications to ships, vehicles, planes and hand-held terminals, and for TV and radio broadcasting, for which application of other technologies, such as cable, is impractical or impossible.1 3 Telephony The first and historically the most important application for communication satellites is in international telephony. Fixed-point telephones relay calls to an earth station, where they are then transmitted to a geostationary satellite. 3 An analogous path is then followed on the downlink. In contrast, mobile telephones (to and from ships and airplanes) must be directly connected to equipment to uplink the signal to the satellite, as well as being able to ensure satellite pointing in the presence of disturbances, such as waves onboard a ship.2 4 Improvements in submarine communications cables caused a decline in the use of satellites for fixed telephony in the late 20th century. Hand held telephones (cellular phones) used in urban areas do not make use of satellite communications. Instead they have access to a ground based constellation of receiving and retransmitting stations. 5 Satellite Television and radio Television became the main market, its demand for simultaneous delivery of relatively few signals of large bandwidth to many receivers being a more precise match for the capabilities of geosynchronous co