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1、Technical English,For Information Science and Electronic Engineering,Unit 7,Satellite Communications,Part I,Applications of Communications satellite,3,New Words,4,1,A communications satellite (sometimes abbreviated to comsat) is an artificial satellite stationed in space for the purposes of telecomm
2、unications. Modern communications satellites use a variety of orbits including geostationary orbits, elliptical orbits and low (polar and non-polar) Earth orbits.,停留在太空,现代通信卫星使用多种轨道，包括对地静止轨道、椭圆形轨道、低地球轨道（过极地和不过极地）。,5,2,For fixed (point-to-point) services, communications satellites provide a technolog
3、y 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 o
4、r impossible.1,它们也可用于如船，车辆，飞机和手持终端的移动通信，用于电视和无线电广播，以及应用其它技术，比如电缆，是不现实或不可能的地方。,6,3 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
5、 a geostationary satellite.,固定电话将通话传输到地面站，在那里再发射到对地静止卫星上,7,3,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 p
6、ointing in the presence of disturbances, such as waves onboard a ship.2,相反，移动电话（船和飞机上接收和发送的）必须直接连到设备把信号上传到卫星，又能确保在有干扰时的卫星指向，如船上的电波干扰。,8,4,Improvements in submarine communications cables caused a decline in the use of satellites for fixed telephony in the late 20th century. Hand held telephones (cell
7、ular 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.,海底通信电缆的改进导致了20世纪后期用于固定电话的卫星应用有所下降。,接入地面上用于接收和发射的星罗棋布的基站群,9,5 Satellite Television and radio,Television became the main mark
8、et, 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 comsats.3 Two satellite types are used for North American television and radio: Direct Broadcast Satellite (DBS), and Fixed Servic
9、e Satellite (FSS).,电视成为主要市场，其对相对很少的大带宽信号同时传送给很多接收机的要求更好地匹配了对地静止通信卫星的性能。,10,6,A direct broadcast satellite is a communications satellite that transmits to small DBS satellite dishes (usually 18 to 24 inches in diameter). Direct broadcast satellites generally operate in the upper portion of the microw
10、ave Ku band. DBS technology is used for DTH-oriented (Direct-To-Home) satellite TV services.,直播卫星是一种通信卫星，它直接向小型DBS卫星天线发射。,11,7,Fixed Service Satellites (FSS) use the C band, and the lower portions of the Ku bands. They are normally used for broadcast feeds to and from television networks and local a
11、ffiliate stations, as well as being used for distance learning by schools and universities, business television, videoconferencing, and general commercial telecommunications.4 FSS satellites are also used to distribute national cable channels to the central control units.,它们通常用于向电视网和加盟地方台馈送或收集广播节目，也
12、用于中小学和大学的远程教学、商业电视、视频会议和一般商用通信。,12,8,FSS satellites differ from DBS satellites in that they have a lower RF power output than the latter, requiring a much larger dish for reception (3 to 8 feet in diameter for Ku band, and 12 feet on up for C band), as well as using linear polarization for each of t
13、he transponders RF input and output (as opposed to circular polarization used by DBS satellites).5,FSS与DBS的不同之处在于固定业务卫星具有更低的射频功率输出，需要更大的碟形卫星接收天线，收发器的射频输入和输出也都采用线极化。,13,8,FSS satellite technology was also originally used for DTH satellite TV from the late 1970s to the early 1990s in the United States
14、 in the form of TVRO (TeleVision Receive Only) receivers and dishes.,14,9,This all changed when the first American DBS provider, DirecTV, was established in 1994, stealing the limelight from FSS satellite technology for DTH programming. However, FSS satellites on the C and Ku bands are still used by
15、 cable and satellite channels such as CNN, The Weather Channel, HBO and others, for distribution to cable TV headends, and to the DBS providers themselves who then re-distribute these channels over their own DBS systems.,夺去了FSS卫星技术在DTH节目方面的风头,15,10,Free-to-air satellite TV channels are also usually
16、distributed on FSS satellites in the Ku band. The Intelsat Americas 5, Galaxy 10R and AMC 3 satellites over North America provide a quite large amount of FTA channels on their Ku band transponders.,在他们的Ku波段收发器上提供相当多的FTA频道。,16,11,The American Dish Network DBS service has also used FSS technology due to their need for more capacity to carry local television stations, and for more bandwidth to carry HDTV channels. Satellites for communication have now been launched that have transponders in the Ka
