Who am
I? 卫星通信技术/Satcom {卫星通信基础/Satcom ABC 通信卫星资源/Comsat resource 卫星通信网/Network 链路设计/Link
budget 建站开通/Installation 干扰与异常/Interference 降雨衰耗/Rain
atten 工作频段/Freq bands 轨位协调/Coordination 中国广播卫星/China DBS 常用数据表/Tables 友情参与/Visitor's
area}
技术文章/Articles 科技译文/Translations 其他作品/Others
生命的延续/My family
GSO satellites are locating and operating at geostationary orbit. Geostationary orbit is a circular one, with its revolution period of 24 hours. A GSO satellite appears stationary above the earth, because of its running direction and orbital velocity is as same as the earth’s rotational direction and speed respectively. Therefore, an earth station could point to the satellite at fixed elevation and azimuth angles.
由静止卫星的轨道位置示意图可见，地球的平均半径约为6378公里，静止卫星的轨道高度约为35786公里，轨道半径约为42,164公里。从静止卫星看地球，地球的角直径约为17.4度。
As the sketch map of geostationary orbit shown, the earth’s mean radius is about 6378 km, while the altitude and orbit radius of a GSO satellite is approximately equal to 35786 km and 42164 km respectively. Looking from a GSO satellite, the earth’s diameter is about 17.4º in angle.
Adopting EquiRectengular projection method, the sketch map of the coverage of a GSO satellite located in 105.5ºE longitude presents three green lines denoting service areas of elevation 0º, 15º and 30º respectively. The service area of 0º elevation indicates that a GSO satellite could only cover the area between 80ºN and 80ºS latitude, while higher latitude areas near north pole and south pole would be out of the service. On the other hand, the service area of 15º elevation could cover the area about 140 degrees in longitude, it shows that only three GSO satellites with 120ºorbital space one another could cover most of the earth surface where latitude not higher than 70º.
英国的空军军官Arthur C Clarke于1945年著文提出利用静止卫星提供全球微波通信的设想。
In 1945, Arthur C. Clarke, a British air force officer, conceived the idea to effect global microwave communication by GSO satellites.
The satcom development steps are as follows: The earliest space communication was experienced in mid 1950s, when some balloon-satellites were used as passive reflectors. Around 1960, store and then transit tests were held on low earth orbit (LEO) satellites. In early 1960s, people could communicate across the ocean by mid or high earth orbit (MEO or HEO) satellites. In 1964, GSO satellite was used for live broadcast for Tokyo Olympic Game. Intelsat-1 was launched in 1965 as the first commercial communication satellite, and Soviet Union began its satcom service as well in the same year.
目前，已有第9代Intelsat卫星在轨提供服务。卫星通信成为经济上最为成功的空间技术。仅在东亚和东南亚就有中国、日本、韩国、印度尼西亚、泰国、马来西亚、新加坡、菲律宾等国拥有在轨的静止通信卫星。我国现有5家通信卫星公司，拥有10颗在轨通信卫星，总通信容量约为370个（36MHz）标准转发器。
Nowadays, the 9th generation Intelsat satellites are providing service in orbit, and satcom has become most successful space technology in economy. Only in east and south-east Asia regions, there are 8 countries including China, Japan, Korea, Indonesia, Thailand, Malaysia, Singapore and Philippines which possess communication satellites. And China has now 5 satcom companies with 10 communication satellites in orbit, and the total communication capacity is approximately 370 standard transponders (36MHz bandwidth per transponder).
Satcom is one kind of microwave communication by utilizing communication satellite as a relay station and it has the following properties.
Wide coverage: cost independent of distance, well suitable for broadcasting, easier network expansion and quicker remote installation, but difficult in finding interfering resources
Long distance: long time delay and high loss in propagation
High frequency: wider bandwidth and larger capacity, but higher rain attenuation in some frequency bands
High cost: more expensive in installation and operation which including earth station equipments and antennas, the network management system, installation expense, and the rent for transponder resource
卫星通信的分类 / Classifications for satcom
According to services types:
FSS, Fixed Satellite Service, satcom service for fixed earth stations
BSS, Broadcasting Satellite Service, broadcasting service for audio, video and data
MSS, Mobile Satellite Service, communication service for mobile users
According to operational frequency bands:
L-band, 1-2GHz, for mobile communication and audio broadcastingr
S-band, 2-3GHz, for mobile communication and video broadcasting
C-band, 4-6GHz, for fixed communication and audio broadcasting
X-band, 7-8GHz, for fixed communication (especially for government and military service)
Ku-band, 10-14GHz, for fixed communication and broadcasting
Ka-band, 17-31GHz, for fixed and mobile communication
According to orbital altitude:
LEO, Low Earth Orbit, lower than 5000 km
MEO, Medium Earth Orbit, between 5000 and 20000 km
HEO, High Earth Orbit, higher than 20000 km
According to orbital types:
Type: circle orbit and ellipse orbit
Inclined angle: equator orbit, inclined orbit and polar orbit
According to transponder types:
Transparent channel or bend pipe: frequency conversion and carrier transmission, without demodulate and other process
On-broad processing: including carrier demodulation and switching, even beam exchange
Store and transit: normally for LEO mini-satellite, the received signals will be stored temporarily, and then transmitted back to the earth at desired time and location
由通信卫星的频率极化计划图可见，通信卫星的整个工作频段通常被分为多个子频段。每个子频段都由一套滤波、变频和放大电路构成独立的传输通道，相关的电路设备被称为通信转发器。
透明信道方式的通信转发器只对信号作滤波、变频和放大处理：（接收天线定向接收上行信号，低噪声放大器对上行信号进行预放大，）输入带通滤波器选择上行信号中的相关频率分量，混频器对信号作上行/下行频率转换，信道放大器用于调整转发器的增益，功率放大器对输出信号作功率放大，输出带通滤波器限制带外噪声对相邻转发器的影响，（发送天线定向发送下行信号。）
返回页首卫星通信所用的放大倍数和传输损耗等的数值都很大，不便于用真数表示和比较。如果用以10为底的对数，即贝尔（Bel）表示时，又因单位过大而不很方便。常用单位为分贝（dB，decibel），即贝尔的1/10。采用对数后，还可以将乘除运算简化为加减运算。
卫星通信采用定向天线聚集信号能量，克服超长距离传输带来的极大损耗。卫星通信地球站常用抛物面反射天线。通信广播卫星多采用抛物面结构的波束赋型天线。
卫星转发器的三个主要参数为G/T、SFD与EIRP。G/T和SFD反映卫星接收系统在其服务区内的性能，它们与卫星接收天线的增益分布线性相关。EIRP反映转发器的下行功率，它与卫星发送天线的增益分布线性相关。
卫星天线增益随天线指向与工作频率而变。因此，转发器参数随服务区内的不同地点而变，同一地点的不同转发器参数也有差异。特定地点的转发器参数可从城市参数列表或等值线分布图中查到 。
G/T为接收系统的品质因数（figure of merit）。它是接收天线增益G与接收系统噪声温度T之比值，单位为dB/k。G/T的计算公式为
饱和通量密度SFD为，当转发器被推到饱和工作点时，上行载波在接收天线口面所达到的通量密度。它反映卫星转发器对上行功率的需求量，单位为dBW/m2。SFD与G/T的关系为
由对比同一颗通信卫星的C频段EIRP分布图和Ku频段EIRP分布图可知，C频段转发器的服务区大，通常覆盖几乎所有的可见陆地，适用于远距离的国际或洲际业务；Ku频段转发器的服务区小，通常只覆盖一个大国或数个小国，只适用于国内业务。C频段转发器的EIRP通常为36到42dBW，G/T通常为-5到+1dB/k，地面天线的口径一般不小于1.8米；Ku频段转发器的EIRP通常为44到56dBW，G/T通常为-2到+8dB/k，地面天线口径有可能小于1米。另一方面，C频段因为电波传播通常不受气候条件的影响，适用于可靠性较高的业务；Ku频段转发器则因电波传播可能遭受降雨衰耗的影响，只适用于建网条件较差、天线尺寸和成本受限的业务。
卫星转发器通常采用行波管功率放大器。行波管放大器是一种非线性放大器，放大器输入功率与输出功率的关系可由I/O关系曲线表示。图中的纵坐标和横坐标分别为放大器的输出功率和输入功率。曲线的顶点对应于放大器的饱和输出功率。曲线从左至右可被大致分为三个区。左侧为线性区，输出功率和输入功率呈线性关系，最高点为放大器的线性工作点。线性工作点和饱和点之间为非线性区，输出功率的增幅低于输入功率的增幅。饱和点的右侧为过饱和区，输出功率将随输入功率的增大而下降。
饱和输出功率与曲线上某个点的输出功率之差值为该功率点的输出回退值（OBO，Output Back-off），饱和输入功率与某个实际输入功率的差值为该功率点的输入回退值（IBO，Input Back-off）。I/O关系曲线以饱和功率，即曲线的顶点所对应的最大功率为参考点。饱和功率点的输出回退值和输入回退值均为0。
转发器在多载波工作时，将产生互调分量，降低工作性能。为了避免互调干扰，所有载波的总功率应该不超过转发器的线性功率，以使转发器工作在线性条件下。转发器线性工作点的OBO和IBO分别为转发器的线性OBO和线性IBO。
放大器的线性工作点越接近于饱和点，多载波条件下的最大输出功率就越高。采用行波管放大器的转发器线性OBO通常为4.5dB。部分加装线性器的转发器，可以提高多载波条件下的转发器总输出功率，其线性OBO通常为3dB。
卫星通信ABC系列：E1-卫星通信概述，E2-通信卫星介绍，E3-卫星操作者与技术管理，E4-卫星操作者的通信业务
The Innovation of Satcom Industry in China
Who am
I? 卫星通信技术/Satcom {卫星通信基础/Satcom ABC 通信卫星资源/Comsat resource 卫星通信网/Network 链路设计/Link
budget 建站开通/Installation 干扰与异常/Interference 降雨衰耗/Rain
atten 工作频段/Freq bands 轨位协调/Coordination 中国广播卫星/China DBS 常用数据表/Tables 友情参与/Visitor's
area}
技术文章/Articles 科技译文/Translations 其他作品/Others
生命的延续/My family
