Satellite Basics

Satellite Basics ( 卫星基本知识 )

http://www.marinesatellitesystems.com/index.php?page_id=98

22,300 A Space Odyssey - The Geostationary Clarke Orbit ( 22,300太空漫游-对地静止克拉克轨道 )

In 1945 the renowned science fiction author of "2001 A Space Odyssey", Arthur C Clarke,  proposed that communications satellites placed in orbit at a height of 22300 miles above the earth, would remain in synchronous orbit with the earth, or parked in the same spot above the equator, allowing earth based stations to communicate around the world via satellite. This magic distance causes the satellite to orbit the earth at the same speed as the earths rotation. Satellites that are closer to the earth must orbit faster than the earth to maintain orbit, and satellites that are further from earth orbit  slower than the rotation of the earth.

( 1945年，《 2001年太空漫游》的著名科幻小说作者亚瑟·克拉克（Arthur C Clarke）提出，放置在距地球22300英里高的轨道上的通信卫星应与地球保持同步轨道，或停放在同一轨道上 位于赤道上方的位置，使地球站可以通过卫星在世界范围内通信。 这个神奇的距离使卫星以与地球自转相同的速度绕地球旋转。 距地球较近的卫星必须比地球快运行以维持轨道，而距地球较远的卫星则要慢于地球自转。 )


While Arthur C Clarke may not have been responsible for the science behind the orbit, he was certainly influential with the use of this orbit over the past 65 years for satellite communications and TV distribution as we know it today.

( 尽管亚瑟·克拉克（Arthur C Clarke）可能没有对轨道背后的科学负责，但他在过去65年中对于今天我们所知的卫星通信和电视发行的使用无疑具有影响力。 ) 

 

Some Satellite History ( 卫星历史 )

On August 31st, 1962, President John F Kennedy signed into effect the Communications Satellite Act that began the competitive commercial use of satellites and establishing a communications system utilizing space satellites, and contributing to world peace and understanding.
( 1962年8月31日，总统约翰·肯尼迪（John F Kennedy）签署了《通信卫星法》，该法开始了对卫星的商业竞争性使用，并建立了利用太空卫星的通信系统，并为世界和平与谅解做出了贡献。 )
In 1967 President Johnson reported that the Act has brought mankind to the threshold of a full-time global communications service to which all nations of the world may have equal access. Comsat had been joined by 17 other nations for the creation of INTELSAT, (International Telecommunications Satellite Consortium).

( 1967年，约翰逊总统（Johnson总统）报道说，该法案使人类达到了专职的全球通信服务的门槛，世界上所有国家都可以平等地使用该服务。 Comsat与其他17个国家一起创建了INTELSAT（国际电信卫星联盟）。 )

 

GEO - Geostationary Earth Orbit ( GEO-对地静止地球轨道 )

In order to order to overcome Earth's gravity, and remain stationary in the sky, synchronized with the rotation of the earth, geostationary satellites must maintain an orbit  of 22,300 miles (37,000Km) above the equator, spaced just 2 degrees apart, and  often even less. This results in the satellite orbiting the earth at exactly the same rate that the earth revolves, thus appearing to the observer to be stationary in the sky.
( 为了克服地球的重力并保持静止不动，并与地球自转同步，对地静止卫星必须在赤道上方保持22,300英里（37,000Km）的轨道，相距仅2度，甚至减。这导致卫星以与地球公转的速率完全相同的速度绕地球公转，因此对观察者来说似乎是静止在天空中的。 )
2 Degree Spacing: ( 2度间距： )
Two degrees may seem like a small separation but they are actually about 700 miles apart. Each satellite is traveling at about 7000 miles per hour to complete a journey of more than 165 000 around the earth each day. The earth is rotating at the same rate, so the satellite appears to be stationary and we can point our antennas at the same spot in the sky.
( 两度似乎很小的距离，但实际上相距约700英里。每颗卫星每小时以大约7000英里的速度运行，每天完成围绕地球的超过165 000的旅程。地球以相同的速度旋转，因此卫星似乎静止不动，我们可以将天线指向天空中的同一位置。 )
Because of this vast distance, large, directional,dish reflector antennas are required to communicate with geostationary satellites. In the marine world, these antennas must be stabilized to point precisely at the correct satellite, as the ship moves and turns below.
( 由于距离如此之长，因此需要大型，定向，碟形反射器天线才能与对地静止卫星通信。在海洋世界中，当船在下方移动和转弯时，必须稳定这些天线，以精确地指向正确的卫星。 )
In reality, the orbits are  elliptical not circular, so the satellite is actually traveling in an ellipse, further away from the earth in apogee and closer to the earth in perigee.
( 实际上，轨道是椭圆形的而不是圆形的，因此卫星实际上是椭圆形的，在远地点远离地球，在近地点更接近地球。 )
There is also a significant time delay of about half a second  for the signal to travel from earth to the satellite and back.
( 信号从地球传播到卫星再返回的过程也存在大约半秒的显着时间延迟。 )

 

Inclined Orbits ( 倾斜轨道 )

Like a spinning top, the satellite orbits tend to wobble a bit and it is necessary for the satellite operators to periodically command the satellite to fire rocket thrusters to correct the orbit, both in a east -west direction as well as the north south direction in order to maintain the correct position in the sky.

( 就像旋转的陀螺一样，卫星轨道往往会有些摇摆，卫星运营商有必要定期命令卫星发射火箭推进器以校正该轨道的东西向和南北向。为了在天空中保持正确的位置。 )

As the satellite ages over 15 or 20 years and begins to run out of fuel, while still fully functional electronically,  the satellite operator can let the satellite go into an inclined orbit, where they use the remaining fuel to keep the satellite in the correct orbital longitude, while letting the orbit drift north and south. The orbit remains consistent but since the Earth is rotating the satellite appears to move north and south in a figure eight pattern in the sky.
( 随着卫星的寿命超过15或20年，并开始用尽燃料，同时仍具有电子功能，卫星运营商可以让卫星进入倾斜轨道，利用剩余的燃料将卫星保持在正确的轨道上经度，同时让轨道向北和向南漂移。轨道保持一致，但是由于地球在旋转，卫星似乎以天空中的八字形向南和向北移动。 )
This extends the usefulness of the satellite beyond it's scheduled end of life, but the antenna equipment on the ground needs to have the ability to target and search for the satellite that appears from Earth to be moving up and down in a figure 8 pattern. This is not very practical for antennas that are bolted down on land, but can be used very economically for marine antennas that have inherent tracking and searching capabilities.
( 这将使卫星的实用性超出其预定的使用寿命，但是地面上的天线设备需要具有瞄准和搜索从地球出现的，以图8形式上下移动的卫星的能力。对于用螺栓固定在陆地上的天线，这不是很实际，但是对于具有固有跟踪和搜索功能的船用天线，可以非常经济地使用。 )
As the satellite arc fills up with new satellites, satellite slot positions are becoming more and more of a premium, so these days there is a greater tendency to replace aging satellites with the latest and greatest, rather than trying to squeeze the value out of the older equipment.
( 随着新卫星的不断涌现，卫星插槽的位置正变得越来越重要，因此，如今，越来越有可能用最新的和最大的卫星来替代老化的卫星，而不是试图从中挤出价值。旧设备。 )
Inclined satellites can be of value to users in the polar regions for a few hours each orbit, as the satellite dips toward the pole increasing coverage. 

( 倾斜的卫星对每个极地地区的用户来说都是有价值的，因为它会向极地倾斜，从而增加覆盖范围，因此每个轨道运行几个小时。 ) 

 

LEO: Low Earth Orbit Satellites ( LEO：低地球轨道卫星 )

In order for Low Earth Orbit satellites to remain only a few hundred miles above the surface of the earth, it is necessary for them to move much faster with reference to the earth.
( 为了使“低地球轨道”卫星保持在距地球表面仅几百英里的位置，有必要使它们相对于地球运动得更快。 )
Lower orbits appear to move faster than the rotation of the earth, and higher orbits slower. 
( 较低的轨道似乎比地球自转快，而较高的轨道则慢。 )
Low Earth Orbit (LEO) satellites, like the Iridium fleet, are only a few hundred miles above the surface of the earth, allowing small, handheld terminals (like overgrown cell phones) with omnidirectional antennas to be used. To maintain this low orbit, the satellites are constantly moving, rapidly around the earth.
( 像铱星舰队一样，低地球轨道（LEO）卫星仅在地球表面上方几百英里处，因此可以使用带有全向天线的小型手持终端（如杂草丛生的手机）。为了维持这一低轨道，卫星不断在地球周围快速移动。 )

The disadvantage of LEO satellites is that they are not parked in one spot, relative to the earth, and are actually rotating rapidly around the earth. LEO satellites are rising and setting and zipping across the sky, so to provide uninterrupted service, you need to be able to see more than one satellite at any given time, with the capability to hand off the call from one setting satellite to a new one that might be rising.
( LEO卫星的缺点是它们没有停在相对于地球的一个地点，而是实际上绕地球快速旋转。 LEO卫星正在上升，并在天空中设置和滑动，因此，要提供不间断的服务，您需要在任何给定时间都能看到多于一颗卫星，并且能够将呼叫从一颗设置中的卫星切换到新的一颗可能会上升。 )
Iridium and Globalstar are two of the major players that have pursued this complex technology, not without many commercial and technical problems.
( 铱和Globalstar是追求这一复杂技术的两个主要参与者，并非没有很多商业和技术问题。 )
More about LEO systems later.

( 稍后更多有关LEO系统的信息。 )

 

MEO: Medium Earth Orbit ( MEO：中地球轨道 )

 Medium Earth Orbit satellites, like the proposed O3B Network have satellites in a circular orbit about 5000 miles above the equator.

( 像拟议的O3B网络一样，中地球轨道卫星的卫星也位于赤道上方约5000英里的圆形轨道上。 )

The O3B Network uses 8 satellites with 10 steerable spot beams that light up certain areas on the surface of the earth as the satellite revolves around the earth.

( O3B网络使用8颗具有10个可控点波束的卫星，当卫星绕地球旋转时，它们会照亮地球表面上的某些区域。 )

 

Global beams, Hemi beams, Spot beams ( 整体光束，半光束，点光束 )

 

Finding Satellite information on the internet ( 在互联网上查找卫星信息 )

tracksat.com

Satsig.com

Lyngsat.com

Lyngsat maps.com

Satbeams.com

http://en.kingofsat.net/search.php

 

Antenna gain dBi ( 天线增益dBi )

The gain of a directional antenna is compared to an omni-directional (or isotropic) antenna.
 ( 将定向天线的增益与全向（或各向同性）天线进行比较。 )
The isotropic antenna radiates equally in all directions and has a gain of 1 (0dB). The directional antenna focuses the energy in one direction, thus having a gain  which is expressed as a logarithmic ratio compared to the isotropic antenna as dBi.

( 各向同性天线在所有方向上均等地辐射，增益为1（0dB）。 定向天线将能量集中在一个方向上，因此具有增益，与各向同性天线相比，增益以对数比表示为dBi。 )



Note that the above diagram should be imagined in 3D, with a spherical isotropic radiation and conical directional pattern.
( 请注意，以上图表应以3D形式想象，并具有球形各向同性辐射和圆锥形方向性图案。 )
The isotropic antenna is a theoretical antenna (like a ball) , radiating in a perfect sphere in all directions. This is different to a real world omni-directional antenna that radiates in a donut shape with the radiating element at the center.
( 各向同性天线是一种理论上的天线（如球），在一个完美的球体中向各个方向辐射。 这与真实的全向天线不同，该天线以甜甜圈形状辐射，辐射元件位于中心。 )
A typical 1 meter parabolic antenna has a gain of about 40 dBi at Ku-Band  frequencies.

( 典型的1米抛物面天线在Ku频带频率下的增益约为40 dBi。 )

 

EIRP or E.I.R.P. ( EIRP(Effective Isotropic Radiated Power) 有效全向辐射功率 )

Effective Isotropic Radiated Power which is the effective power radiated  from a directional antenna compared to a theoretical isotropical (omnidirectional) antenna.
( 有效各向同性辐射功率，即与理论上的各向同性（全向）天线相比，从定向天线辐射的有效功率。 )
This includes the power of the transmitter and the antenna gain and it is expressed in dBW.

( 这包括发射机的功率和天线增益，以dBW表示。 )

 

EIRP or dBW on a satellite footprint map. ( 卫星足迹图上的EIRP或dBW。 )

The dBW EIRP lines that you see on a satellite coverage footprint map indicate the downlink signal strength that you can expect to receive at that spot on the map.
( 您在卫星覆盖区覆盖图上看到的dBW EIRP线表示可以期望在该图上的该点接收到的下行链路信号强度。 )

It is a bit confusing to talk about power ( EIRP = Effective Isotropic Radiated Power) when talking about the downlink receive signal, but it is referring to the effective power radiated from the satellite toward that point on the earth.
( 在谈论下行链路接收信号时，谈论功率（EIRP =有效各向同性辐射功率）有点令人困惑，但它指的是从卫星向地球上那个点辐射的有效功率。 )
The required uplink power needed toward the satellite (the receive characteristics of the satellite) is expressed as G/T in dB/K on a different type of map.
( 指向卫星所需的所需上行链路功率（卫星的接收特性）在不同类型的地图上表示为以dB/K为单位的G/T。 )
The amount of EIRP will determine the minimum size of antenna you need to receive the signal.
( EIRP的数值将决定您需要接收信号的最小天线尺寸。 )
Very basically, a 1 meter antenna will need an minimum EIRP of about 42 dBW and a 45cm antenna would need a minimum EIRP of 49 dBW.
( 基本上，一米长的天线将需要至少约42 dBW的EIRP，而45厘米长的天线将需要至少49 dBW的EIRP )

1.5m(60")	1.2m(50")	1m(40")	76cm(30")	60cm(24")	45cm(18")	36cm(14")	33cm(13")	28cm(11")
40 dBW	41 dBW	42 dBW	43 dBW	45 dBW	48 dBW	50 dBW	51 dBW	53 dBW

G/T Satellite Uplink Map ( G/T卫星上行链路地图  )

The uplink power that you need to transmit toward the satellite is expressed as G/T in dB/K on a different type of map. These maps actually show the receive characteristics of the satellite, but from this you can calculate the EIRP needed to satisfy the link.
( 您需要向卫星传输的上行链路功率在另一种类型的地图上以G/T（dB/K）表示。 这些图实际上显示了卫星的接收特性，但是从中可以计算出满足链路所需的EIRP。 )
It is important to remember that the transmit pattern might be different from the receive pattern. The receive characteristics of the satellite are shown as G/T

( 重要的是要记住，传输模式可能与接收模式不同。卫星的接收特性用G/T表示 )

 

Ka-Band Pointing Accuracy ( Ka波段指向精度 )

BEAMWIDTH AND POINTING ACCURACY ( 亮度和点精度 )
I read this somewhere on the web so don't quote me: ( 我在网络上的某处阅读了此内容，所以不要引用我的话： )

Bearing in mind that 3dBs is half the signal strength, at C-band the signal typically drops 3dBs at about 0.75 degrees off the satellite, at Ku band it drops 3dBs at about 0.65 degrees off, but at Ka-band the 3dB  point is as close as 0.4 degrees of the satellite. This means that Ka Band maritime terminals need to point significantly closer in accuracy than C or Ku terminals.
( 记住，3dB是信号强度的一半，在C波段，信号通常在离卫星0.75度处下降3分贝，在Ku波段，它在大约0.65度处下降3分贝，但在Ka波段，3dB点与卫星的0.4度接近。这意味着Ka波段的海上终端需要比C或Ku终端更精确。 )

 

ESV- Earth Station aboard Vessel ( ESV-船上地球站 )

A FCC term  used in regulations, describing VSAT terminals aboard vessels
( 法规中使用的FCC术语，描述了船上的VSAT终端 )
http://www.satellitetoday.com/enterprise/maritime/FCC-Jurisdiction-for-Maritime-Communications_34706.html
http://www.gvf.org/docs/broadband2010/Day_1_1450-1600_Robert_Hanson.pdf

 

Comsat Mobile Communications ( Comsat移动通信 )

 

Inmarsat ( 海事卫星组织 )

 

MEO - Medium Earth Orbit ( MEO-中地球轨道 )

Medium Earth Orbit ( 中地球轨道 )

 

Satellite Latency ( 卫星延迟 )

The time delay between data leaving the satellite and reaching the ground network and vice versa.
( 数据离开卫星与到达地面网络之间的时间延迟，反之亦然。 )
Because the satellite is parked 22, 300 miles above the earth, there is a physical time delay for the signal to travel to and from the satellite. This is a fixed quantity, due to the speed of light, that cannot be changed  (unless Einstein was wrong:-).
( 由于卫星停在距离地球300英里的22号处，因此信号往返于卫星的传播存在物理时间延迟。由于光速，这是一个固定量，无法更改（除非爱因斯坦错了：-）。 )
Satellite latency is about 240 ms each way, or a total of almost 500ms. Combine that with the terrestrial network latencies and your typical round trip ping times to the satellite are about 720ms. If the satellite hub is located across the ocean from the destination you are pinging, you can expect as much as 1000ms round trip delay. This is due to the added latency of the undersea fiber cable.
( 每种方式的卫星等待时间约为240毫秒，或总计近500毫秒。将其与地面网络延迟相结合，到卫星的典型往返ping时间约为720ms。如果卫星集线器位于您要ping通的目的地的大洋彼岸，则可以预期多达1000ms的往返延迟。这是由于海底光缆增加了等待时间。 )
Latency can effect the data throughput on TCP/IP traffic because each packet sent needs to wait for an acknowledgement before sending the next. Several spoofing techniques are used to fool the TCP/IP into thinking it received the acknowledgement so that it sends the next packet without waiting. This can speed things up a bit, but you can never overcome the physical delay between the ground and the satellite.
( 延迟会影响TCP / IP流量的数据吞吐量，因为发送的每个数据包都需要先等待确认，然后才能发送下一个。几种欺骗技术被用来欺骗TCP / IP，使其认为已接收到确认，因此它无需等待即可发送下一个数据包。这样可以加快速度，但是您永远无法克服地面与卫星之间的物理延迟。 )
On voice calls, you will sometimes have to wait a second after the other person finishes speaking, to avoid speaking over each other.
( 在语音通话中，有时您需要在对方完成发言后等待一秒钟，以免彼此交谈。 )
There is no way around this satellite delay when using GEO satellites, and don't let any sales folk tell you there is. Low Earth Orbit satellites like Iridium and Globalstar have significantly less latency as they are just a few hundred miles above the Earth.

( 使用GEO卫星时，无法避免这种卫星延迟，也不要让任何销售人员告诉您。铱星和全球之星等低地球轨道卫星的等待时间大大缩短，因为它们仅距地球几百英里。 ) 

 

Low Earth Orbit LEO ( 低地球轨道LEO )

 

Medium Earth Orbit MEO ( 中地球轨道MEO )

 

Symbol Rate ( 符号率 )

The symbol rate = Data Rate x Inverse of FEC x Modulation index.

( 符号率=数据率x FEC的倒数x调制指数。 )

The Modulation index: QPSK=0.5 , 8PSK = 0.333, 16QAM=0.25
( 调制指数：QPSK = 0.5，8PSK = 0.333，16QAM = 0.25 )
Let's take 41250 kbps with 3/4 rate FEC and QPSK as an example:

( 让我们以3/4速率FEC和QPSK的41250 kbps为例： )

41250 x 4/3 x 0.5 = 27500 ksps (kilo symbols per second)

( 41250 x 4/3 x 0.5 = 27500 ksps（每秒千比特） )

 

SCPC - Single Carrier Per Channel ( SCPC-每个频道一个载波 )

SCPC - Dedicated, One-on-one satellite circuits. ( SCPC-专用的一对一卫星电路。 )

Single Carrier Per Channel circuits provide a dedicated channel between ship and shore wherein the full bandwidth is always on and available to you alone, whether you are using it or not. Satellite space segment is pricey by any standards, due to the extreme cost of developing, building, deploying, and operating a satellite in space. Therefore, you would need good reasons to justify a dedicated service just for you, unless you were a very heavy, around the clock, bandwidth user with an unlimited budget. There are several variations of SCPC where a shipping company might secure dedicated bandwidth and then share the bandwidth, using various technolgies, across their fleet of ships.

( 每通道单载波电路提供了船与岸之间的专用信道，无论您是否使用它，总带宽始终处于打开状态，并且仅对您可用。 由于在太空中开发，建造，部署和运营卫星的极高成本，按任何标准衡量，卫星太空市场的价格都是昂贵的。 因此，您将有充分的理由为您量身定制专用服务，除非您是一个非常忙碌的全天候带宽无限预算的用户。 SCPC有多种变体，船运公司可以确保专用带宽，然后在其船队中使用各种技术共享带宽。 )

 

TDM -Time Division Multiplexing ( TDM-时分复用 )

Not to be confused with TDMA, Time-division multiplexing  is a type of digital multiplexing in which two or more signals are transferred  simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel.
( 不要与TDMA混淆，时分复用是一种数字复用，其中两个或多个信号作为一个通信通道中的子通道同时传输，但实际上是在通道上轮流进行。 )
The time domain is divided into recurring timeslots, one for each sub-channel. A  byte of data block of channel 1 is transmitted during timeslot 1, channel 2 during timeslot 2, etc.
( 时域分为循环时隙，每个子信道一个。 通道1的一个数据块的字节在时隙1期间发送，通道2在时隙2的期间发送等等。 )
TDM is Multiplexing technique where each channel is assigned a timeslot in a frame whether they use it or not, and TDMA is multiple access technology when several channels are randomly assigned timeslots on demand, and when needed.

( TDM是一种多路复用技术，其中为每个通道分配是否在一个帧中使用一个时隙，而TDMA是一种多路访问技术，当几个通道根据需要以及在需要时被随机分配时，TDMA是一种多址技术。 )

 

TDMA - Time Division Multiple Access ( TDMA-时分多址 )

TDMA - Time Division Multiple Access: TDMA is the common form of securely sharing bandwidth, where each second or millisecond is sliced up into microseconds and shared between several users. This is timeshare in the sky except that you are not buying a week or two per year, but rather a few milliseconds every second. While you are downloading your Internet, or speaking on the phone, you don't even realize that there are several other users doing the same thing on the same satellite link. When there are fewer people using the link, there are more timeslots and more bandwidth available for you, and when there are more people, you will have a bit less.
( TDMA-时分多址：TDMA是安全共享带宽的常见形式，其中每秒或毫秒被分成几微秒，并在多个用户之间共享。 这是天空中的分时度假，除了您不是每年购买一两个星期，而是每秒购买几毫秒。 当您下载Internet或通过电话讲话时，您甚至没有意识到还有其他几个用户在同一卫星链接上执行相同的操作。 当使用该链接的人数减少时，将为您提供更多的时隙和更多的带宽，而当人数更多时，您的资源就会减少一些。 )

 

Downlink ( 下行链接 )

Link from the satellite to the ground station

( 从卫星到地面站的链接 )

 

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