Unit20Understanding the GPS（认识GPS）
What Is GPS?
The global Positioning System(GPS)operated by the U.S.Department OF Defense(DOD) is a satellite-based system that can be used to locate positions anywhere on the earth.（全球定位系统（GPS），由美国国防部【Department OF Defense】管理【operated运转、操作】，是一个基于人造卫星的系统，可以用来在全球任何地方定位）
GPS provides continuous(24hours/day),real-time,3-dimensional positioning,navigation and timing worldwide.（GPS提供全世界范围内的全天候，实时，三维坐标，导航和授时的功能。）
Any person with a GPS receiver can access the system,and it can be used for any application that requires location coordinates.（任何拥有一台GPS接收机的人都可以使用【access】这套系统，可以应用于任何需要位置坐标的工作中【be used for用来做】）The GPS system consists of three segments:⑴the space segment:the GPS satellites themselves,⑵the control segment,operated by the https://www.doczj.com/doc/5d2090948.html,itary,and⑶the user segment, which includes both military and civilian users and their GPS equipments.（GPS由【consist of由……组成】三个部分【segment部分、节】组成：⑴太空部分：GPS卫星自己，⑵地面控制部分，由美国军方【military原意军方的，这里词性转变了——军方n.】控制，⑶用户【user】部分，包括军方和民用【civilian民用；civil民用的—civil engineering】用户和他们的装备。）
Space Segment:The GPS Constellation
The first GPS satellite was launched by the U.S.Air Force in early1978.（第一颗GPS卫星由美国空军【Air Force】在1978年的早些时候发射的）
There are now24satellites orbiting the earth at an altitude of about10,900miles.（现在有24颗卫星在绕地球大约10900英里的高度运行）
The high altitude insures that the satellite orbits are stable,precise and predictable,and that the satellites’motion through space is not affected by atmospheric drag.（这么高的高度确保了卫星轨道是稳定、精确和可知的【predictable可预言的、可预知的】，并且确保了卫星穿过太空的运行不受到大气阻力【atmospheric drag】的影响）
There are four satellites in each of6orbital planes.（6个轨道平面【orbital plane】中每个有4颗卫星。）
Each plane is inclined55degrees relative to the equator,which means that satellites cross the equator tilted at a55degrees angle.（每个平面相对于【relative to】赤道面倾斜【incline】55度，这就意味着这些卫星以55度角穿过赤道）
The system is designed to maintain full operational capability even if two of the24 satellites fail.（这套系统被设计成即使24颗卫星的2颗故障也能维持【maintain】全工作能力【operational capability】）
These24satellites make up a full GPS constellation.（这24颗卫星组成了一个完整的GPS星座）
The GPS satellites are powered primarily by sun-seeking solar panels,with nicad batteries providing secondary power.（GPS卫星主要由太阳定向【sun-seeking】太阳能电池板【solar panels】提供动力【power供以动力】，镍镉蓄电池【nicad batteries】提供备用【secondary第二的、副的】动力）
Each GPS satellite has four atomic clocks on board,only one of which is in use at a time. These highly accurate atomic clocks enable GPS to provide the most accurate timing system that exists.（每一个GPS卫星有四个原子钟【on board在船上，这里就是在卫星上的意思】，某一时刻只有一台在使用。这些高精度原子钟能够使GPS提供最准确的时间系统【that exist持续、存在】）
Control Segment:U.S.DOD
The U.S.Department Of Defense maintains a master control station at Falcon Air Force Base in Colorado Springs.（美国国防部维持【maintain】了一个主控站，在科罗拉多斯普林斯【Springs】的佛肯【Falcon猎鹰】空军基地）
There are four other monitor stations located in Hawaii,Ascension Island,Diego Garcia and Kwajalein.（还有4个监控站座落在夏威夷、阿松森岛【大西洋】、迭哥加西亚岛【印度洋】、卡瓦加兰岛【北太平洋】）
The DOD stations measure the satellite orbits precisely.（DOD站【指5个监控站】精确测量卫星轨道）
Any discrepancies between predicted orbits and actual orbits are transmitted back to the satellites.（任何预报轨道和实际轨道之间的偏差【discrepancy差异】被传回卫星。）The satellites can then broadcast these corrections,along with the other position and timing data,so that a GPS receiver on the earth can precisely establish the location of each satellite it is tracking.（然后卫星可以广播这些改正值——连同【along with连同……一起】位置和时间数据一起，以便地球上的GPS接收机可以精确确定它所跟踪【track】的每个卫星的位置）
User Segment:Military and Civilian GPS Users（用户部分：军方和民用GPS用户）The https://www.doczj.com/doc/5d2090948.html,itary uses GPS for navigation,reconnaissance,and missile guidance systems.（美国军方把GPS用在导航、侦察和导弹制导【missile guidance】系统上）Civilian use of GPS developed at the same time as military uses were being established, and has expanded far beyond original expectations.（GPS的民用在军用建立起之时就发展起来了，并且已经发展的远远超过起初的期望）
There are civilian applications for GPS in almost every field,from surveying to transportation to natural resource management to agriculture.（GPS的民用用途几乎用在每个领域，从测量到交通到自然资源管理到农业）
Most civilian uses of GPS,however,fall into one of three categories:navigation, positioning and timing.（然而，GPS的大部分民用用途，分为【fall into】三种类型【category】：导航、定位和授时。）
How Does GPS Work?
A GPS receiver calculates its position by a technique called satellite ranging,which involves measuring the distance between the GPS receiver and the GPS satellites it is tracking.（GPS接收机利用一项被称为卫星测距【satellite ranging】的技术，计算它自己的位置，涉及到GPS接收机到它所跟踪的GPS卫星的距离测量）
The range(the range a receiver calculates is actually a pseudorange,or an estimate of range rather than a true range)or distance is measured as elapsed transit time.（距离（接收机测的距离实际上是伪距，或者说是距离估值，而不是真实距离）是用传播时间来测的【elapse流逝v.】【本句意思是距离是用测时间的方法得到的】）
The position of each satellite is known,and the satellites transmit their positions as part of the“messages”they send via radio waves.（每个卫星的位置已知，并且卫星将它们的位置作为它们用无线电波发出的“讯文【导航电文】”的一部分传送【transmit】出去）The GPS receiver on the ground is the unknown point,and must compute its position based on the information it receives from the satellites.（地面GPS接收机是未知点，必需基于它从卫星收到的这些信息计算它的位置。）
The first step in measuring the distance between the GPS receiver and a satellite requires measuring the time it takes for the signal to travel from the satellite to the receiver.（测量GPS接收机和卫星的第一步，要求【require】测出【它把信号来传播】从卫星到接收机的信号【signal】传播时间。）
Once the receiver knows how much time has elapsed,the travel time of the signal multiplies the speed of light(because the satellite signals travel at the speed of light,
approximately186,000miles per second)to compute the distance.（一旦接收机知道用【elapse流逝】了多少时间，用信号的传播时间乘【multiply】上光速（因为卫星信号以光速传播，大约【approximately】186,000英里每秒）来计算距离。）
Distance measurements to four satellites are required to compute a3-dimensional(latitude, longitude and altitude)position.（计算一个三维（纬度、精度和高程）位置需要【be required to被……需要】4颗卫星的距离值）
In order to measure the travel time of the satellite signal,the receiver has to know when the signal left the satellite and when the signal reached the receiver.（为了测出卫星信号的传播时间，接收机需要知道信号是什么时候离开卫星的和什么时候到达接收机的）Knowing when the signal reaches the receiver is easy,the GPS receiver just“checks”its internal clock when the signal arrives to see what time it is.（确定信号到达接收机的时间容易，GPS接收机只需在信号到达时“检查”自己内置的钟，看看是什么时间）But how does it“know”when the signal left the satellite?（但是怎样“知道”信号何时离开卫星的呢？）
All GPS receivers are synchronized with the satellites so they generate the same digital code at the same time.（所有GPS接收机与卫星是同步的【synchronize同步v.】，因此它们在同一时间产生同样的信号码）
When the GPS receiver receives a code from a satellite,it can look back in its memory bank and“remember”when it emitted the same code.（当GPS接收机收到一个从卫星来的码时，它可以在它的内存条【memory bank】中回忆【回溯】并“记起”它发出【emit 因为接收机不向卫星发射信号，因此这里应翻译为产生】的相同码）
This little“trick”allows the GPS receiver to determine when the signal left the satellite.（这个小小的“技巧”【trick窍门、恶作剧、诡计】使得GPS接收机能够确定信号是
何时离开卫星的）
Once the receiver has the distance measurements,it’s basically a problem of geometry.（一旦接收机测出了距离，基本上【basically】就只有几何上的问题了）
If it“knows”where the four satellites are,and how far it is from each satellite,it can compute its location through trilateration.Here’s an illustration of how it works.（如果知道了4颗卫星的位置，距每颗卫星的距离【it指接收机而不是卫星】，就可以通过三边测量来计算它的位置。这里有个例子【illustration例子】说明它如何工作的）The GPS receiver“locks on”to one satellite and calculates the range to be12,000miles.（GPS接收机“锁定”一颗卫星并计算出距离为12,000英里）
This fact helps narrow the receiver location down,but it only tells us that we are somewhere on a sphere which is centered on the satellite and has a12,000mile radius.（这把接收机的位置限定【narrow使……变窄，这里就是限制的意思】下来，但是它只能告诉我们在一个以这个卫星为中心，半径12,000英里的球面上）
Many of the locations on that sphere are not on earth,but out in space.（这个球面的许多位置不在地球上，而在太空中）
Now,consider that the receiver picks up a signal from a second satellite and calculates the range between the receiver and the satellite to be10,000miles.（现在，考虑这个接收机从第二颗卫星获得【pick up】一个信号，计算出接收机到这颗卫星的距离是10,000英里）
That means we are also somewhere on a sphere with a10,000mile radius with the second satellite at the center.（这就意味着我们也在一个以第二颗卫星为中心、半径10,000英里的球面上的某处【somewhere】）
We must,therefore,be somewhere where these two spheres intersect.（因此，我们肯定在
这两个球面相交【intersect】的某处。）
When the two spheres intersect,a circle is formed,so we must be somewhere on that circle.（当这两个球面相交时，一个圆圈形成了，因此我们肯定在这个圆圈的某处）
If the receiver picks up another satellite,says at11,000miles away,another sphere is formed,and there are only two points where the three spheres intersect.（如果接收机“拾取”了另一个卫星，比方说【say比方说、假定】11,000英里以外，另一个球面形成了，三个球面相交只有两个交点）
Usually the receiver can discard one of the last two points because it is nowhere near the earth.（通常接收机可以丢弃【discard】最后这两点中的一个点，因为它离地球差的远【nowhere near】）
So,we’re left with one point which is the location of the GPS receiver.In practice,a fourth measurement is needed to correct for clock error.（这样我们留下一个点，它是GPS接收机的位置。在实际当中，需要第四个观测值来修正时间误差【不能翻译成钟误差，因为钟误差中文上是指卫星和接收机的钟的误差】）
GPS(II II II))
Uuit21Understanding the GPS(II II))认识GPS(
GPS Error Sources
There are many sources of possible errors that will degrade the accuracy of position computed by a GPS receiver.（有许多或然误差【possible error】来源，这种误差会降低【degrade】GPS接收机计算的位置的准确度）【that从句修饰errors】
The travel time of GPS satellite signals can be altered by atmospheric effects;when a GPS signal passes through the ionosphere and troposphere it is refracted,causing the speed of the signal to be different from the speed of a GPS signal in space.（GPS卫星信号的传播时间会因大气影响而改变【alter】；当GPS信号穿过电离层【ionosphere】和对流层【troposphere】时会发生折射【refract】，因为信号速度与在太空中信号的速度不同。）
Sunspot activity also causes interference with GPS signals.（太阳黑子活动也对GPS信号产生干扰【interference干涉、干扰】）
Another source of error is measurement noise,or distortion of the signal caused by electrical interference or errors inherent in the GPS receiver itself.（另一个误差源是测量噪声，或者是由于电干扰【electrical interference】或GPS接收机自己固有的误差引起的信号失真【distortion】）【即，电干扰和接收机固有误差都是失真原因】Errors in the ephemeris data(the information about satellite orbits)will also cause errors in computed positions,because the satellites weren’t really where the GPS receiver “thought”they were(based on the information it received)when it computed the positions.（星历数据（关于卫星轨道的信息）误差也会引起位置的误差，因为当计算位置时，卫星不真正地在接收机“认为”它们在的地方（基于它接受的信息所知道））
Small variations in the atomic clocks(clock drift)on board the satellites can translate to large position errors;a clock error of1nanosecond translates to1foot or0.3meters user error on the ground.（卫星原子钟内的很小的变化【variation变奏、变化】（钟漂【drift 漂流；应该是钟偏clock offset】）能转化成很大的位置误差；一纳秒【nanosecond】的钟差能造成1英尺或0.3米的地面用户误差）
Multipath effects arise when signals transmitted from the satellites bounce off a reflective surface before getting to the receiver antenna.（当从卫星发射的信号在到达接收机天线【antenna天线】之前，被一个反射面【reflective surface】反射【bounce off】时，多路径效应【Multipath effect】出现了【arise】）
When this happens,the receiver gets the signal in straight line path as well as delayed path (multiple paths).The effect is similar to a ghost or double image on a TV set.（当这种情况发生时，接收机接收直线路径来的信号，也【as well as】接收延迟路线（多路径）来的信号。其效果与电视的重影【ghost】或重像类似）
How to Reduce GPS Errors（如何减小GPS的误差）
You’ve probably heard people talk about getting2to5meters accuracy with a GPS receiver,or even centimeter or millimeter accuracy.（你可能听说过有人说GPS接收机可以达到2到5米的准确度，甚至是厘米或毫米级精度）
Is there a way to cancel out the errors and get satisfied accuracy?（有方法低偿【cancel out】误差并获得满意的精度吗？）
The answer is yes,but the level of accuracy depends on the type of equipment you are using.（回答是肯定的，但是准确度的级别依赖于你使用的装备的类型）
The following discussion describes a technique used to achieve2to5meters accuracy using mapping(resource)grade receivers.（下面的讨论描述了一种用于绘图（资源）级【grade】接收机的技术，能够获得2到5米的准确度）
Some mapping grade receivers are even capable of sub-meter accuracy,but the increased accuracy comes at a price.（一些绘图级的接收机甚至能够达亚米级精度，但是精度的提高是以很高代价带来的【at a price以很高代价adv.】
Survey grade receivers are the most accurate,capable of centimeter or even millimeter accuracy,depending on the equipment,but they use more advanced techniques to achieve this level of accuracy and,naturally,are more expensive.（测地级接收机最精确，凭【depend on】装备能够达到厘米甚至是毫米级精度，但是它们是使用更先进的技术才得到这样的精度水平，自然地，就更昂贵。）
Recreational grade receivers usually can receive real-time differential corrections,but they cannot store a file that can be differentially corrected using post-processing methods.（休闲【Recreational娱乐的、休闲的】级的接收机通常可以接收实时差分改正，但是它们不能存储文件，【that从句修饰file】以便可以用后处理【post-processing后处理】方法差分【differentially】改正。）
Differential Correction（差分改正）
Differential correction is a method used to reduce the effects of selective available(SA) and other sources of GPS positioning error(differential correction cannot correct for multipath or receiver error;it counteracts only the errors that are common to both reference and roving receivers).（差分改正是一种用来减小选择可用性政策(SA)影响和其它GPS定位误差源的方法（差分改正不能修正多路径效应或接收机误差；它只能抵消【counteract抵消】基准接收机【reference receiver】和流动接收机【roving receiver】所共有的误差）
It requires,in addition to your“roving”GPS receiver,a GPS receiver on the ground in a known location to act as a static reference point.（这需要——除了【in addition to除……之外】你的“流动”GPS接收机之外——一个GPS接收机在地面上一个已知的位置担当【act as】静态【static】参考点）【如果要直译的话，可以两边加破折号把中间的句子夹起来】
This type of setup is often called a GPS base station.Since the base station“knows”where it is,it can compute the errors in its position calculations(in reality,it computes timing errors)and apply them to any number of roving receivers in the same general area.（这种设备经常被称为GPS基准站。由于基准站“知道”在哪儿，它就可以计算它自己的位置误差（实际上，它是计算定时误差【timing error】），并将它们应用于【apply to 将……应用于】许多【any number of许多】同一区域的流动接收机）
This requires that the base and rover receivers“see”the same set of satellites at the same time.（这需要基准接收机和流动接收机同时“看到”【锁定】相同一批卫星）
The base station,depending upon how it is configured,can correct roving GPS receiver data in one(or both)of two ways.（基准站，依照如何配置【configure】，可以用两种中
的一种方法（或同时用两种）对GPS流动接收机的数据进行修正）
⑴in the first method,called real-time differential correction or real-time differential GPS (DGPS),the base station transmits(usually via radio link)error correction messages to other GPS receivers in the local area.（第一种方法，称为实时差分改正或实时差分GPS （DGPS），基准站发射（通常经由【via经由、通过】无线电联系）误差改正信息到其它当地的GPS接收机）
In this case,the positions you read on your GPS receiver while you are out collecting data are the corrected positions.（这样，当你外出收集数据时在你的GPS接收机上读出的位置就是改正后的位置。）
⑵the second method,called post-processed differential correction,is performed on a computer after the roving receiver data are collected.（第二种方法，称为后处理差分改正，在流动站数据收集完后经由计算机完成的【perform执行、完成】）
While you are out in the field collecting data,the positions you read on your roving GPS receiver are uncorrected.（当你在野外收集数据时，你在GPS流动接收机上读出的位置是未经修正的【uncorrected】）
It is not until you take the rover files back to the office and process them using differential correction software and data from the base station file,that you get corrected positions.（直到你把流动站文件带回办公室并利用差分改正软件和基准站的文件数据进行处理，你才能得到改后位置）
The base station file contains information about the timing errors.（基准站文件包括同步误差信息）
This information allows the differential correction software to apply error corrections to the roving receiver file during processing.（这种信息可以在处理时使差分改正软件把误差改正施于流动接收机文件上。）【apply to把……施于……】
Since the base and rover receivers have to“see”the same set of satellites at the same time, the base file has to start before the rover file starts,and end after the rover file ends(a base station is normally set up to track all satellites in view,insuring that it will“see”at least the four satellites that the roving receiver is using to compute positions).（由于基准站接收机和流动站接收机必需同时“看到”同一组卫星，基准站文件必需开始于流动站文件之前，结束于流动站文件之后（基准站通常设置为跟踪所有视野内的卫星，以保证至少“看到”流动站接收机用来计算位置的4颗卫星））
Post-processed differential correction,then,requires both base and rover receivers that are capable of collecting and storing files.（后处理差分改正，要求基准站接收机和流动接收机都能收集和存储文件。）
Most recreational grade receivers cannot collect and store files that can be differentially corrected.（多数休闲级接收机不能收集和存储可以用来差分改正的文件）
How Accurate Is GPS?（GPS准确度如何？）
The accuracy that can be achieved using GPS depends on the type of equipment used,the time of observation,the positions of the satellites being used to compute positions,the model and the software you used.（使用GPS可以达到的准确度依赖于使用装置的类型、观测时间、用来计算位置的卫星的位置、你所使用的模式和软件）
In general,recreational and mapping grade receivers using C/A code without differential correction are accurate to between30and100meters.（大体上讲【In general】，使用C/A 码没有差分改正的休闲和绘图级接收机精度在30到100米之间。）
Many people using recreational grade receivers don’t realize they cannot get highly accurate readings using them autonomously(without differential correction).（许多用休闲类接收机的人没有意识到他们直接使用（没有差分改正）它们不能取得高精度的读数。）【autonomously原意为自治地，也就是不受干扰地，这里可以翻译为直接地】
Unfortunately,some manufacturers’ads are misleading and only confuse the situation.（不幸地是，有些厂商【manufacturer】的广告误导【mislead】并混淆【confuse】了这种情况）
They claim30meters accuracy,which actually means30meters GEP(circular error probable).（他们宣称有30米的精度，实际上那意味着30米的GEP（圆概然误差【probable error概然误差——测量误差概率为0.5时的误差】）
This means that positions will be within30meters50percent of the time.（这意味着这个位置有50％的机会【time】在30米内）
There is really no way to tell whether you are within30meters or within100or more meters of the true position.（实在不能说你是在真实位置的30米内或者100米内或着更多米内）
Most mapping and recreational grade receivers with differential correction can provide from about2to5meters accuracy.（大多数差分改正的测图和休闲类接收机可以提供2到5米的准确度）
Some receivers use what is called“carrier-smoothed code”to increase the accuracy of the C/A code.（有些接收机利用被称为“载波平滑码”【即消除C/A码，还原载波的技术】来增加C/A码的精度）
This involves measuring the distance from the receiver to the satellites by counting the number of waves that carry the C/A code signal.（这涉及【involve包括、涉及】利用计算承载C/A码信号的载波数来测量从接收机到卫星的距离）
These receivers can achieve10cm to1meter accuracy with differential correction.（这些接收机可以利用差分改正达到10cm到1米的精度）
Dual frequency survey grade receivers using more advanced network survey techniques
can achieve centimeter to millimeter accuracy.（双频【Dual frequency】测量型【测地型】接收机利用更先进的联合【network这里是联合的意思】测量技术，可以达到厘米到毫米的精度）
Unit22Competition in Space Orbit（太空轨道上的竞争）
Around the world,countries are mobilizing to build independent satellite navigation networks,troubled that the Global Positioning System(GPS)is run by the US military and controlled by the US government.（环绕世界【环顾世界】，顾虑【trouble顾虑、忧虑】到GPS由美国军方运作并受到美国政府的控制，许多国家都努力【mobilize筹措……以做什么，这里换做努力】建立独立卫星导航网）
On March7,1995was announced the Russian Federation Government’s decree“On executing works in use of the GLONASS system for the sake of civil users”.（在1995.3.7，俄罗斯联邦政府宣布一项法令【decree】“GLONASS系统民用【for the sake of为了；civil民用的】计划”）【这话是意译，On executing works是执行……工作的意思】It was also reported on“21st century”,2001that the European Union says the US couldn’t guarantee the reliability and availability of the GPS civilian signal,as United States national security needs could potentially outweigh any other needs.（2001年“二十一世纪”也报道说，欧盟说美国不能保证【guarantee保证】GPS民码的可靠性【reliability】和有效性【availability】，由于美国国家安全需要潜在地超过了其它任何需要）“Europe cannot accept reliance on a military system which has the possibility of being cut off,”said Rene Oosterlinck,head of the navigation department at the European Space Agency,fairly summing up the emotions GPS brings about abroad.（“欧洲不能接受依赖一个有可能被切断的军方系统”，Rene Oosterlinck【欧洲航空航天局导航部、伽利略计划主任】说道，欧洲航空局导航部主任，清楚的【fairly清楚的、概括的】概括【sum up概括、总结】了GPS对国外产生【brings about产生】的情绪【按习惯就是国外对
GPS的情绪】。）
In response,Europe will spend billions assembling a civil satellite system called Galileo, scheduled to be operation by2008.（作为反应【response】，欧洲将花费数十亿装配【assemble这里可以译为开发】一个民用卫星系统称为“Galileo”，预期【schedule 预期v.；时间表】在2008年运转【operation运转n.】
For the benefit of students to get some knowledge on the competition in space orbit,brief introduction to GLONASS,GALILEO and BEIDOU is provided as follows.（为了【For the benefit of为了……的利益】使学生们得到一些太空轨道上的竞争的知识，下面提供【provide提供，可以译为列出】了关于GLONASS，GALILEO和北斗的简要介绍）
GLONASS（GLONASS）
The Russian Federation’s GLObal Navigation Satellite System(GLONASS)was developed for the Russian military and was declared operational in1996.（俄联邦的全球导航卫星系统（GLANASS）是由俄罗斯军方开发的，并在1996年宣布运行）
Fully deployed GLONASS constellation is composed of24(21operational+3spares) satellites in three orbital planes whose ascending nodes are120degrees apart.（全配置【deploy配置】的GLONASS星座【constellation星座】由24（21运行＋3备用）颗卫星在三个轨道平面中，其升交点相差【apart分开的】120度）
8satellites are equally spaced in each plane with argument of latitude displacement of45 degrees.（8颗卫星平均分布【equally相等地，space分开、间隔】在升交角距【argument of latitude】相差【displacement位移、变换，这里译为相差就得了】45度的每个平面内）
Besides the planes themselves have15degrees argument of latitude displacement.（此外【Besides此外】，轨道平面它们自己有15度的升交角距差）
Each satellite operates in circular19,100km orbits at an inclination angle of64.8degrees and each satellite completes an orbit in approximately11hours15minutes.（每个卫星在19100公里的圆轨道上运转，倾角为64.8度【指轨道倾角】，每个卫星完成一次绕轨飞行【orbit绕轨飞行n.】大约【approximately】11小时15分钟）
The spacing of satellites in orbits is arranged so that minimums of5satellites are in view to users worldwide.（轨道卫星间隔【spacing】被安排能够使使用者最少【minimum 至少n.】能看到5颗卫星）
GLONASS constellation allows providing continuous and global navigation coverage for performing of successful navigation observations.（GLONASS星座【constellation】能够为成功的【或者圆满的】航行【navigation】观测的执行【performing】提供持续的和全球的导航覆盖【coverage】）
Each satellite transmits radio frequency navigation signal containing navigation message for users.（每个卫星发射包含导航信息的无线电频率导航信号给使用者）
The GLONASS Constellation is operated by Ground-based Control Complex(GCC).（GLONASS星座【Constellation】由地面控制部分【Ground-based Control Complex】（GCC）来操作）
It consists a System Control Center(SCC)and several Command Tracking Stations (CTSs).（它是由一个系统控制中心【System Control Center】（SCC）和几个置零跟踪站【Command Tracking Station】（CTSs）组成【consist由……组成】）
The CTSs track the GLONASS satellites in view and accumulate ranging data and telemetry from the satellites signals.（CTSs跟踪所见到的【in view视野内】GLONASS 卫星，并根据卫星信号收集【accumulate积累】距离修正【ranging距离修正】数据和进行遥测【telemetry遥感勘测n.】）
The information from CTSs is processed at the SCC to determine satellite clock and orbit states and to update the navigation message of each satellite.（CTSs的信息经SCC处理，确定卫星时钟和轨道状态【state】，并上传导航信息给每个卫星）
This updated information is transmitted to the satellites via the CTSs,which are also used for transmitting of control information.（上传的信息经【via】CTSs发射，也用来做校准信息【control information】的发射之用【指GTS也用来做……之用】）
Compared with GPS,GLONASS has the following characteristics:（与GPS相比，GLONASS有以下特点：）
⑴dual-frequency(L1in the range:1597-1617MHz;L2in the range:1240-1260MHz);（
⑴双频（L1范围：1597-1617MHz【megahertz】；L2范围：1240-1260MHz）
⑵each satellite transmits a different frequency on L1and L2;（⑵每个卫星以L1和L2波段发射不同的频率【即频分多址FDMA，每个卫星发射频率都不同，而GPS采用码分多址CDMA，每个卫星噪声码不同】）
⑶a different datum and time reference system to GPS;（⑶与GPS有不同的基准和时间参考系）【GPS使用的是美国的UTC协调世界时，GLONASS使用的是苏联UTC】⑷PPS(Precise Positioning Service)and SPS(Standard Positioning Service)as in the case of GPS;（⑷同GPS一样，有PPS（精密定位服务）和SPS（标准定位服务））
⑸no Selective Availability is implemented.（⑸没有选择可用性【SA政策】的执行【implement执行】）
Although some of the characteristics of GLONASS are very similar to GPS,there are nevertheless significant technical differences,in addition,the level of maturity of the user receiver technology and the institutional capability necessary to support the GLONASS
space and control segment are significantly less than in the case of GPS.（虽然GLONASS 的某些特性与GPS非常相似，然而【nevertheless】仍有重大的【significant】技术差别，另外【in addition】，客户接收机技术的成熟水平【level of maturity，maturity成熟】和用来支持GLONASS空间和控制部分【segment】所必需【necessary】的制度上的【institutional制度上的】性能【指频分多址，使得用户设备非常昂贵】比GPS 有显著的【significantly引人注意地】不如）
GLONASS will continue to be viewed by many user communities as a technically inferior system to GPS,a system concerning which there are many question marks regarding its long-term viability.（GLONASS将继续受到许多用户社团【community团体】的关注，作为比GPS技术上【technically技术上】次等的【inferior次等的】系统）
This uncertainty is stifling much needed market investment in new generation receiver hardware.（这个非常令人郁闷的【stifling令人郁闷的】不可靠的问题【uncertainty不可靠n.】需要市场投资新一代接收机硬件）
Although GLONASS has the potential to rival GPS in coverage and accuracy,this potential is unlikely to be realized in the medium term,and hence for the foreseeable future GLONASS should be considered a complementary system to GPS.（虽然GLONASS有潜力【potential潜力；有潜力的】在覆盖【coverage】和精确度上与GPS 竞争【rival竞争】，这种潜力被认为【realize认识到】在中期【medium中间的term 期间】是不太可能的【unlikely不可靠的、不可能的】，因此【hence】在可预见的【foreseeable可预知的】将来GLONASS将被考虑作为GPS的一个补充【complementary补充的】系统）
The combination of GPS and GLONASS is part of a Global Navigational Satellite System (GNSS).（GPS和GLONASS的联合是全球导航卫星系统（GNSS）的一部分）Although there are GLONASS-only receivers available on the market,these are generally
inferior to GPS products.（虽然市场上有可用的【available】GLONASS接收机，一般【generally】要差于GPS产品）
However,there is a distinct trend to develop receivers that can track and process signals from both the GPS and GLONASS satellites.（可是，有一种明显的【distinct明显的】倾向，发展接收机使之可以跟踪和处理来自GPS和GLONASS卫星的信号）
A combined GPS+GLONASS receiver can track signals from a48-satellite constellation, twice as many as the GPS-only constellation and therefore significantly improving availability.（一台组合【combined组合的】GPS＋GLONASS接收机可以跟踪从一个48颗卫星星座传来的信号，是GPS星座的两倍，因此【therefore】显著地【significantly 引人注目地】提高了其实用性【availability】）
GALILEO（伽利略）
Satellite navigation users in Europe today have no alternative other than to take their positions from US GPS or Russian GLONASS satellites.（现在，欧洲卫星导航的使用者除了使用美国的GPS或俄罗斯的GLONASS卫星定位，没有其它选择了【no alternative other than to除……没有选择，alternative可选择的东西】）
Yet the military operators of both systems give no guarantee to maintain an uninterrupted service.（然而这两个系统的军方操控者都不给出保证维持【maintain】不间断的【uninterrupted】服务）
As far back as the early1990s,the European Union saw the need for Europe to have its own global satellite navigation system.（早在【As far back as】90年代早期，欧盟看到欧洲需要自己的全球卫星导航系统）
GALILEO,an independent system under civilian control which will be guaranteed to operate at all times,is a joint initiative of the European Commission(EC)and the
European Space Agency(ESA).（GALILEO，一个被控制为民用的独立系统，将可以保证一直【at all time一直】运行，由欧共体【European Commission】（EC）和欧洲航天局【European Space Agency】（ESA）共同发起【joint共同的initiative发起n.】）The mission is now at the point of moving from definition to full-scale development.（从一个概念【definition定义n.】到全面的【full-scale全面的】发展，这项任务【mission】现在接近【at the point of】运行了）
⑴the GALILEO infrastructure is being implemented in three phases.（⑴GALILEO基础设施【infrastructure】由3个阶段实现【implement实现；执行】）
⑵development and In-Orbit Validation(2002-2005).（⑵开发和轨道内检测【实验卫星轨道检测】【Validation确认n.】（2002－2005）
⑶deployment(2006-2007).（⑶部署阶段【deployment部署】（2006－2007））
⑷commercial Operations(from2008).（⑷商业运行（从2008开始））
The first experimental satellite,part of the so-called GALILEO System Test Bed(GSTB) will be launched in the second semester of2005.（第一颗实验卫星，号称【so-called】伽利略系统实验床【Test Bed】（GSTB）的一部分，将在2005下半年【second semester 第二个学期】发射）
The objective of this experimental satellite is to characterize the critical technologies, which are already under development under ESA contracts.（这颗实验卫星的目标是定性【characterize表现……特性】关键性【critical重要的】技术，已经在ESA合同【contract】发展之下了）
Thereafter up to four operational satellites will be launched in the timeframe2005-2006to validate the basic GALILEO space and related ground segment.（其后【Thereafter】轮到【up to轮到、该由】4颗运作卫星将在2005－2006时间框架内发射，确认【validate