Unit 20 Understanding the GPS

1、Unit 20 Understanding the GPS（认识 GPS）What Is GPS?（什么是 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

2、的系统，可以用来在全球任何地方定位）GPS provides continuous (24 hours/day), real-time, 3-dimensional positioning, navigationand 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 anyapplication that requires location coordinates.（任何拥有一台 GP

3、S 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: the space segment: the GPS satellitesthemselves, the control segment, operated by the U.S. military, and the user segment,which includes both military and civilian users and their GPS equipments.（

4、GPS 由【consistof 由组成】三个部分【segment 部分、节】组成：太空部分：GPS 卫星自己，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，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（太空部分：GPS 星座【或 GPS 星群】）The first GPS satellite was launched by the U.S. Air Force in early 1978.（

5、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 now 24 satellites orbiting the earth at an altitude of about 10,900 miles.（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, andthat the satellites motion through space is not af

6、fected 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【atmosphericdrag】的影响）There are four satellites in each of 6 orbital planes.（6 个轨道平面【orbital plane】中每个有 4 颗卫星。）Each plane is inclined 55 degrees relative to the equator, which means that satellites crossth

7、e equator tilted at a 55 degrees 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 the 24satellites 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】）These 24 satellite

8、s 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 batteriesproviding 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 satel

9、lite 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 timingsystem 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.

10、DOD Monitoring（控制部分：美国国防部监控）The U.S. Department Of Defense maintains a master control station at Falcon Air ForceBase 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 Garciaand Kwajalein

11、.（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 thesatellites.（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 broa

12、dcast these corrections, along with the other position andtiming data, so that a GPS receiver on the earth can precisely establish the location of eachsatellite 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 U

13、sers （用户部分：军方和民用 GPS 用户）The U.S. military 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

14、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 totransportation 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,positio

15、ning 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?（GPS 如何工作）A GPS receiver calculates its position by a technique called satellite ranging, whichinvolves measuring the distance between the GPS receiver and the GPS satellites it istracking.（GPS 接收机利用一项被称为卫星测距【 s

16、atellite 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 ofrange 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

17、方法得到的】）The position of each satellite is known, and the satellites transmit their positions as part ofthe “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 positionbased on the

18、 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 requiresmeasuring 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

19、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 signalmultiplies the speed of light (because the satellite signals travel at the speed of light,approximately 186,000 miles 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

20、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 a 3-dimensional (latitude,longitude and altitude) position.（计算一个三维（纬度、精度和高程）位置需要【berequired to 被需要】4 颗卫星的距离值）In order to measure the travel time of the satellite signal,

21、 the receiver has to know whenthe 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” itsinternal clock when the signal arrives to see what time it is.（确定

22、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 digitalcode 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

23、 GPS receiver receives a code from a satellite, it can look back in its memorybank 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

24、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, its 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 cancompute its location t

25、hrough trilateration. Heres 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 be 12,000 miles.（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 re

26、ceiver location down, but it only tells us that we aresomewhere on a sphere which is centered on the satellite and has a 12,000 mile 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

27、上，而在太空中）Now, consider that the receiver picks up a signal from a second satellite and calculates therange between the receiver and the satellite to be 10,000 miles.（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 a 10,000 mile radius with th

28、e secondsatellite 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

29、在这个圆圈的某处）If the receiver picks up another satellite, says at 11,000 miles away, another sphere isformed, 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 becau

30、se it is nowhere near theearth.（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, were left with one point which is the location of the GPS receiver. In practice, a fourthmeasurement 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

31、钟的误差】）Uuit21 Understanding the GPS (II) 认识 GPS(II)GPS Error Sources (GPS 误差来源)There are many sources of possible errors that will degrade the accuracy of positioncomputed 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

32、signals can be altered by atmospheric effects; when a GPSsignal passes through the ionosphere and troposphere it is refracted, causing the speed ofthe 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【refrac

33、t】，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 byelectrical 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【el

34、ectrical 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 errorsin computed positions, because the satellites werent really where the GPS receiver“thought” they were (based on the information

35、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 tolarge position errors; a clock error of 1 nanosecond translates to 1 foot or 0.3 meters userer

36、ror 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 reflectivesurface 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

37、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

38、）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）Youve probably heard people talk about getting 2 to 5 meters accuracy with a GPSreceiver, 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 satisfi

39、ed accuracy?（ 有 方 法 低 偿 【cancelout】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 areusing.（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 achieve 2 to 5 meters accuracyusing mapping (resource) grade receivers.（下面的讨论

40、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 increasedaccuracy 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 millime

41、teraccuracy, depending on the equipment, but they use more advanced techniques to achievethis 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 different

42、ial corrections, but theycannot 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

43、used to reduce the effects of selective available (SA)and other sources of GPS positioning error (differential correction cannot correct formultipath or receiver error; it counteracts only the errors that are common to bothreference 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

44、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 aknown 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【a

45、ct 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” whereit is, it can compute the errors in its position calculations (in reality, it computes timingerrors) and apply them to any number of roving receivers in the same

46、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 sametime.（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 up

47、on how it is configured, can correct roving GPS receiverdata 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）in the first method, called real-time differential correction or real-time differential GPS(DGPS), the base station transmits (usually via radio link

48、) error correction messages toother 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 dataare 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

49、后的位置。） the second method, called post-processed differential correction, is performed on acomputer 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 GPSreceiver are unc

50、orrected.（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 differentialcorrection 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

51、 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 tothe 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 “

52、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 basestation is normally set up to track all satellites in view, insuring that it will “see” at leastthe four satellites that the roving receiver is using to

53、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 arecapable 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

54、recreational grade receivers cannot collect and store files that can be differentiallycorrected.（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, thetime of observation, the positions of the satellites being

55、 used to compute positions, themodel 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 differentialcorrection are accurate to between 30 and 100 meters.（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

56、机精度在 30 到 100 米之间。）Many people using recreational grade receivers dont realize they cannot get highlyaccurate 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

57、 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 claim 30 meters accuracy, which actually means 30 meters GEP (circular errorprobable).（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 within 30 meters 50 percent 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 within 30 meters or within 100 or moremeters 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 re