外文翻譯及文獻：全球定位系統(tǒng)的介紹 Introduction to the Global Positioning System

1、英文翻譯introduction to the global positioning system what is gps?the global positioning system (gps) is a location system based on a constellation of about 24 satellites orbiting the earth at altitudes of approximately 11,000 miles. gps was developed by the united states department of defense (dod), fo

2、r its tremendous application as a military locating utility. the dods investment in gps is immense. billions and billions of dollars have been invested in creating this technology for military uses. however, over the past several years, gps has proven to be a useful tool in non-military mapping appl

3、ications as well.gps satellites are orbited high enough to avoid the problems associated with land based systems, yet can provide accurate positioning 24 hours a day, anywhere in the world. uncorrected positions determined from gps satellite signals produce accuracies in the range of 50 to 100 meter

4、s. when using a technique called differential correction, users can get positions accurate to within 5 meters or less.today, many industries are leveraging off the dods massive undertaking. as gps units are becoming smaller and less expensive, there are an expanding number of applications for gps. i

5、n transportation applications, gps assists pilots and drivers in pinpointing their locations and avoiding collisions. farmers can use gps to guide equipment and control accurate distribution of fertilizers and other chemicals. also，gps is used for providing accurate locations and as a navigation too

6、l for hikers, hunters and boaters.many would argue that gps has found its greatest utility in the field of geographic information systems (gis). with some consideration for error, gps can provide any point on earth with a unique address (its precise location). a gis is basically a descriptive databa

7、se of the earth (or a specific part of the earth). gps tells you that you are at point x,y,z while gis tells you that x,y,z is an oak tree, or a spot in a stream with a ph level of 5.4. gps tells us the where. gis tells us the what. gps/gis is reshaping the way we locate, organize, analyze and map o

8、ur resources.trilateration - how gps determines a location？in a nutshell, gps is based on satellite ranging - calculating the distances between the receiver and the position of 3 or more satellites (4 or more if elevation is desired) and then applying some good old mathematics. assuming the position

9、s of the satellites are known, the location of the receiver can be calculated by determining the distance from each of the satellites to the receiver. gps takes these 3 or more known references and measured distances and triangulates an additional position.as an example, assume that i have asked you

10、 to find me at a stationary position based upon a few clues which i am willing to give you. first, i tell you that i am exactly 10 miles away from your house. you would know i am somewhere on the perimeter of a sphere that has an origin as your house and a radius of 10 miles. with this information a

11、lone, you would have a difficult time to find me since there are an infinite number of locations on the perimeter of that sphere.second, i tell you that i am also exactly 12 miles away from the abc grocery store. now you can define a second sphere with its origin at the store and a radius of 12 mile

12、s. you know that i am located somewhere in the space where the perimeters of these two spheres intersect - but there are still many possibilities to define my location.adding additional spheres will further reduce the number of possible locations. in fact, a third origin and distance (i tell you am

13、8 miles away from the city clock) narrows my position down to just 2 points. by adding one more sphere, you can pinpoint my exact location. actually, the 4th sphere may not be necessary. one of the possibilities may not make sense, and therefore can be eliminated.for example, if you know i am above

14、sea level, you can reject a point that has negative elevation. mathematics and computers allow us to determine the correct point with only 3 satellites.based on this example, you can see that you need to know the following information in order to compute your position:a) what is the precise location

15、 of three or more known points (gps satellites)? b) what is the distance between the known points and the position of the gps receiver?how the current locations of gps satellites are determined？gps satellites are orbiting the earth at an altitude of 11,000 miles. the dod can predict the paths of the

16、 satellites vs. time with great accuracy. furthermore, the satellites can be periodically adjusted by huge land-based radar systems. therefore, the orbits, and thus the locations of the satellites, are known in advance. todays gps receivers store this orbit information for all of the gps satellites

17、in what is known as an almanac. think of the almanac as a bus schedule advising you of where each satellite will be at a particular time. each gps satellite continually broadcasts the almanac. your gps receiver will automatically collect this information and store it for future reference.the departm

18、ent of defense constantly monitors the orbit of the satellites looking for deviations from predicted values. any deviations (caused by natural atmospheric phenomenon such as gravity), are known as ephemeris errors. when ephemeris errors are determined to exist for a satellite, the errors are sent ba

19、ck up to that satellite, which in turn broadcasts the errors as part of the standard message, supplying this information to the gps receivers.by using the information from the almanac in conjuction with the ephemeris error data, the position of a gps satellite can be very precisely determined for a

20、given time.computing the distance between your position and the gps satellitesgps determines distance between a gps satellite and a gps receiver by measuring the amount of time it takes a radio signal (the gps signal) to travel from the satellite to the receiver. radio waves travel at the speed of l

21、ight, which is about 186,000 miles per second. so, if the amount of time it takes for the signal to travel from the satellite to the receiver is known, the distance from the satellite to the receiver (distance = speed x time) can be determined. if the exact time when the signal was transmitted and t

22、he exact time when it was received are known, the signals travel time can be determined.in order to do this, the satellites and the receivers use very accurate clocks which are synchronized so that they generate the same code at exactly the same time. the code received from the satellite can be comp

23、ared with the code generated by the receiver. by comparing the codes, the time difference between when the satellite generated the code and when the receiver generated the code can be determined. this interval is the travel time of the code. multiplying this travel time, in seconds, by 186,000 miles

24、 per second gives the distance from the receiver position to the satellite in miles.four (4) satellites to give a 3d positionin the previous example, you saw that it took only 3 measurements to triangulate a 3d position. however, gps needs a 4th satellite to provide a 3d position. why?three measurem

25、ents can be used to locate a point, assuming the gps receiver and satellite clocks are precisely and continually synchronized, thereby allowing the distance calculations to be accurately determined. unfortunately, it is impossible to synchronize these two clocks, since the clocks in gps receivers ar

26、e not as accurate as the very precise and expensive atomic clocks in the satellites. the gps signals travel from the satellite to the receiver very fast, so if the two clocks are off by only a small fraction, the determined position data may be considerably distorted.the atomic clocks aboard the sat

27、ellites maintain their time to a very high degree of accuracy. however, there will always be a slight variation in clock rates from satellite to satellite. close monitoring of the clock of each satellite from the ground permits the control station to insert a message in the signal of each satellite

28、which precisely describes the drift rate of that satellites clock. the insertion of the drift rate effectively synchronizes all of the gps satellite clocks.the same procedure cannot be applied to the clock in a gps receiver. therefore, a fourth variable (in addition to x, y and z), time, must be det

29、ermined in order to calculate a precise location. mathematically, to solve for four unknowns (x,y,z, and t), there must be four equations. in determining gps positions, the four equations are represented by signals from four different satellites.the gps error budgetthe gps system has been designed t

30、o be as nearly accurate as possible. however, there are still errors. added together, these errors can cause a deviation of +/- 50 -100 meters from the actual gps receiver position. there are several sources for these errors, the most significant of which are discussed below:atmospheric conditionsth

31、e ionosphere and troposphere both refract the gps signals. this causes the speed of the gps signal in the ionosphere and troposphere to be different from the speed of the gps signal in space. therefore, the distance calculated from signal speed x time will be different for the portion of the gps sig

32、nal path that passes through the ionosphere and troposphere and for the portion that passes through space.as mentioned earlier, gps signals contain information about ephemeris (orbital position) errors, and about the rate of clock drift for the broadcasting satellite. the data concerning ephemeris e

33、rrors may not exactly model the true satellite motion or the exact rate of clock drift. distortion of the signal by measurement noise can further increase positional error. the disparity in ephemeris data can introduce 1-5 meters of positional error, clock drift disparity can introduce 0-1.5 meters

34、of positional error and measurement noise can introduce 0-10 meters of positional error.ephemeris errors should not be confused with selective availability (sa), which is the intentional alteration of the time and ephemeris signal by the department of defense. a gps signal bouncing off a reflective

35、surface prior to reaching the gps receiver antenna is referred to as multipath. because it is difficult to completely correct multipath error, even in high precision gps units, multipath error is a serious concern to the gps user.measuring gps accuracyas discussed above, there are several external s

36、ources which introduce errors into a gps position. while the errors discussed above always affect accuracy, another major factor in determining positional accuracy is the alignment, or geometry, of the group of satellites (constellation) from which signals are being received. the geometry of the con

37、stellation is evaluated for several factors, all of which fall into the category of dilution of precision, or dop.dop is an indicator of the quality of the geometry of the satellite constellation. your computed position can vary depending on which satellites you use for the measurement. different sa

38、tellite geometries can magnify or lessen the errors in the error budget described above. a greater angle between the satellites lowers the dop, and provides a better measurement. a higher dop indicates poor satellite geometry, and an inferior measurement configuration.some gps receivers can analyze

39、the positions of the satellites available, based upon the almanac, and choose those satellites with the best geometry in order to make the dop as low as possible. another important gps receiver feature is to be able to ignore or eliminate gps readings with dop values that exceed user-defined limits.

40、 other gps receivers may have the ability to use all of the satellites in view, thus minimizing the dop as much as possible.全球定位系統(tǒng)的介紹 什么是gps？全球定位系統(tǒng)（）是一種基于顆高度大約英里的地球軌道衛(wèi)星的定位系統(tǒng)。是美國國防部（）因為其在軍事定位裝置方面巨大的應(yīng)用而開發(fā)的。國防部對的投入是極大的。已經(jīng)有數(shù)十億美元的投資為了開發(fā)這種軍事應(yīng)用技術(shù)。然而，過去一段時間以來，已經(jīng)被證實是在測繪非軍事地圖應(yīng)用方面十分有用的工具。gps衛(wèi)星的軌道足夠高以避免以土地為基礎(chǔ)的系

41、統(tǒng)相關(guān)的問題，還可以在世界上任何地方提供每天24小時準確的定位。在裸眼可視位置gps衛(wèi)星信號產(chǎn)生的定位精度為到米。當(dāng)使用差分技術(shù)時，用戶可以得到精度為5米以下的定位。今天，許多行業(yè)都促使國防部改變。由于gps單位正變得更小，更便宜，gps的應(yīng)用正在不斷增加。在交通運輸應(yīng)用方面，gps協(xié)助飛行員和司機準確定位它們的位置，避免碰撞。農(nóng)民可以使用gps引導(dǎo)設(shè)備并且控制化肥和其他化學(xué)品的準確分布。此外，gps用于提供準確的位置，并作為徒步旅行者，獵人和船民的導(dǎo)航工具。很多人認為，gps已經(jīng)在地理信息系統(tǒng)（gis）領(lǐng)域發(fā)揮最大的應(yīng)用。在考慮到一些誤差的情況下，gps可以提供地球上任何一點的唯一一個地址（

42、它的精確位置）。 gis從根本上說是地球的一個描述的數(shù)據(jù)庫（或地球的特定部分）。 gps會告訴你，你是在點x，y，z，而gis告訴你，x，y，z點是一棵橡樹，或河流中的一個ph值5.4的點。 gps告訴我們，“在哪里”， gis告訴我們“是什么”。 gps/ gis正在重塑定位，管理，分析，并且映射我們的資源。 三邊定位gps是怎樣定位的？簡單地說，gps是基于衛(wèi)星測距 -計算接收器和3顆或更多顆衛(wèi)星（4個或更多，如果需要高程）之間的距離，然后用一些以前的正確數(shù)字進行計算。假設(shè)衛(wèi)星的位置是已知的，通過確定每個衛(wèi)星到接收機的距離，可以計算出接收機的位置。 gps用這3個或更多的已知的參考和測量距

43、離然后“三角測量”出額外的位置。作為一個例子，假設(shè)我要你根據(jù)我給你提供的很少的一些線索讓你在一個固定的位置找到我。首先，我告訴你，我離你的房子正好是10英里遠。你會知道我在一個以你的房子為圓心，半徑10英里的球形邊界的地方。只有這些信息，你很難找到我，因為在這球形的邊界上有無數(shù)的位置點。第二，我告訴你，我也正好離abc雜貨店12英里遠?，F(xiàn)在，你可以畫出一個以雜貨店為圓心，半徑12英里的球形。你知道，我所在的地方，在兩個球形空間的周長交叉的地方 - 但我的位置還是有很多的可能性。添加更多的范圍將進一步減少可能的地點。事實上，第三個圓心和距離（我告訴你是8英里遠的城市時鐘）使我的位置縮小到只有2點

44、。再增加一個范圍，你可以找出我的確切位置。其實，第四球體可能不是必需的。其中一個可能是沒有意義的，并因此可以被消除。例如，如果你知道我是海平面上，你可以拒絕一個海拔為負的點。數(shù)學(xué)和計算機讓我們能夠只用3顆衛(wèi)星確定正確的點。基于這個例子，你可以明白，你需要知道以下信息，以便計算你的位置：a）3個或更多的已知點（gps衛(wèi)星）的精確位置是什么？b）已知點的位置和gps接收器之間是多少距離？如何確定gps衛(wèi)星當(dāng)前位置？gps衛(wèi)星在高度為11000公里的軌道繞地球飛行。國防部可以非常準確地預(yù)測衛(wèi)星的路徑與時間的關(guān)系。此外，衛(wèi)星還可以定期通過巨大的陸基雷達系統(tǒng)進行調(diào)整。因此，軌道，衛(wèi)星的位置，都是預(yù)先已知

45、的。今天的gps接收器存儲所有的gps衛(wèi)星的軌道信息，被稱為星歷。你的星歷像“巴士時刻表”，提醒你每顆衛(wèi)星在一個特定的時間點在什么地方。每個gps衛(wèi)星不斷廣播星歷。你的gps接收器會自動收集這些信息，并將其存儲以供將來參考。國防部不斷地監(jiān)視衛(wèi)星的軌道預(yù)測值的偏差。任何偏差（由于自然的大氣現(xiàn)象，如重力），被稱為星歷誤差。當(dāng)被確定為存在衛(wèi)星星歷誤差，誤差被發(fā)送備份到該衛(wèi)星，依次將錯誤作為標準的消息的一部分廣播，提供這種信息給gps接收機。通過使用星歷表的信息誤差數(shù)據(jù)，可以很精確地在一個給定的時間確定gps衛(wèi)星的位置。計算你的位置和gps衛(wèi)星之間的距離gps通過測量衛(wèi)星和接收機之間無線電信號（gps信號）傳輸所花費的時間來確定衛(wèi)星和接收機的距離。無線電波以光速傳播，這是大約每秒186,000英里的速度。因此，如果信號從衛(wèi)星到接收機的時間量是已知的，衛(wèi)星到接收機的距離（距離=速度時間）可以被確定。如果信號發(fā)送和收到的時間確定，信號的傳輸時間才能確定。為了做到這一點，衛(wèi)星和接收機使用非常精確的同步時鐘，以便它們在完全