全球卫星定位系统外文翻译.doc

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1、外文文献及译文文献、资料题目：Global Positioning System外文文献：Global Positioning SystemThe Global Positioning System (GPS) is a global navigation satellite system (GNSS) developed by the United States Department of Defense and managed by the United States Air Force 50th Space Wing. It is the only fully functional GN

2、SS in the world, can be used freely, and is often used by civilians for navigation purposes. The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military

3、applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.It uses a constellation of between 24 and 32 medium Earth orbit satellites

4、 that transmit precise radiowave signals, which allow GPS receivers to determine their current location, the time, and their velocity. Its official name is NAVSTAR GPS. Although NAVSTAR is not an acronym,1 a few backronyms have been created for it.Since it became fully operational in 1993, GPS has b

5、ecome a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, scientific uses, and hobbies such as geocaching. Also, the precise time reference is used in many applications including the scientific study of earthquakes and as a required time synchroniza

6、tion method for cellular network protocols such as the IS-95 standard for CDMA. GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the users exact location. Essentially,

7、the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the users position and display it

8、 on the units electronic map. A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the users 3D position (latitude, longitude and altitude).

9、 Once the users position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more. History The first satellite navigation system, Transit, used by the United States Navy, was first succe

10、ssfully tested in 1960. Using a constellation of five satellites, it could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite which proved the ability to place accurate clocks in space, a technology that GPS relies upon. In the 1970s, the

11、ground-based Omega Navigation System, based on signal phase comparison, became the first worldwide radio navigation system.The design of GPS is based partly on similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War

12、 II. Additional inspiration for the GPS came when the Soviet Union launched the first man-made satellite, Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputniks radio transmissions. They discovered that, because of the Doppler effect, the frequency of the

13、signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion.After Korean Air

14、 Lines Flight 007 was shot down in 1983 after straying into the USSRs prohibited airspace,3 President Ronald Reagan issued a directive making GPS freely available for civilian use as a common good.4 The satellites were launched between 1989 and 1993.Initially the highest quality signal was reserved

15、for military use, while the signal available for civilian use was intentionally degraded (Selective Availability, SA). Selective Availability was ended in 2000, improving the precision of civilian GPS from about 100m to about 20m.Of crucial importance for the function of GPS is the placement of atom

16、ic clocks in the satellites, first proposed by Friedwardt Winterberg in 1955.5 Only then can the required position accuracy be reached.Timeline In 1972, the US Air Force Central Inertial Guidance Test Facility (Holloman AFB) conducted developmental flight tests of two prototype GPS receivers over Wh

17、ite Sands Missile Range, using ground-based pseudo-satellites. Satellite numbersBlockLaunch PeriodSatellites launchedCurrently in serviceI1978198510+110II1985199090IIA199019971913IIR1997200412+1112IIR-M200520097+126IIF200920110+1020IIIA2014?0+1230IIIB0+830IIIC0+1630Total59+21+122+363311Failed2In pre

18、paration3Planned.(Last update: 16 December 2008) In 1978 the first experimental Block-I GPS satellite was launched. In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 that strayed into prohibited airspace due to navigational errors, killing all 269 people on board, U.

19、S. President Ronald Reagan announced that the GPS would be made available for civilian uses once it was completed.910 By 1985, ten more experimental Block-I satellites had been launched to validate the concept. On February 14, 1989, the first modern Block-II satellite was launched. In 1992, the 2nd

20、Space Wing, which originally managed the system, was de-activated and replaced by the 50th Space Wing. By December 1993 the GPS achieved initial operational capability.11 By January 17, 1994 a complete constellation of 24 satellites was in orbit. Full Operational Capability was declared by NAVSTAR i

21、n April 1995. In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive12 declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset. In 1998, U.S. Vice

22、 President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety. On May 2, 2000 Selective Availability was discontinued as a result of the 1996 executive order, allowing users to receive a non-de

23、graded signal globally. In 2004, the United States Government signed an agreement with the European Community establishing cooperation related to GPS and Europes planned Galileo system. In 2004, U.S. President George W. Bush updated the national policy and replaced the executive board with the Natio

24、nal Space-Based Positioning, Navigation, and Timing Executive Committee. November 2004, QUALCOMM announced successful tests of Assisted-GPS for mobile phones.13 In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performanc

25、e. On September 14, 2007, the aging mainframe-based Ground Segment Control System was transitioned to the new Architecture Evolution Plan.14 The most recent launch was on March 15, 2008.15 The oldest GPS satellite still in operation was launched on November 26, 1990, and became operational on Decemb

26、er 10, 1990.16 Basic concept of GPSA GPS receiver calculates its position by precisely timing the signals sent by the GPS satellites high above the Earth. Each satellite continually transmits messages containing the time the message was sent, precise orbital information (the ephemeris), and the gene

27、ral system health and rough orbits of all GPS satellites (the almanac). The receiver measures the transit time of each message and computes the distance to each satellite. Geometric trilateration is used to combine these distances with the location of the satellites to determine the receivers locati

28、on. The position is displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units also show derived information such as direction and speed, calculated from position changes.It might seem three satellites are enough to solve for positio

29、n, since space has three dimensions. However, even a very small clock error multiplied by the very large speed of light17the speed at which satellite signals propagateresults in a large positional error. Therefore receivers use four or more satellites to solve for x, y, z, and t, which is used to co

30、rrect the receivers clock. While most GPS applications use the computed location only and effectively hide the very accurately computed time, it is used in a few specialized GPS applications such as time transfer, traffic signal timing, and synchronization of cell phone base stations.Although four s

31、atellites are required for normal operation, fewer apply in special cases. If one variable is already known (for example, a ship or plane may have known elevation), a receiver can determine its position using only three satellites. Some GPS receivers may use additional clues or assumptions (such as

32、reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a degraded position when fewer than four satellites are visible (see 18, Chapters 7 and 8 of 19, and 20). Position calculation introduction To provide an introductory des

33、cription of how a GPS receiver works, measurement errors will be ignored in this section. Using messages received from a minimum of four visible satellites, a GPS receiver is able to determine the satellite positions and time sent. The x, y, and z components of position and the time sent are designa

34、ted as where the subscript i is the satellite number and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received , the GPS receiver can compute the indicated transit time, . of the message. Assuming the message traveled at the speed of light, c, the distance traveled, can be

35、 computed as . Knowing the distance from GPS receiver to a satellite and the position of a satellite implies that the GPS receiver is on the surface of a sphere centered at the position of a satellite. Thus we know that the indicated position of the GPS receiver is at or near the intersection of the

36、 surfaces of four spheres. In the ideal case of no errors, the GPS receiver will be at an intersection of the surfaces of four spheres. The surfaces of two spheres, if they intersect in more than one point, intersect in a circle. A figure, Two Sphere Surfaces Intersecting in a Circle, is shown below

37、.Two sphere surfaces intersecting in a circleThe article, trilateration, shows mathematically that the surfaces of two spheres, intersecting in more than one point, intersect in a circle.A circle and sphere surface in most cases of practical interest intersect at two points, although it is conceivab

38、le that they could intersect at zero points, one point, or in the very special case in which the centers of the three spheres are colinear (i.e. all three on the same straight line) the sphere surface could intersect the entire circumference of the circle. Another figure, Surface of Sphere Intersect

39、ing a Circle (not disk) at Two Points, shows this intersection. The two intersections are marked with dots. Again trilateration clearly shows this mathematically. The correct position of the GPS receiver is the intersection that is closest to the surface of the earth for automobiles and other near-E

40、arth vehicles. The correct position of the GPS receiver is also the intersection which is closest to the surface of the sphere corresponding to the fourth satellite. (The two intersections are symmetrical with respect to the plane containing the three satellites. If the three satellites are not in t

41、he same orbital plane, the plane containing the three satellites will not be a vertical plane passing through the center of the Earth. In this case one of the intersections will be closer to the earth than the other. The near-Earth intersection will be the correct position for the case of a near-Ear

42、th vehicle. The intersection which is farthest from Earth may be the correct position for space vehicles.)Correcting a GPS receivers clockThe method of calculating position for the case of no errors has been explained. One of the most significant error sources is the GPS receivers clock. Because of

43、the very large value of the speed of light, c, the estimated distances from the GPS receiver to the satellites, the pseudoranges, are very sensitive to errors in the GPS receiver clock. This suggests that an extremely accurate and expensive clock is required for the GPS receiver to work. On the othe

44、r hand, manufacturers prefer to build inexpensive GPS receivers for mass markets. The solution for this dilemma is based on the way sphere surfaces intersect in the GPS problem.It is likely that the surfaces of the three spheres intersect, since the circle of intersection of the first two spheres is

45、 normally quite large, and thus the third sphere surface is likely to intersect this large circle. It is very unlikely that the surface of the sphere corresponding to the fourth satellite will intersect either of the two points of intersection of the first three, since any clock error could cause it

46、 to miss intersecting a point. However, the distance from the valid estimate of GPS receiver position to the surface of the sphere corresponding to the fourth satellite can be used to compute a clock correction. Let denote the distance from the valid estimate of GPS receiver position to the fourth s

47、atellite and let denote the pseudorange of the fourth satellite. Let . Note that is the distance from the computed GPS receiver position to the surface of the sphere corresponding to the fourth satellite. Thus the quotient, , provides an estimate of(correct time) - (time indicated by the receivers o

48、n-board clock), and the GPS receiver clock can be advanced if is positive or delayed if is negative. System detailUnlaunched GPS satellite on display at the San Diego Aerospace museumSystem segmentationThe current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).21Space segmentSee also: GPS satelliteand List of GPS satellite launchesA visual example of the GPS constellation in motion