本科毕业论文外文文献及译文

本科毕业论文外文文献及译文文献、资料题目：Global Positioning System文献、资料来源：http:/en.wikipedia.org/wiki/Global_Positioning_System文献、资料发表（出版）日期：2009.01.03院 （部）： 土木工程学院专 业： 土木工程班 级： 测绘052姓 名： 学 号： 指导教师： 翻译日期： 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 GNSS 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 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 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 become 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 synchronization 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, 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 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). 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 successfully 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 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 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 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 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 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 atomic 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 White 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 preparation3Planned.(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.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 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 in 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 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-degraded 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 National 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 performance. 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 December 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 general 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 location. 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 position, 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 correct 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 satellites 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 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 description 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 designated 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 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 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.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 conceivable 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 Intersecting 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-Earth 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 the 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-Earth 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 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 other 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 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 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 satellite 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 on-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 with the Earth rotating. Notice how the number of satellites in view from a given point on the Earths surface, in this example at 45N, changes with time.The space segment (SS) comprises the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three circular orbital planes,22 but this was modified to six planes with four satellites each.23 The orbital planes are centered on the Earth, not rotating with respect to the distant stars.24 The six planes have approximately 55 inclination (tilt relative to Earths equator) and are separated by 60 right ascension of the ascending node (angle along the equator from a reference point to the orbits intersection).25 The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earths surface.26Orbiting at an altitude of approximately 20,200 kilometers about 10 satellites are visible within line of sight (12,900 miles or 10,900 nautical miles; orbital radius of 26,600km (16,500 mi or 14,400 NM), each SV makes two complete orbits each sidereal day.27 The ground track of each satellite therefore repeats each (sidereal) day. This was very helpful during development, since even with just four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.As of March 2008update,28 there are 31 actively broadcasting satellites in the GPS constellation, and two older, retired from active service satellites kept in the constellation as orbital spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.29Control segment The flight paths of the satellites are tracked by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA).30 The tracking information is sent to the Air Force Space Commands master control station at Schriever Air Force Base in Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the United States Air Force (USAF). Then 2 SOPS contacts each GPS satellite regularly with a navigational update (using the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellites internal orbital model. The updates are created by a Kalman filter which uses inputs from the ground monitoring stations, space weather information, and various other inputs.31Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked unhealthy, so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again.User segmentGPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as those shown here from manufacturers Trimble, Garmin and Leica (left to right).The users GPS receiver is the user segment (US) of the GPS. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007update, receivers typically have between 12 and 20 channels.32A typical OEM GPS receiver module measuring 1517 mm.GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of a RS-232 port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include Wide Area Augmentation Syste