最多9999字，除非一单词两字，才能贴够4000词，看这个中意不英文论文我这里很多，需要别的话再说。选自ATI在线课程The Global Positioning System by Robert A Nelson On a recent trip to visit the Jet Propulsion Laboratory， I flew from Washington， DC to Los Angeles on a new Boeing 747-400 The geographical position of the plane and its relation to nearby cities was displayed throughout the flight on a video screen in the passenger When I arrived in Los Angeles， I rented a car that was equipped with a The navigator guided me to my hotel in Pasadena， displaying my position on a map and verbally giving me directions with messages like way exit ahead on the right followed by a left ? When I reached the hotel， it announced that I had arrived at my Later， when I was to join a colleague for dinner， I found the restaurant listed in a menu and the navigator took me This remarkable navigation capability is made possible by the Global Positioning System (GPS) It was originally designed jointly by the US Navy and the US Air Force to permit the determination of position and time for military troops and guided However， GPS has also become the basis for position and time measurement by scientific laboratories and a wide spectrum of applications in a multi-billion dollar commercial Roughly one million receivers are manufactured each year and the total GPS market is expected to approach $ 10 billion by the end of next The story of GPS and its principles of measurement are the subjects of this EARLY METHODS OF NAVIGATION The shape and size of the earth has been known from the time of The fact that the earth is a sphere was well known to educated people as long ago as the fourth century BC In his book On the Heavens， Aristotle gave two scientifically correct First， the shadow of the earth projected on the moon during a lunar eclipse appears to be Second， the elevations of stars change as one travels north or south， while certain stars visible in Egypt cannot be seen at all from G SPACE SEGMENT There have been several generations of GPS The Block I satellites， built by Rockwell International， were launched between 1978 and They consisted of eleven prototype satellites， including one launch failure， that validated the system The ten successful satellites had an average lifetime of 76 The Block II and Block IIA satellites were also built by Rockwell I Block II consists of nine satellites launched between 1989 and Block IIA， deployed between 1990 and 1997， consists of 19 satellites with several navigation In April 1995， GPS was declared fully operational with a constellation of 24 operational spacecraft and a completed ground The 28 Block II/IIA satellites have exceeded their specified mission duration of 6 years and are expected to have an average lifetime of more than 10 Block IIR comprises 20 replacement satellites that incorporate autonomous navigation based on crosslink These satellites are being manufactured by Lockheed M The first launch in 1997 resulted in a launch The first IIR satellite to reach orbit was also launched in The second GPS 2R satellite was successfully launched aboard a Delta 2 rocket on October 7， One to four more launches are anticipated over the next The fourth generation of satellites is the Block II follow-on (Block IIF) This program includes the procurement of 33 satellites and the operation and support of a new GPS operational control The Block IIF program was awarded to Rockwell (now a part of Boeing) Further details may be found in a special issue of the Proceedings of the IEEE for January， CONTROL SEGMENT The Master Control Station for GPS is located at Schriever Air Force Base in Colorado Springs， CO The MCS maintains the satellite constellation and performs the stationkeeping and attitude control It also determines the orbit and clock parameters with a Kalman filter using measurements from five monitor stations distributed around the The orbit error is about 5 GPS orbits are derived independently by various scientific organizations using carrier phase and post- The state of the art is exemplified by the work of the International GPS Service (IGS)， which produces orbits with an accuracy of approximately 3 centimeters within two The system time reference is managed by the US Naval Observatory in Washington， DC GPS time is measured from Saturday/Sunday midnight at the beginning of the The GPS time scale is a composite aper clock that is synchronized to keep step with Coordinated Universal Time (UTC) and International Atomic Time (TAI) However， UTC differs from TAI by an integral number of leap seconds to maintain correspondence with the rotation of the earth， whereas GPS time does not include leap The origin of GPS time is midnight on January 5/6， 1980 (UTC) At present， TAI is ahead of UTC by 32 seconds， TAI is ahead of GPS by 19 seconds， and GPS is ahead of UTC by 13 Only 1，024 weeks were allotted from the origin before the system time is reset to zero because 10 bits are allocated for the calendar function (1，024 is the tenth power of 2) Thus the first GPS rollover occurred at midnight on August 21， The next GPS rollover will take place May 25， SIGNAL STRUCTURE The satellite position at any time is computed in the user receiver from the navigation message that is contained in a 50 bps data The orbit is represented for each one hour period by a set of 15 Keplerian orbital elements， with harmonic coefficients arising from perturbations， and is updated every four This data stream is modulated by each of two code division multiple access， or spread spectrum， pseudorandom noise (PRN) codes: the coarse/acquisition C/A code (sometimes called the clear/access code) and the precision P The P code can be encrypted to produce a secure signal called the Y This feature is known as the Anti-Spoof (AS) mode， which is intended to defeat deception jamming by The C/A code is used for satellite acquisition and for position determination by civil The P(Y) code is used by military and other authorized GPS MODERNIZATION In 1996， a Presidential Decision Directive stated the president would review the issue of Selective Availability in 2000 with the objective of discontinuing SA no later than In addition， both the L1 and L2 GPS signals would be made available to civil users and a new civil 23 MHz signal would be To satisfy the needs of aviation， the third civil frequency， known as L5， would be centered at 45 MHz， in the Aeronautical Radio Navigation Services (ARNS) band， subject to approval at the World Radio Conference in According to Keith McDonald in an article on GPS modernization published in the September， 1999 GPS World， with SA removed the civil GPS accuracy would be improved to about 10 to 30 With the addition of a second frequency for ionospheric group delay corrections， the civil accuracy would become about 5 to 10 A third frequency would permit the creation of two beat frequencies that would yield one-meter accuracy in real A variety of other enhancements are under consideration， including increased power， the addition of a new military code at the L1 and L2 frequencies， additional ground stations， more frequent uploads， and an increase in the number of These policy initiatives are driven by the dual needs of maintaining national security while supporting the growing dependence on GPS by commercial When these upgrades would begin to be implemented in the Block IIR and IIF satellites depends on GPS Besides providing position， GPS is a reference for time with an accuracy of 10 nanoseconds or Its broadcast time signals are used for national defense， commercial， and scientific The precision and universal availability of GPS time has produced a paradigm shift in time measurement and dissemination， with GPS evolving from a secondary source to a fundamental reference in The international community wants assurance that it can rely on the availability of GPS and continued US support for the The Russian Global Navigation Satellite System (GLONASS) has been an alternative， but economic conditions in Russia have threatened its continued Consequently， the European Union is considering the creation of a navigation system of its own， called Galileo， to avoide relying on the US GPS and Russian GLONASS The Global Positioning System is a vital national Over the past thirty years it has made the transition from concept to reality， representing today an operational system on which the entire world has become Both technical improvements and an enlightened national policy will be necessary to ensure its continued growth into the twenty-first
