IntroductiontoNAVSTAR GPS.ppt
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1、Introduction to NAVSTAR GPS,Charlie Leonard, 1999 (revised 2001, 2002),Feasibility studies begun in 1960s. Pentagon appropriates funding in 1973. First satellite launched in 1978. System declared fully operational in April, 1995.,The History of GPS,How GPS Works,Control Segment,Space Segment,User Se
2、gment,Three Segments of the GPS,Monitor Stations,Ground Antennas,Master Station,Kwajalein Atoll,US Space Command,Control Segment,Hawaii,Ascension Is.,Diego Garcia,Cape Canaveral,Space Segment,Military. Search and rescue. Disaster relief. Surveying. Marine, aeronautical and terrestrial navigation. Re
3、mote controlled vehicle and robot guidance. Satellite positioning and tracking. Shipping. Geographic Information Systems (GIS). Recreation.,User Segment,Position and coordinates.The distance and direction between any two waypoints, or a position and a waypoint.Travel progress reports.Accurate time m
4、easurement.,Four Basic Functions of GPS,Position is Based on Time,T + 3,Distance between satellite and receiver = “3 times the speed of light”,T,Signal leaves satellite at time “T”,Signal is picked up by the receiver at time “T + 3”,Pseudo Random Noise Code,Receiver PRN,Satellite PRN,Time Difference
5、,What Time is It?,Zulu Time,Military Time (local time on a 24 hour clock),Universal Coordinated Time,Greenwich Mean Time,Local Time: AM and PM (adjusted for local time zone),GPS Time + 13*,* GPS Time is ahead of UTC by approximately 13 seconds,Signal From One Satellite,The receiver is somewhere on t
6、his sphere.,Signals From Two Satellites,Three Satellites (2D Positioning),Triangulating Correct Position,Three Dimensional (3D) Positioning,Selective Availability (S/A),The Defense Department dithered the satellite time message, reducing position accuracy to some GPS users. S/A was designed to preve
7、nt Americas enemies from using GPS against us and our allies. In May 2000 the Pentagon reduced S/A to zero meters error. S/A could be reactivated at any time by the Pentagon.,Sources of GPS Error,Standard Positioning Service (SPS ): Civilian UsersSource Amount of Error Satellite clocks: 1.5 to 3.6 m
8、eters Orbital errors: 1 meter Ionosphere: 5.0 to 7.0 meters Troposphere: 0.5 to 0.7 meters Receiver noise: 0.3 to 1.5 meters Multipath: 0.6 to 1.2 meters Selective Availability (see notes) User error: Up to a kilometer or more Errors are cumulative and increased by PDOP.,Receiver Errors are Cumulati
9、ve!,Sources of Signal Interference,Using GPS Receivers for Positioning and Navigation,GPS Navigation Terminology,Active GOTO Waypoint,Location Where GOTO Was Executed,GPS Navigation: On the Ground,Position Fix,A position is based on real-time satellite tracking. Its defined by a set of coordinates.
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