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.
10、It has no name. A position represents only an approximation of the receivers true location. A position is not static. It changes constantly as the GPS receiver moves (or wanders due to random errors). A receiver must be in 2D or 3D mode (at least 3 or 4 satellites acquired) in order to provide a pos
11、ition fix. 3D mode dramatically improves position accuracy.,Waypoint,A waypoint is based on coordinates entered into a GPS receivers memory. It can be either a saved position fix, or user entered coordinates. It can be created for any remote point on earth. It must have a receiver designated code or
12、 number, or a user supplied name. Once entered and saved, a waypoint remains unchanged in the receivers memory until edited or deleted.,Planning a Navigation Route,Start,= Waypoint,How A Receiver Sees Your Route,GPS Waypoint Circle of Error,X,GPS Dilution of Precision and Its Affects On GPS Accuracy
13、,GPS Satellite Geometry,Satellite geometry can affect the quality of GPS signals and accuracy of receiver trilateration. Dilution of Precision (DOP) reflects each satellites position relative to the other satellites being accessed by a receiver. There are five distinct kinds of DOP. Position Dilutio
14、n of Precision (PDOP) is the DOP value used most commonly in GPS to determine the quality of a receivers position. Its usually up to the GPS receiver to pick satellites which provide the best position triangulation. Some GPS receivers allow DOP to be manipulated by the user.,Ideal Satellite Geometry
15、,N,S,W,E,Good Satellite Geometry,Good Satellite Geometry,Poor Satellite Geometry,N,S,W,E,Poor Satellite Geometry,Poor Satellite Geometry,Differential GPS,True coordinates = x+0, y+0 Correction = x-5, y+3,DGPS correction = x+(30-5) and y+(60+3) True coordinates = x+25, y+63,Real Time Differential GPS
16、,NDGPS Ground Stations,National Differential Global Positioning System,Yellow areas show overlap between NDGPS stations. Green areas are little to no coverage. Topography may also limit some areas of coverage depicted here.,NDGPS Ground Stations,National Differential Global Positioning System,Yellow
17、 areas show overlap between NDGPS stations. Green areas are little to no coverage. Topography may also limit some areas of coverage depicted here.,Wide Area Augmentation System,Geostationary WAAS satellites,GPS Constellation,WAAS Control Station (West Coast),Local Area System (LAAS),WAAS Control Sta
18、tion (East Coast),How good is WAAS?,With Selective Availability set to zero, and under ideal conditions, a GPS receiver without WAAS can achieve fifteen meter accuracy most of the time.*,Under ideal conditions a WAAS equipped GPS receiver can achieve three meter accuracy 95% of the time.*,* Precision depends on good satellite geometry, open sky view, and no user induced errors.,
