1、Introduction to the Global Positioning System,Pre-Work,GPS for Fire Management - 2004,Describe at least three sources of GPS signal error, and identify ways to mitigate or reduce those errors. Identify the three segments of the Global Positioning System and describe the purpose of each. Describe two
2、 technologies that enhance the accuracy of GPS positioning and collected position data. Describe the process a GPS receiver uses to triangulate its position. Identify the four main functions of the GPS. Successfully complete the Pre-Work Test.,Pre-Work Objectives,A brief history of the Global Positi
3、oning System Segments of the GPS A primer on how the GPS works Problems with the GPS Advancements in the GPS,Unit A Lesson 1 Pre-Work Outline,The History of GPS,How the GPS Works,Control Segment,Space Segment,User Segment,Three Segments of the GPS,Monitor Stations,Ground Antennas,Master Station,Kwaj
4、alein 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. Remote controlled vehicle and robot guidance. Satellite positioning and tracking. Ship
5、ping. 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 measurement.,Four Primary Functions of GPS,Position is Based on Time,T + 3,Then dista
6、nce between satellite and receiver = “3 times the speed of light”,T,Radio waves travel at the speed of light. If GPS signal leaves satellite at time “T”,and is picked up by the receiver at time “T + 3.”,Pseudo Random Noise Code,Receiver PRN,Satellite PRN,Time Difference,What Time is it Anyway?,Zulu
7、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 currently ahead of UTC by 13 seconds.,Signal From One Satellite,The receiver is somewhere on this sphere.,Signals From T
8、wo 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 prevent Americas enemies from u
9、sing 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 meters Orbital errors: 1 me
10、ter 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.,Sources of Signal Interference,Receiver Errors are Cum
11、ulative!,GPS Dilution of Precision and Its Affects On GPS Accuracy,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
12、a receiver. There are five distinct kinds of DOP. Position Dilution of Precision (PDOP) is the DOP value used most commonly in GPS to determine the quality of a receivers position. It is usually up to the GPS receiver to pick satellites which provide the best position triangulation. More advanced GP
13、S receivers can filter out poor DOP values.,Ideal Satellite Geometry,N,S,W,E,Good Satellite Geometry,Poor Satellite Geometry,N,S,W,E,Poor Satellite Geometry,Differential GPS,Realtime Post process,True coordinates = x+0, y+0 Correction = x-5, y+3,DGPS correction = x+(30-5) and y+(60+3) True coordinat
14、es = x+25, y+63,Real Time Differential GPS,USCG 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.,USCG NDGPS Ground Stations,Na
15、tional 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.,Wide Area Augmentation System,Geostationary WAAS satellites,GPS Constellation,WAAS Control Station (W
16、est Coast),Local Area System (LAAS),WAAS Control Station (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.,
