GPS Overview
Chad Hall Trimble Navigation Ltd Mapping & GIS Division 1
Overview of the Presentation History of GPS Three Segments of GPS How to Get a Position Errors Sources in GPS Differential GPS Post-Processing Real-time Correction Accuracy Requirements Question & Answer
Your location is: 37o 23.323’ N 122o 02.162’ W
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History of the Global Positioning System Started development in 1973 First four satellites launched in 1978 Full Operational Capacity (FOC) reached in July 1995 System cost of $12 billion Maintained by the Department of Defense (DoD) GPS provides both civilian and military positioning globally GPS comprised of three “segments” Space Segment (the satellites) Ground Segment (the ground control network) User Segment (GPS receivers and their users)
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GPS Space Segment
GPS Constellation 24 satellites (Space Vehicles or SVs) 10,900 nautical miles above Earth One revolution every 12 hours 6 orbital planes (55° inclination) 4 satellites in each plane 4
GPS Control Segment
Ground Control & Monitoring Stations 1 Master Control - Falcon AFB, CO 4 Monitoring Stations Responsible for monitoring the health of satellites and the upload of navigation messages for broadcast
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GPS User Segment Multiple Applications
•GIS / CAD data capture •Vehicle tracking •Surveying •Construction •Pipeline mapping •Utility asset collection •Agriculture •Photogrammetry •Recreation (hiking etc.)
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How to Get a Position
Need signal from at least four satellites for a 3D position One satellite provides a time reference Distance to three remaining satellites is determined by observing the GPS signal travel time from satellite to the receiver With three known points, and distances to each, we can determine the GPS receiver’s position (trilateration) 7
Errors Sources in GPS Obstruction Atmospheric Delay Multipath Selective Availability (S/A) Before May 2000 = 100m After May 2000 = 15m
Question: Is 15 meters of error good enough accuracy for mapping and survey professionals? Answer: No, reason why one must post-process or use a real-time differential correction source in order to achieve better accuracies. 8
Post-Processing Satellites
Rover (GPS Receiver) 15 m of accuracy Base Station (GPS receiver) at a known location (1 cm of accuracy)
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Real-Time Differential Correction Satellites
Satellite Correction Service
Cell Phone Corrections
Coastguard Beacon
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Example of Uncorrected File (15 meters)
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Example of Corrected File (<1 meter)
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Recreational Grade GPS
15 m 13
Mapping Grade GIS
Satellite Correction Service
Coastguard Beacon
2-3 m Cellular
Post-Process 14
Mapping Grade GIS
Satellite Correction Service
Coastguard Beacon
<1m Cellular
Post-Process 15
High Accuracy GPS
Cellular
RTK Base Station
1-2 cm 16
Where Do You Need to Be?
1-2 cm
<1m 2-3 m 15 m 17
Questions?
Your location is: 37o 23.323’ N 122o 02.162’ W
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