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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


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) ‹


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



GPS User Segment Multiple Applications


•GIS / CAD data capture •Vehicle tracking •Surveying •Construction •Pipeline mapping •Utility asset collection •Agriculture •Photogrammetry •Recreation (hiking etc.)


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)


Real-Time Differential Correction Satellites

Satellite Correction Service

Cell Phone Corrections

Coastguard Beacon


Example of Uncorrected File (15 meters)


Example of Corrected File (<1 meter)


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


RTK Base Station

1-2 cm 16

Where Do You Need to Be?

1-2 cm

<1m 2-3 m 15 m 17


Your location is: 37o 23.323’ N 122o 02.162’ W