how does gps work ?. objectives to describe: the 3 components of the global positioning system how...
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How Does GPS Work ?
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Objectives
To Describe:
• The 3 components of the Global Positioning System
• How position is obtaining from a radio timing signal
• Obtaining instantaneous positions
• High precision GPS and how it works
• The method of operation
• Errors and data quality issues with GPS
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The Global Positioning System
• 24+ satellites
• 20,200 km altitude
• 55 degrees inclination
• 12 hour orbital period• 5 ground control stations• Each satellite passes
over a ground monitoring station every 12 hours
• 3 Components = a) Ground Control Segment; b) Space Segment; and c) User Segment
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Obtaining a Position From a Radio Timing Signal
• Precise atomic clocks are needed to measure travel time
• Getting perfect timing means synchronizing the satellite with the receiver
• The travel time for a satellite directly overhead is about 0.06 seconds
• The difference in sync of the receiver time minus the satellite time is the travel time
• a GPS receiver determines the travel time of a signal from a satellite by comparing the "pseudo random code" it's generating, with an identical code in the signal from the satellite
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Instantaneous positioning with GPS
Accuracy of• +/- 10 m (30 ft) error (horizontal)• +/- 15 m (45 ft) error (vertical)
Your location is:37o 23.323’ N
122o 02.162’ W
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High-precision GPS
D
-Use the carrier phase, not pseudorange-Dual-frequency receivers (to remove ionosphere)-High-precision orbital information-Good monuments-Multiple stations (to remove satellite clock variations)-Sophisticated processing software-Collect lots of data
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High-Precision Systems for Field Surveying
Trimble 5700 geodetic receiver with flat antenna
Movable tripod with choke-ring antenna
Monument stability and centering and leveling the antenna are critical !
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Method of Operation
• Global positioning system (GPS)
• Receiver determines location, speed, direction, and time
• 3 satellite signals are necessary to locate the receiver in 3D space
• 4th satellite is used for time accuracy
• Position calculated within sub-centimeter scale
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Atmospheric Delays
Ionosphere (use dual frequency receivers) Troposphere (estimate troposphere)
Troposphere
Ionosphere
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Multipath ‘echoes’
• Similar issue to ‘ghost’ images on TV• Causes inaccurate measurements• If we were outside right now, what would cause
potential multipath errors?
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Summary
• 3 components to GPS are: 1) Ground Control Segment; 2) Space Segment; and 3) User Segment
• Radio signals are matched in satellite and receiver to obtain travel times used to calculate distance and position
• High Precision GPS can obtain millimeter level accuracy (but at a cost)
• Atmospheric delays and multipath can cause GPS inaccuracies