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Published byElwin Sullivan Modified over 9 years ago
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GPS Receivers: Basics and Selection T.S. Stombaugh, J.D. Luck and S.A. Shearer Biosystems and Agricultural Engineering University of Kentucky
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Global Positioning System Space-based radio navigation facility System of 24 Rockwell satellite vehicles (plus a few obsolete) maintained by the DoD Altitude is 12,500 miles Radio signals broadcast from satellites allow users to calculate position
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Distance to Satellite Lightening vs. Thunder Radio signals travel at the speed of light Let’s put some numbers on it Altitude = 12,500mi Speed of light = 186,000 mi/s Transmission Time = 0.067 s What happens if we are off a little? 0.001 s * 186,000 mi/s = 186 mi
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Triangulation
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Satellite Position Satellites broadcast several pieces of information including Atomic clock corrections Ephemerides - parameters to describe position and velocity of each satellite within its orbit
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GPS Errors Clock Ephemeris Satellite Configuration Atmospheric interference Multipath Selective Availability (SA)
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Dilution of Precision Low DOPHigh DOP
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Multipath Error
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Atmospheric Interference
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Selective Availability The intentional dithering of timing and ephemeris information by the DoD intended to prevent hostile use of GPS technology.
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Local Area DGPS Establish your own base station Radiobeacon (USCG) National DGPS (NDGPS) Local Area Augmentation System (LAAS) RTK Arrays RTK Networks More on these later….
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Roving Receiver Correction Signal Base Station
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USCG Radiobeacon Correction
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Wide Area DGPS Sources Commercial satellite providers Omnistar Racal Deere Wide Area Augmentation System (WAAS) Global DGPS (GDPS)
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Roving Receiver Correction Signal Base Station
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RTK DGPS Real Time Kinematic or, Carrier Phase DGPS Accuracy of Mobile Rovers: under 2.0 cm horiz. and 5.0 cm vert. Accuracy of Static Surveying Units: under 1.0 cm horiz. and 2.5 cm vert.
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RTK Correction Signal RTK Roving Receiver RTK Base Station GPS Satellites
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Static Accuracy 1 m
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Terms Error – difference between measurement and actual value Accuracy – lack of error Precision – random errors Bias – systematic errors
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Reporting Methods Visualize circles that contain the majority of data points: CEP – 50% of measurements 95% CEP - 95% of measurements 2DRMS – 95.4 to 98.2% of measurements 1 Sigma – 66.7% of measurements 2 Sigma – 95% of measurements Be certain that you compare apples to apples -- be aware of accuracies reported as CEP or 1 Sigma!
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Static Accuracy – 95% CEP 0.83 m
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Dynamic Testing
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Ideal Path 15 m
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Test Results
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Filter Evaluation No Filter Filter
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Purchase Considerations Update rate (1, 5 or 10 Hz)? Accuracy? Correction signal capabilities (Radiobeacon, commercial satellites, or WAAS)? Compatibility (RS-232 or CAN; data format)? Reliability? Service after the sale? Expected life?
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Interfacing User Interface On-unit programming External programming Data Interface – Software NMEA 0183 Data Interface – Hardware RS-232 CAN
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Satellite Positioning Systems GPS (United States) GLONASS (Russia) EGNOS (European) - GALILEO
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Summary Update rate Accuracy Correction Method Compatibility Reliability Service Useful Life
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