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GEO 241: GPS Global Positioning System. What is GPS?. Satellite based navigation system Developed by the Dept. of Defense $12 billion system Began construction in mid ’70s Developed for military operations, but with a provision for civilian use
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What is GPS? • Satellite based navigation system • Developed by the Dept. of Defense • $12 billion system • Began construction in mid ’70s • Developed for military operations, but with a provision for civilian use • Consists of 24 (28) high orbit satellites sending out coded radio signals that are picked up by receivers that calculate position
How does GPS Work? • Space Segment • Control Segment • User Segment Image courtesy of http://www.aero.org/publications/GPSPRIMER/GPSElements.html
Space Segment • 24+ NAVSTAR(NAVigation Satellite Timing And Ranging) • Orbit is 20,200 km • 1 revolution in 12 hours • Satellite spec: • 1900lbs • 12 ft with solar panels extened • 12 hour orbital periods Image courtesy of http://www.nmt.edu/~mreece/gps/whatisgps.html
Control Segment • Receives/Transmits Information to SV • Master Control Station – Colorado Springs • Monitoring stations distributed throughout the world. Image courtesy of http://www.nmt.edu/~mreece/gps/whatisgps.html
Control Segment cont. • Receives/Transmits Information to SV • Master Control Station – Colorado Springs • Monitoring stations distributed throughout the world. Image courtesy of http://www.colorado.edu/geography/gcraft/
User Segment • Applications • Location • Navigation • Mapping • Tracking Image courtesy of http://www.nmt.edu/~mreece/gps/whatisgps.html
Satellite Signal • GPS satellites transmit information via radio signals on 2 frequencies • Radio waves are electromagnetic waves, like light, and travel at 186,000 miles per second • A satellite overhead will transmit its signal to us in 6/100ths of a second • Most receivers measure in nanoseconds (0.000000001 second)
Satellite Signal cont. • Signals cannot penetrate water, soil, buildings and other obstacles. Image courtesy of students.washington.edu/ ruby42/me.htm
Satellite Almanac • Sent along with timing and position information. • Prediction of all satellite orbit. • Needed to run mission planning using certain software. • Some GPS receivers will receive and store the almanac. • Valid for about 30 days.
Satellite Ranging • Speed of light x time = distance • EX. Car traveling at 30 mi/hr • In 1 hr, what is the distance the care traveled? • 30mi/hr * 1 hr = 30mi • Speed of the radio signal • Equals the speed of light (300,000 km per second or 186,000 miles) 50 millisecond 40 millisecond
Measuring the distance from a satellite • Speed of light x time = distance • Radio waves are electromagnetic waves, like light, and travel at 186,000 miles per second • Each satellite is equipped with an atomic clock • Most receivers measure in nanoseconds (0.000000001 second)
Measuring the distance from a satellite Why is it so important to know the distance of the satellite?
Triangulation 50 millisecond 40 millisecond
GPS Errors (typical) • Satellite clock error 2 feet • Ephemeris error 2 feet • Receiver error 4 feet • Atmospheric delay 12 feet • Selective Availability (if on) 25 feet • Multipath errors and obstructions*
Differential GPS • A receiver placed at a known location calculates the combined error in the satellite range data • That correction can be applied to all other receivers in the same locale, to eliminate virtually all error in their measurements. • This can be done in “real time” or by “post-processing” the data after collection
Differential GPS Figure Courtesy of Trimble Navigation Limited
Differential GPS Methods • Postprocessed Differential • Error are corrected after data has been collected • Real-time Differential • Errors are corrected in the field • WAAS (Wide Area Augmentation Signal) • Errors are corrected in the field
WAAS (Wide Area Augmentation Signal) • Developed by Federal Aviation Administration • Free differential correction • 1 - 3 meter accuracy • Will allow aircraft navigation with GPS • There are 2 geostationary WAAS satellites (east & west coasts of U.S.)
Accuracy • High accuracy requires more rigorous data collection parameters. • Logging Interval • the frequency position data is collected • Elevation Mast • The amount of sky viewable • Signal-To-Noise Ratio • Strength satellite signal compare to noise • Position of Dilution of Precision • Satellite Geometry
Logging Interval • Logging interval defines the frequency at which a position is stored. • Point features – Set at 1 second • Line/Area features – should match the base station logging interval. Logging intervals may also depend on speed of travel: • If walking – 5 seconds • If driving – 1 second
Elevation Mask • Elevation masks restrict your receiver to using only those satellites above a certain elevation in the sky. • This ensures that a base station can always see all the satellites used by the rover. • The default elevation mask for a rover is 15* • Lowing masks increase # of satellites, increases atmospheric delay and increases effects of multipath.
Signal to Noise Ratio Mask • As the proportion between the signal and the noise decreases, data is distorted by the noise. • The higher the value of the SNR, the better. • Quality is degraded if it falls below 6.0 • Typical SNRs range between 10 and 25