GEO 241: GPS Global Positioning System

1. GEO 241: GPSGlobal Positioning System

2. 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

3. How does GPS Work? • Space Segment • Control Segment • User Segment Image courtesy of http://www.aero.org/publications/GPSPRIMER/GPSElements.html

4. 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

5. 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

6. 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/

7. User Segment • Applications • Location • Navigation • Mapping • Tracking Image courtesy of http://www.nmt.edu/~mreece/gps/whatisgps.html

8. 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)

9. Satellite Signal cont. • Signals cannot penetrate water, soil, buildings and other obstacles. Image courtesy of students.washington.edu/ ruby42/me.htm

10. 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.

11. 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

12. 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)

13. Measuring the distance from a satellite Why is it so important to know the distance of the satellite?

14. Triangulation 50 millisecond 40 millisecond

15. Triangulation

16. Triangulation

17. Triangulation

18. Triangulation

19. ERRORS

20. 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*

21. 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

22. Differential GPS Figure Courtesy of Trimble Navigation Limited

23. 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

24. 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.)

25. 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

26. 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

27. 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.

28. 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