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Differential GPS and Error correction

Differential GPS and Error correction. By Gatien, Nb_gatien@yahoo.fr. Outline. 1- Intro: GPS Basics 2- GPS uses today 3- GPS accuracy 4- Errors correction 5- Differential correction. 1- GPS Basics.

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Differential GPS and Error correction

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  1. Differential GPS and Error correction By Gatien, Nb_gatien@yahoo.fr

  2. Outline • 1- Intro: GPS Basics • 2- GPS uses today • 3- GPS accuracy • 4- Errors correction • 5- Differential correction

  3. 1- GPS Basics • GPS = Global Positioning System = a process used to establish a position at any point on the globe • A constellation of • 28 satellites orbiting the earth • Determine • - (a) the precise location (longitude, latitude and height co-ordinates) • - (b) the precise time (Universal Time Coordinated,UTC)

  4. 1- GPS Basics (continued) • At least 4 satellites are needed to determine a location

  5. 1- GPS Basics (continued)

  6. 2-The uses of GPS today (1) • Science and research • Commerce and industry • Agriculture and forestry • Communications technology • Tourism and sport • Military • …

  7. 2-The uses of GPS today

  8. Agriculture and forestry

  9. 2-The uses of GPS today Science and research

  10. 2-The uses of GPS today Transport….

  11. 3- GPS Accuracy Errors …

  12. 4- Sources of GPS errors Various elements contribute to errors…

  13. 4- Sources of GPS errors Satellite Clock : One billionth of a second (one nanosecond) of inaccuracy in a satellite clock  in about 30 centimeters . For this reason, the satellites are equipped with very accurate (Cesium) atomic clocks Satellite Orbit Error : the accuracy of our computed position also depends on how accurately we know the location of the satellites (the points of references). - satellites orbit are monitored continuously from ground stations – Positions are then transmitted to GPS

  14. 4- Sources of GPS errors • Atmospheric Errors: Ionosphere and Troposphere: • the speed of light varies due to atmospheric conditions. • The upper layer of the atmosphere, called the ionosphere, contains charged particles that slow down the code and speed up the carrier. • The lower level of the atmosphere, which contains water vapors, is called the troposphere. It has the effect of slowing down both code and carrier. Selective Availability: The Man-Made Errors : Errors in the satellite clock, the satellite orbit, the ionosphere, the troposphere, the multipath, and the receiver typically amount to less than 10 meters of range error which, under typical GDOPs of about 2, results in a position accuracy of about 20 meters.

  15. 4- Sources of GPS errors Geometric Dilution Of Precision (GDOP) : The problem is that the speed of light varies due to atmospheric conditions. The upper layer of the atmosphere, called the ionosphere, contains charged particles that slow down the code and speed up the carrier. The lower level of the atmosphere, which contains water vapors, is called the troposphere. It has the effect of slowing down both code and carrier .

  16. 4- Sources of GPS errors Multipath :If the indirect path is considerably longer than the direct path (more than 10 meters) such that the two patterns of signals can be separated, then the multipath effect can be substantially reduced by signal processing techniques. . Receiver Errors : Receivers may introduce some errors (mm – cm)

  17. 5- Differential correction Differential Mode Assume you have two receivers not too far from each other. The errors due to the satellite clock, the satellite orbit, the ionosphere, the troposphere and SA affect both receivers the same way and with the same magnitude. If we knew the exact location of one receiver, we could use that information to calculate errors in the measurement and then report these errors (or correction values) to the other receiver, so that it could compensate for them. This technique is called Differential mode.

  18. 5- Differential correction

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