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# How GPS Works

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For EPS Thursday Meeting.
(English Presentation Space)

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### How GPS Works

1. 1. Daniel – EPS 2010 How GPS Works
2. 2. What does GPS stand for again? GPS = Global Positioning System
3. 3.  Trilateration  Measuring distance  Accuracy of time and location of satellites
4. 4. Trilateration  A method for determining the intersections of three sphere surfaces given the centers and radii of the three spheres. Trilateration Triangulation
5. 5. Simplifying the problem Let’s assume several factors for easy understanding.  GPS Satellites are on the ground (3D  2D)  We know the exact location of satellites  We can calculate the distance from each of the satellites
6. 6. Start with first satellite 200km Satellite 1 Your location is somewhere on the circumference of the circle.
7. 7. Second satellite gives two points 200km Satellite 1 B 50km A Satellite 2 Your location must be one of the intersection. (Point A or B)
8. 8. Third satellite sets the location 200km Satellite 1 B 50km A Satellite 2 150km Satellite 3 Now you know you are on point A.
9. 9. Real trilateration in 3D space The intersection of two sphere is circle. (not two points as in 2D)
10. 10. Real trilateration in 3D space Knowing the distances from three satellites gives you two points.
11. 11. Speed Equation  GPS receiver uses speed equation to calculate the distance to satellites Distance = Speed × Duration (Time)
12. 12. Example of speed equation  If you don’t know the distance between pitcher and catcher, you can calculate it with the speed and duration of the ball Distance ??? Speed : 150km/h  150km/h ≈ 40m/s  If it took 1 second, the distance is about 40m.
13. 13. GPS satellites use radio signal  Instead of throwing balls, the GPS satellites send radio wave and GPS receivers catch them.  Radio wave fly at the same speed of light. (about 300,000 km/s)
14. 14. Signal from GPS satellites Each satellites continually transmits messages including :  the time the message was transmitted  presise orbital information (location of itself, ephemeris)  rough orbits of all GPS satellites (the almanac)
15. 15. Precise calculation of time
16. 16. • The key idea of positioning by GPS is ‘Trilateration’ • Two factors for trilateration 1. the location of each satellite (at least three satellites) 2. the distance from the satellite • Distance = speed of light × duration time • Satellites send radio signal including time and location