1. Global Positioning Systems

2. Why GPS? Challenges of finding exact location by traditional methods Astronomical observation Adjustments based on gravity variation Development of datums An exact location is found by measuring back to the nearest control point += Challenges: tedious monuments move! really not practical over large areas A continent- sized series of control points

3. A new approach … GPS US Dept of Defense, cir. 1994 24 satellites ~11,000 miles above earth Transmit precise location and time, 24/7

4. Three Segments of GPS: Satellites Receivers Ground stations

5. The Satellites 24 satellites: 4 “visible” at any one time, anywhere Super-accurate atomic clocks Constantly broadcast radio signals

6. The Receiver Receives signals from satellites Identifies transmitting satellites Calculates distance from each satellite Calculates present location

7. How a GPS receiver calculates distance Receives from satellite the precise time the signal was broadcast Receiver also has a clock, so … Calculates time for signal to arrive A radio signal always travels at the same speed … the “speed of light” Travel time  Speed of light = distance from satellite “Speed of light” = 186,000 miles/second e.g.: 1/10 sec x 186,000 mi/sec = 18,600 miles

8. Triangulation Usually one of the two points measured can be discarded because it doesn’t make sense

9. Triangulation, 2 (quadangulation?)  4 th satellite: Identifies error in clock of GPS receiver, and allows receiver to compute the correction needed  The receiver computes its position almost instantaneously  Gives location in BOTH lat-long and UTM

10. The Ground Stations 5 precisely-located stations that monitor and correct… the orbit of the satellites … and the accuracy of their clocks These allow the satellites to broadcast their location correctly

11. GPS unit displays of satellites

12. Accuracy of the GPS: Limitations Selective Availability Turned off on May 2, 2000 Created inaccuracy of up to 150 meters Typical accuracy for today’s “consumer GPS units”: 10 – 15 meters

13. Sources of error in GPS Precision of receiver Satellite clock errors Atmospheric conditions Lower atmosphere – weather – water vapor Upper atmosphere (ionosphere) – ions – electrically charged particles Slows down satellite signals: Result is like clock error How can we compensate for all these errors?

14. Visibility of satellites Other sources of error in GPS: Positions of satellites Better Worse VISIBILITY ISSUES How can we compensate for all these errors?

15. A solution: Differential GPS Stationary GPS receiver, + radio transmitter, at known location 1.Receives signals from all visible satellites 2.Calculates timing delays in signal from each satellite 3.Radio transmits this info to nearby “roving” GPS receivers 4.Roving receiver can correct for same timing delays Base station (e.g. WAAS) Error with Differential GPS: WAAS < 3 meters Custom base station < 1 meter