Global Positioning System (GPS)

1. Global Positioning System (GPS) • Satellite Location • 20 Satellites in system • Each carries very accurate clock • Sends a coded signal every millisecond • Ground Stations • Track satellite locations • Send signals to satellites to correct errors

2. Local Receivers • Generate same signals as satellite • Compare differences to determine distance to satellite • Clocks not nearly as accurate • Some errors occur

3. GPS Location • Location found by comparing signals • Time Difference x Light Speed = Distance • Clocks accurate to nanosecond (1 x 10-9 s) • Accuracy = (186,282 mi/s)(5280 ft/mi)(1 x 10-9 s)= 0.984 ft • How do we locate to the cm?

4. Multiple Satellites • Two – point falls on a circle • One – distance only • Three – one of two points • 4 or more – average to eliminate error

5. Who Uses GPS? • Surveyors • Military • Agriculture • Railroads • Corporate fleets • Auto Security

6. Where Are We? • Longitude • Starts at Greenwich • 180° E or W • Longitude • Starts at Equator • 90° N or S • Moorhead • N 46°52’ Latitude, W 96°45’ Longitude

7. Convert to Grid • Projections • Lambert’s Conformal • Transverse Mercator • GPS uses UTM • Error limited to 1:10,000 • Computer models convert Lat, Long to universal grid

8. Sources of Error • Atmospheric • Built-in Compensation • Receivers • More satellites, better averaging • Selective Availability • Intentional Error – DOD • Same error at any given time • Eliminate by differential positioning

9. Relative Positioning • Dynamic • Two receivers, one set over known • Both collect data over time • Computer determines correction after • Real-time Kinetic • Permanent receiver over know • Transmit real-time corrections to field • Farmers, transit authorities

10. PDOP • Position Dilution of Precision • Numeric scale indicating precision • Depends on position, number of satellites • Satellite alignment could limit precision • PDOP = 1: Best precision • PDOP = 5: Very poor – try later