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# Global positioning system

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### Global positioning system

1. 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. 2. Local Receivers Generate same signals as satellite  Compare differences to determine distance to satellite  Clocks not nearly as accurate  Some errors occur
3. 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. 4. Multiple Satellites One – distance only Two – point falls on a circle Three – one of two points 4 or more – average to eliminate error
5. 5. Who Uses GPS? Surveyors Military Agriculture Railroads Corporate fleets Auto Security
6. 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. 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. 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. 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. 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