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
 One – distance only
 Two – point
falls on a circle
 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