The document describes how the Global Positioning System (GPS) works by using a network of 20 satellites that transmit coded signals to receivers on Earth. GPS locates receivers by comparing the time differences between satellite signals, which allows calculation of distance. At least four satellites are needed to determine location on Earth through triangulation. GPS is used by various industries and provides location accuracy down to centimeters when correcting for different sources of error.

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