The document describes how the Global Positioning System (GPS) works by using satellites and receivers. It explains that GPS uses 20 satellites that transmit coded signals to receivers, which then use the time differences between satellite signals to calculate the receiver's distance from each satellite and triangulate its position. The document also discusses some applications of GPS like surveying, as well as sources of error and techniques to improve accuracy like using multiple satellites and differential positioning.

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