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GPS

GPS. The Global Positioning System. The Global Positioning System. A network of satellites that continuously transmits coded information, which makes it possible to precisely identify locations on earth by measuring distance from the satellites. The Global Positioning System.

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GPS

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  1. GPS The Global Positioning System

  2. The Global Positioning System • A network of satellites that continuously transmits coded information, which makes it possible to precisely identify locations on earth by measuring distance from the satellites.

  3. The Global Positioning System • Became fully operational in 1995 • 24 satellites in orbit • Each orbits the earth once every 12 hours at an altitude of 12,600 miles (most satellites are at approximately 100 miles).

  4. GPS Segments • GPS consists of three segments: • Control Segment • Space Segment • User Segment

  5. Control Segment • Master Control Station at Colorado Springs • Numerous monitoring stations and antennas • Monitoring stations track all satellites in view. This info is processed at the MCS to determine orbits and update each satellite’s navigation message. • Insures transmitted signal is as error-free as possible

  6. Space Segment • NAVSTAR satellites built by Rockwell International and Hughes • Cost: Approximately $40 million each • Well over $10 billion total cost to date.

  7. User Segment • Consists of antennas and receiver-processors that provide positioning, velocity, and precise timing to the user.

  8. How GPS Works • GPS is based on satellite ranging • We can calculate our position on earth by measuring our distance from a group of satellites in space • Satellites continually broadcast a signal which our GPS units receive • A distance to the satellite is calculated by timing how long it takes the satellite signal to reach our GPS unit. • Uses an atomic clock and a type of radio signal • We can calculate an accurate 3D ground position using 4 satellites

  9. Accuracy • Accuracy varies greatly depending on the type of GPS unit used and the quality of the satellite signal • Ideal conditions, clear view of sky: 5–15 meters • Heavy forest canopy: up to 30 meters or more • WAAS corrected: < 3 meters • DGPS: base station accurately surveyed • Military, construction uses

  10. Factors that affect accuracy • Number of visible satellites • Satellite signal strength • Errors in clocks and orbits • Multipath • Quality of receiver • Satellite geometry • Number of observations recorded

  11. Number of visible satellites • The more satellites a GPS receiver can track, the better the accuracy. Satellite signals can be blocked by: • Topography • Dense forest canopy • Buildings

  12. GPS Signal Strength • Atmospheric Interference • The GPS signal slows as it passes through the atmosphere • Signals from satellites low in the sky pass through more atmosphere • Tree canopy can also reduce the strength

  13. Multipath • Reflection of the GPS signal off buildings, large rock surfaces or water bodies before it reaches the receiver • Multipath signals have traveled a longer distance which produces errors in signal timing

  14. Satellite Geometry • Ideal satellite geometry exists when the satellites are located at wide angles relative to each other • Poor satellite geometry results when the satellites are located in a tight grouping

  15. Number of observations recorded • Waypoint averaging • Allow receiver to receive signal at same location for long period • Accuracy increased most after 30 minutes

  16. Using a GPS Unit • Learning to use a GPS Unit is easy • Use the manual • http://www.garmin.com/manuals/GPSMAP76S_QuickStartGuide.pdf • Practice

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