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GPS
- 1. Global Positioning System An easy to use technology for everyone
- 2. What is GPS ? A very precise positioning system • Developed and maintained by the US Department of Defense (DOD) • Satellite Based * 24 satellites * 20,200 km high orbit
- 3. BUT! Although it is a very precise geographic positioning system •It is very easy to get youself into trouble •Why? •Because you (probably) don’t understand how it works •And that leads to garbage
- 4. Characteristics of GPS • Free • Precise • Reliable • Anytime & anywhere • All weather • Unlimited user capacity Almost!
- 5. Segments of GPS 1. Space Segment A constellation of 24 satellites 2. Monitor Station A network of earth-based facilities 3. Users & Equipment Source:Trimble
- 6. Segments of GPS Source:Trimble 1. Space Segment A constellation of 24 satellites
- 8. How GPS Works ……… Uses measurements from 4+ satellites Distance = travel time x speed of light Source:Trimble
- 9. Determining GPS Position • Suppose the distance from Satellite A to our position is 11,000 miles • At this point we could be located anywhere on the specified sphere Satellite A + • Next, let us take another measurement from a second satellite, Satellite BSatellite B + • Now our position is narrowed down to the intersection of theses two sphere
- 10. Satellite C + Determining GPS Position Satellite A Satellite B + + • Taking another measurement from a 3rd satellite narrows our position down even further, to the two points • So by ranging from 3 satellites we can narrow our position to just two points in space • These points are located where the 3rd sphere cuts through the the intersection of first two spheres
- 11. Satellite C + How do we decide which one is our true location? Satellite A Satellite B + + • We could make a 4th measurement from another satellite to determine the true point • However, GPS receivers use a 4th satellite to precisely locate our position • We can eliminate one of the two points that gives a ridiculous answer • The ridiculous point may be too far from the earth OR
- 13. How accurate is GPS? Depends on some variables • Design of receiver • Relative positions of satellites, technically known as PDOP (Position dilution of precision) • Postprocessing • Time spent on measurement
- 14. GPS Receivers
- 15. GeoXT Versus Garmin Mostly used for GIS data collection Mostly used for recreational purposes
- 16. Methods of data collection Three methods of positioning • Autonomous 10-20 meters • Differential 2-5 meters • Phase Differential centimeter 2-5m10-20 m cm
- 17. How accurate is GPS? Depends on some variables • Design of receiver • Relative positions of satellites, often known as DOP (Dilution of Precision) • Postprocessing • Time spent on measurement
- 18. WAAS • Wide Area Augmentation System. • It provides FREE GPS differential correction data for visible satellites • Developed & operated by the FAA (Federal Aviation Administration) for flight navigation but it’s available free to GPS users • WAAS-enabled receivers can provide sub-meter level accuracy anywhere in most locations of the US and southern Canada.
- 19. WAAS • Differential corrections are computed from ground stations and then uploaded to geostationary satellites for broadcasting • WAAS-enabled GPS receiver automatically uses such correction data to enhance the positional accuracy
- 21. Multipath • When GPS signals arrive at the receiver having traveled different paths
- 22. What is a PDOP? • Position Dilution of Precision Good PDOP Poor PDOP
- 23. SNR (signal-to-noise ratio) • SNR determines the signal strength relative to noise • GPS position is degraded if the SNR of one or more satellites in the constellation falls below certain range Signal Strength Indicators
- 24. Using Garmin12 Quit ENTER Goto PAGE MARK Scroll keypad Cycle through pages Save a waypoint On/Off switch
- 27. Common use of GPS A. GIS data collection & mapping B. Navigation C. Recreation
- 29. GPS for Navigation • GPS in PDA • Are getting popular in car • Comes with voice guidance
- 31. GPS in Farmland
- 32. Future GPS A. GPS in USA B. GLONASS program from Russia C. GALILEO from European countries
Editor's Notes
- At the cost of 15 billion dollar of your tax money
- At the cost of 15 billion dollar of your tax money
- Speed of the light: 186,000 mile/sec 300,000 km/sec 3x10^8 meter/sec
- SNR is also known as signal strength
- When SA was on, the position could be anywhere on 1st, 2nd, Alpine, or Spruce street. With SA removed, the position is located with in the road width of +/- 15 feet. (road = 35’, the circle is of 20’ radius). Before the location was within 100 meters, that is the point is about a football field away. Now it’s within the 10 meters. Removal of SA made GPS more useful and handy for our day to day use in cell phone, car, watch etc.
- Farm equipment capable of producing yield map during harvesting shows how crop yield varies across the field. Allowing farmers to plan fertilizer application for future crops.