What Is GPS


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Breakdown of GPS best practices when working on the North Country Trail. Matt Rowbotham, NCTA - 2009

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What Is GPS

  1. 1. GPS Overview <ul><li>Matt Rowbotham, GIS Coordinator </li></ul><ul><li>North Country Trail Association </li></ul>
  2. 2. Introduction <ul><li>Global Positioning System (GPS) consists of three parts. </li></ul><ul><ul><li>The Space Segment </li></ul></ul><ul><ul><li>The Control Segment </li></ul></ul><ul><ul><li>The User Segment </li></ul></ul>
  3. 3. Global Positioning System Control Segment Space Segment User Segment
  4. 4. Space Segment <ul><li>A network of 24 – 32 satellites orbiting Earth. </li></ul><ul><li>These satellites continually transmit time-marked radio (microwave) signals back to Earth. </li></ul>
  5. 5. Control Segment <ul><li>A number of tracking stations located around the world. These stations monitor and send corrective data via the satellites. </li></ul>
  6. 6. User Segment <ul><li>The user segment consists of the people and receivers on the ground. </li></ul>
  7. 7. How does it work?
  8. 8. How does it work <ul><li>The signal received on the ground from a satellite contains information on that satellite’s position as well as the time the signal was sent. </li></ul>
  9. 9. How does it work Speed of Light x Travel Time Distance Time signal left satellite Time current
  10. 10. How does it work <ul><li>Once a GPS receiver has locked onto the signals from two different satellites it can begin to estimate a position </li></ul>
  11. 11. How does it work <ul><li>One on the surface of the earth and one in deep space (which of course is not accurate). </li></ul>
  12. 12. How does it work <ul><li>With three different satellites it can use this information to calculate (trilateration) a position </li></ul>
  13. 13. How does it work <ul><li>With the addition of a fourth satellite signal the GPS receiver can calculate a three-dimensional position (3D), including altitude. </li></ul>
  14. 14. Accuracy
  15. 15. Accuracy <ul><li>The level of accuracy we expect varies between recreational grade units (Garmin, Magellan, etc) and resource grade units (Trimble, Lecia, etc.) </li></ul><ul><li>According to Garmin their receivers are on average accurate to 15 meters (~ 50 feet). </li></ul><ul><li>The Trimble ProXT receiver owned by the NCTA is capable (with user correction) of measurements accurate down to submeter (< 3 feet), nearing a 1 centimeter accuracy in ideal conditions. </li></ul>
  16. 16. What level of accuracy do we need? <ul><li>In the US, national accuracy standards for large scale maps (~1:20,000) can be considered to be roughly +/- 3 feet. </li></ul><ul><li>But… </li></ul><ul><ul><li>Location of information on the North Country Trail is fairly obvious. </li></ul></ul><ul><ul><li>Trimble GPS is cumbersome and has a steep learning curve, as well as limited access. </li></ul></ul><ul><ul><li>There is widespread availability of recreational grade GPS units. </li></ul></ul>
  17. 17. What level of accuracy do we need? <ul><li>To balance high demand for up to date information with the need for accurate information, accept the lower accuracy potential of recreational grade GPS receivers. </li></ul><ul><li>Strive to push these units to the maximum accuracy potential. </li></ul><ul><li>This can be accomplished by better understanding this technology and incorporating best practices and error correction procedures whenever possible. </li></ul>
  18. 18. Reduced Accuracy and Errors
  19. 19. Reduced Accuracy <ul><li>There are many factors at play that can affect the accuracy of position estimates being made by a GPS receiver. </li></ul><ul><li>These can be grouped into two generalized categories </li></ul><ul><ul><li>Arrangement of satellites in the sky at a given time in your area (satellite geometry). </li></ul></ul><ul><ul><li>Signal interference between the satellite and receiver. </li></ul></ul>
  20. 20. Satellite Geometry <ul><li>Not only is the number of satellites that your GPS receiver has locked onto important, but the arrangement of these satellites to one another and to your receiver can affect the precision of your measurement as well as amplify other errors. </li></ul>
  21. 21. Satellite Geometry <ul><li>Satellite geometry is considered good when one satellite is directly above the receiver, and three or more satellites are equally spaced around the horizon . </li></ul>
  22. 22. Satellite Geometry <ul><li>Satellite geometry is considered good when one satellite is directly above the receiver, and three or more satellites are equally spaced around the horizon . </li></ul>
  23. 23. Signal Interference <ul><li>A reduction in accuracy will occur when the length of time it takes a signal to travel between a satellite and GPS receiver is affected. </li></ul><ul><li>The factors that can slow down the speed of the signal include </li></ul><ul><ul><li>Particles and weather systems in the atmosphere. </li></ul></ul><ul><ul><li>Buildings and other landscape and surface features. </li></ul></ul>
  24. 24. Signal Interference Ionosphere Solid Structures Metal Electro-magnetic Fields
  25. 25. Errors <ul><li>You will have slightly different readings for the same point each time you take a measurement, including trying to navigate back to the point. </li></ul>
  26. 26. Wide-Area Augmentation System (WAAS)
  27. 27. WAAS <ul><li>WAAS is a system of satellites and ground stations that provide GPS signal corrections. </li></ul><ul><li>The system was developed for aviation use, but is available to everyone. </li></ul><ul><li>The two WAAS satellites show up on Garmin GPS units as satellite numbers 48 and 51. </li></ul><ul><li>Reception of a WAAS corrected signal is indicated on the Garmin 76CSx by the letter “D” on the signal strength bar for a given satellite on the Satellite Page of the unit. </li></ul>
  28. 28. WAAS If WAAS signal intermittent- WAAS correction for SV 19 No WAAS correction for SV 11
  29. 29. WAAS WAAS satellites GPS Constellation WAAS Control Station (Position known) GPS receiver (Position with errors) 25 Reference Stations (Position known)
  30. 30. WAAS <ul><li>In certain instances Garmin units may select a 2D WAAS corrected signal over a more accurate 3D non-WAAS corrected signal, which is potentially less accurate. </li></ul><ul><li>Unfortunately for the NCTA, WAAS reception seems to be rather limited in the more remote areas of the northern tier of the continental US. </li></ul><ul><li>WAAS should be disabled if if you’re getting poor WAAS reception. </li></ul>
  31. 31. Enhancement <ul><li>External Antenna </li></ul><ul><ul><li>Map76S more accurate under canopy </li></ul></ul><ul><li>Hat/ Helmet Mounts </li></ul><ul><li>Sleeve Mounts </li></ul><ul><li>Vehicle Brackets </li></ul>
  32. 32. Taking Measurements
  33. 33. Taking Measurements <ul><li>For the North Country Trail we are interested in both points and lines. </li></ul><ul><li>GPS units must be able to connect to a computer to transfer large amounts of data </li></ul><ul><li>If no connection, you can still work with a small number of points (no lines), using the G ood P encil S ystem. </li></ul>
  34. 34. Taking Measurements <ul><li>Tips </li></ul><ul><ul><li>Always clear Track log before you start working (line) </li></ul></ul><ul><ul><li>Use Average feature for taking points </li></ul></ul><ul><ul><li>If just recording with GPS for NCTA you can work in WGS 84 </li></ul></ul><ul><ul><li>Backup and save often! </li></ul></ul>
  35. 35. Transferring Information
  36. 36. MN DNR Garmin <ul><li>The Minnesota DNR has put together a freely available software package that makes sharing data between our mapping system and GPS units extremely easy. </li></ul><ul><li>Step by step instructions in handout </li></ul><ul><li>Note: Be aware of datum and projects when sending data to NCTA. I prefer unprojected, WGS 84. </li></ul>
  37. 37. Submission Forms
  38. 38. Questions?