Basic GPS

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Basic GPS

  1. 1. GPS Basics A very brief overview! by Monica Spicker 2007 Some figures are copyrighted and used with the kind permission of Dr. Peter H. Dana. B M O C
  2. 2. Several Systems B M O C US: NAVSTAR - NAV igation S ystem with T iming A nd R anging, DOD Russia: GLONASS GLO bal N avigation S atellite S ystem. No yet complete European Union: Galileo (complimentary to US GPS) – started in 2003. Recently joined by various middle and far eastern countries. Japan: local regional system - Quasi-Zenith Satellite System (QZSS) with 3 satellites China: Twinstar Rapid Positioning System or Beidou Navigation System – pairs geosynchronous satellites, compatible with Galileo
  3. 3. US GPS History <ul><li>1978: First GPS satellite launch </li></ul><ul><li>1983: GPS revealed (kept secret until now) </li></ul><ul><li>1994: All 24 satellites operational </li></ul><ul><li>1999: Global Positioning System modernized with two new civil signals added to future GPS satellites, significantly enhancing the service provided to civil, commercial, and scientific users worldwide. </li></ul><ul><li>2000: Selective availability permanently turned off </li></ul><ul><li>2004: Legislation signed to keep civilian service free world wide </li></ul>B M O C
  4. 4. Parts of a GPS System B M O C
  5. 5. Control System B M O C Image copied from http://www.aero.org/publications/crosslink/summer2002/04.html
  6. 6. Satellites <ul><li>6 orbital planes ensure at least 4 or more satellites available at almost all times. </li></ul>B M O C <ul><li>About 31 satellites </li></ul><ul><li>Cesium or Rubidium clocks. </li></ul><ul><li>Very high orbits </li></ul><ul><li>Several replaced every year. </li></ul>
  7. 7. Receiver <ul><li>Cost reflects: </li></ul><ul><ul><li>Antenna quality </li></ul></ul><ul><ul><li>Clock quality </li></ul></ul><ul><ul><li>Memory </li></ul></ul><ul><ul><li>Software </li></ul></ul><ul><li>Look for </li></ul><ul><ul><li>Patch antenna </li></ul></ul><ul><ul><li>Easy to read menus </li></ul></ul><ul><ul><li>WAAS enabled </li></ul></ul><ul><ul><li>Computer hookup </li></ul></ul><ul><ul><li>Technical support </li></ul></ul>B M O C Mapping grade $3000 - $25000 Submeter with post processing Recreational: $100 - $500 10 m accuracy
  8. 8. <ul><li>All are 12 channel – can receive signals from 12 satellites at once. </li></ul><ul><li>One frequency and coarse acquisition code (C/A code) </li></ul><ul><li>Mapping/surveying/military grade units are dual frequency and receive other information that “recreational” receivers cannot. </li></ul>What Do Civilian Units Receive? B M O C
  9. 9. Other Accessories <ul><li>Computer cable </li></ul><ul><ul><li>Download and upload maps, routes and waypoint. </li></ul></ul><ul><ul><li>Upgrade over the internet </li></ul></ul><ul><li>Computer Mapping Software </li></ul><ul><ul><li>Can map out tracks and waypoints </li></ul></ul><ul><ul><li>Can plan and upload routes and waypoints </li></ul></ul><ul><li>Receivers may also have </li></ul><ul><ul><li>Digital compasses </li></ul></ul><ul><ul><li>Barometric Altimeters </li></ul></ul><ul><ul><li>FRS Radio </li></ul></ul><ul><ul><li>Other features. </li></ul></ul><ul><li>Carry extra batteries! </li></ul><ul><ul><li>Lithium last longer and not cold sensitive. </li></ul></ul><ul><ul><li>NiMH are rechargeable and not cold sensitive. </li></ul></ul>B M O C
  10. 10. The Fundamental Principal: <ul><li>Speed * Time = Distance </li></ul><ul><li>Radio waves are electromagnetic radiation, and travel the constant speed of light. </li></ul><ul><li>The receiver measures how long it takes for a signal to reach it, and then computes the distance to the satellite. </li></ul><ul><li>Universal Time (UT or GMT) is the “clock” time used. </li></ul>B M O C
  11. 11. Determining position : 3 satellites for position. 4 needed for elevation B M O C 1 satellite: Solution is a hollow sphere 3 satellites: 2 points on a line. One pt is “in space”and thrown out 2 satellites: Solution is a circle. distance GPS satellite
  12. 12. Three okay, four or more better <ul><li>In theory, only three satellites are needed to acquire an accurate position fix. </li></ul><ul><li>In practice, four or more satellites mainly because of timing error that arises from a variety of sources. </li></ul><ul><li>The core of a good understanding of GPS is understanding these error sources. </li></ul>B M O C
  13. 13. Sources of error: Amt of error B M O C Satellite Geometry 4 – 6m* Multi-path errors Variable Ionosphere (charged particles) < 2m Troposphere (the dense part) < 2m Receiver errors (timing, clock, antenna) < 2m Satellite clock errors < 1m Ephemeris errors (satellite position) < 1m * Planning programs are available to predict times with best geometry. Afternoons in northern hemisphere generally have poorest geometry. Orbits have widest gaps in the north.
  14. 14. Best Satellite Geometry <ul><li>One satellite directly above </li></ul><ul><li>Three low and spread around </li></ul>B M O C
  15. 15. GPS cannot “see” through objects! B M O C Some of the newer satellites and receivers can receive through thinner solid objects like cars, buildings and anything wet like tree canopies, people, heavy rain, etc. Terrain is still too big through which to get a signal!
  16. 16. Multipath error B M O C Increases the length of time for a signal to reach the receiver.
  17. 17. Multipath errors B M O C <ul><li>Smooth surfaces act as mirrors to GPS signals. </li></ul><ul><li>“ Smooth” means anything as smooth or smoother than a coarse gravel road. </li></ul><ul><li>Open water is particularly reflective. </li></ul><ul><li>Reflectance leads to multipath errors... </li></ul>
  18. 18. B M O C TEST 1. Why is this equatorial rainforest wildlife biologist sitting on a horse in the middle of a river? 2. What problems is she likely to be having (with her GPS unit)?
  19. 19. Initializing the receiver <ul><li>Setup : CRITICAL! </li></ul><ul><ul><li>Map Datum* </li></ul></ul><ul><ul><li>Grid System* </li></ul></ul><ul><ul><ul><li>It is possible to create your own grid and or datum! </li></ul></ul></ul><ul><ul><li>Distance units </li></ul></ul><ul><ul><li>Other features or options </li></ul></ul>B M O C
  20. 20. Map Datums B M O C <ul><li>Earth model to reference and compute coordinates </li></ul><ul><li>GPS based on World Geodetic System 1984 </li></ul><ul><ul><li>(WGS 84) </li></ul></ul><ul><li>USGS quad maps based on North American Datum 1927 (NAD27) </li></ul><ul><li>Some US maps are based on NAD83 (=WGS84) </li></ul><ul><li>Canadian maps are based on NAD83 </li></ul><ul><li>Other countries have their own datums </li></ul><ul><li>GPS Receiver set up must match map. </li></ul>
  21. 21. What happens when datum is not set correctly ! B M O C
  22. 22. Elevations <ul><li>Calculated from a mathematical Earth model called the ellipsoid. </li></ul><ul><li>Elevations on a map are based on mean sea level shape (Geoid) </li></ul><ul><li>Can be very different and you won’t know where or when it’s different and by how much. </li></ul><ul><li>Relative elevations will be correct but absolute elevations may not agree with the map! </li></ul><ul><li>GPS receivers with barometric altimeter use both methods. </li></ul>B M O C
  23. 23. Geoid vs Ellipsoid B M O C Image Sources: Potsdam University and Swisstopo
  24. 24. Real Time Correction by Radio Beacon B M O C
  25. 25. B M O C Beacons operated by the U.S. Coast Guard & U.S. Army Corps of Engineers. Many inland states have own beacons. In most of the world, inland areas do not have access to this type of data.
  26. 26. WAAS <ul><li>The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) developed the Wide Area Augmentation System for use in precision flight approaches. </li></ul><ul><li>25 ground reference stations in US monitor GPS satellite data and calculate corrections. </li></ul><ul><li>The correction is broadcast through one of two geostationary satellites. </li></ul><ul><li>Currently, WAAS coverage is only in US and parts of southern Canada. </li></ul><ul><li>Any receiver with a WAAS antenna can receive the signal. </li></ul><ul><li>Must enabled through setup. </li></ul>B M O C
  27. 27. Waypoints <ul><li>Input coordinates of a desired point </li></ul><ul><li>Saved coordinates of a field point </li></ul><ul><li>Can be combined into a set route. </li></ul>B M O C
  28. 28. Navigation <ul><li>Need to have stored waypoints. </li></ul><ul><li>Travel in a straight line from waypoint to waypoint (route). </li></ul><ul><li>Feedback is distance and direction - you may need a compass! </li></ul><ul><li>Can save waypoints along the way and follow back (reverse route). </li></ul><ul><li>Straight line may not be safest - use a map! </li></ul>B M O C
  29. 29. Rule To Remember! <ul><li>A GPS receiver is an accessory , NOT A REPLACEMENT for a map, compass altimeter and good, basic navigation skills! </li></ul>B M O C
  30. 30. Some Hints <ul><li>Check batteries and settings before setting out. </li></ul><ul><li>Use lithium batteries or NiMH when available. </li></ul><ul><li>Practice with and know all needed screens on your receiver. </li></ul><ul><li>Have all nav equipment together and readily available. Tie your GPS and compass onto yourself. </li></ul><ul><li>Store as waypoints all checkpoints, decision points and rally points during planning and in the field. </li></ul><ul><li>Know the distance and direction between pre-planned checkpoints, etc. should the GPS fail. </li></ul><ul><li>Waypoint your return point before leaving. </li></ul><ul><li>Keep track of your location at all times. </li></ul><ul><li>Move into an opening close to your position if you cannot receive a good position where you are. </li></ul><ul><li>Unless actively navigating with GPS or required to keep a track log, only turn it on to take a position, if unsure of location. </li></ul><ul><ul><li>If keeping a track log, clear out all old tracks before setting out. </li></ul></ul>B M O C

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