1. Part II: The Global Positioning System A constellation of 24 high- altitude satellites Cost: over $12 billion US tax dollars

2. GPS Satellites More than 24 up at all times. Three atomic clocks per satellites. Built to be resistant to blocking and jamming. Rockwell: 11,000 miles 17 feet 1900 lbs.

3. Ranging, why 3 needed? GPS Satellite

5. Relative Timing Each satellite sends out the same sequence of signals The receiver calculates the shift between identical parts of the code Analogy: Stand on 20 yard line of stadium – two friend yell to you from each goal line at the same moment. Time difference

6. GPS Uses Natural Resource based Animal/plant locations Soil/water sampling locations Human-based: Air and sea navigation Emergency Services Locating Airplane landing systems Car-based direction services.

7. Why is the system not perfect? Error sources Atmospheric Multipath scatter Positional accuracy error Dept. of Defense introduced error Cheap GPS Units Limited satellite channels Poor internal processors. These are the real numbers! What is this?

8. One solution – Differential GPS This system uses two receivers to eliminate much of the signal error This increases accuracy to a few meters with good units.

9. Differential GPS Resources Real-Time Correction What is it? Coast Guard Signals USGS Signals

10. Post-Processed Correction What is it? What is RINEX? State Base Stations www.sso.state.ne. us/gps/ USGS Sites Nice downloads!

11. Where to get information and help. Univ. of Nebraska at Kearney Marc Albrecht: albrechtm@unk.edu and http://gis.unk.edu http://gis.unk.edu Steele Becker: beckers@unk.edu Univ. of Nebraska CALMIT= www.calmit.unl.eduwww.calmit.unl.edu Jim Lacy = jlacy1@un.edu Rick Perk = rperk1@unl.edu Marcus Tooze = mtooze1@unl.edu* Nebraska Natural Resource Commission Steve Rathje = rathje@nrcdec.nrc.state.ne.us* Natural Resource Districts Doug Steinke = steinke@nrcdec.nrc.state.ne.us

12. If you really want to learn more.. Univ. of Neb. At Kearney 2-day ESRI-certified workshops in ArcView UNK classes (Biology 430/830) ESRI (Environmental Systems Research Institute), the people who make ArcExplorer and ArcView - www.esri.com online classes downloadable tutorials.