An Introduction to GPS / GNSS

1. An Introduction to GPS / GNSS Prepared by: In Partnership with: NSF DUE-1205110; 0903270

2. Outline • Terms: GNSS & GPS? • Why do we use GNSS? • What is GNSS? • How does GNSS Work? • What do you need to know about GNSS? • What can you do with GNSS? • How is GNSS used in the real world?

3. GNSS and GPS • GPS = Global positioning system • GPS refers to the constellation of navigation satellites associated with the American System (which is a global system). • For a long time, GPS was the ‘only game in town’, so everything (satellites, receivers, etc.) was referred to as “GPS” • Times are changing… other options (besides GPS) are quickly developing

4. GNSS = Global Navigation Satellite System • GNSS is an umbrella term that includes any satellite navigation system. Options include: • GPS (U.S. | operational since 1994) • GLONASS (Russian | ‘re-operational’ since 2010) • Galileo (European Union | anticipated operation:2019) • Compass(China | Operational in Asia/Pacific since 2012 / anticipated global operation: 2020)

5. Why GNSS? • Many features have addresses and landmarks that are associated with a destination. Fred Jones 332 Elm St. Frog Holler, VA 42534

6. However, there are many features that do not have addresses… There are many MAJOR cities that do not even have STREET NAMES! And then there is the open ocean and sky… Why GNSS?

7. Why GNSS? Location, Location, Location and INFORMATION!!!

8. Navigation is critical Historical Navigational tools have limits: The Sextant – doesn’t work if it is cloudy Lowrance – radionavigation: only worked near land… The military had its own reasons for determining location… Identify targets Friendly fire issues “smart bombs” Pre-GPS

9. What is GNSS… And how does it work?

10. What is GPS? • GPS (and GNSS) is not a single UNIT! • GPS = Global Positioning SYSTEM • GPS was developed by the Department of Defense at a cost of >$12 billion • Funding for the GPS was contingent on making the system available to the public.

11. GPS (and GNSS) is a SYSTEM There are three major components in this system: • Satellites • Ground Control Stations • GNSS Receivers (or units)

12. Satellites • There are 24-32 satellites up there at any given time orbiting the earth at ~11,000 naut. miles. • The DOD knows the EXACT location of each of the satellites at any given moment. • These satellites have VERY accurate clocks on board. • The satellites continuously send radio signals towards earth. • These radio signals are picked up by GPS receivers.

13. Satellites: “A Beehive of Activity” http://www.nasa.gov/multimedia/imagegallery/image_feature_1283.html

14. GPS Control Stations • There are five control stations that monitor the satellites. • Control stations enable information on Earth to be transmitted to the satellites (updates and fine turning). • Control stations continuously track satellites, and update the positions of each satellite. • Without control stations, the accuracy of the system would degrade in a matter of days.

15. GPS Receivers • GPS units are referred to as “receivers”. • They receive information (radio signals) from satellites. • The GPS receiver knows how • long it takes the signal to travel • from the satellite to the • receiver.

16. GPS Receivers • The GPS receiver knows how long it takes the signal to travel from the satellite to the receiver. • The Receiver is therefore able to calculate its distance from the satellite. • Distance = time x velocity • Distance = time x 186,355 mi./sec. • The receiver can calculate the time that signal traveled from the satellite to the receiver. • The receiver is therefore able to determine its exact distance from the satellite.

17. How GPS (and GNSS) Works One satellite…

18. How GPS Works If the GPS receiver only obtains signals from 1 Satellite, then it “knows” that it is located somewhere on this sphere…

19. How GPS Works

20. How GPS Works If the GPS receiver only obtains signals from 2 satellites, then it “knows” that it is located somewhere where these 2 spheres intersect

21. How GPS Works

22. How GPS Works If the GPS receiver obtains signals from 3 satellites, then it “knows” that it is located somewhere where these 3 spheres intersect (2 points)

23. How GPS Works

24. How GPS Works A fourth satellite is required to determine the exact location and elevation.

25. What do you need to know about GNSS?

26. Different “Grades” of GNSS receivers • Recreational Grade GNSS • Accurate to within 5 meters (could be better, but don’t rely on it) • Suitable for hunting, recreational, and some business uses • Lowest cost (smallest, and easiest to use): ~$100-$800 • Mapping Grade GNSS • Accurate to within 1 meter (3 feet) • Requires differential processing (from a base station) • Suitable for many natural resource applications, city planning • $800-$7,000 • Survey Grade GNSS • Accurate to within 1 cm • Suitable for building bridges… • $15,000 -$30,000

27. What you need to know about GNSS? • Signal Accuracy Issues • Selective Availability • Tricks of the Trade • Current Applications of GNSS • Future applications of GNSS

28. GPS: Signal Accuracy There are 2 types of GPS Signals: P-code: (“Precise” code) • This is only available to the military and some selected public officials. • Very precise, not degraded. C-code: (“Civilian” Code). • Less precise • Signal can be degraded (by scrambling the signal) especially in times of conflict. • This is what the GARMIN receivers (and all public GPS receivers) work with…

29. Correcting for errors:Selective Availability • It is possible to correct for inherent signal errors. • This process is called Differential Correction • Here’s how it works…

30. Differential Correction • There are already established base stations established around the U.S. • Surveyors have determined the precise location of these base stations. • Each base station has a GNSS receiver, which collects incoming (error prone) signals. • The true (surveyed) location coordinates are then compared to the GNSS coordinates. • The correction values are then: • Posted to the web for later correction (post-processing); • sent to other GNSS receivers in the field (correction ‘on the fly’).

31. Differential Correction Base station w/ GNSS receiver at known location: Differential Correction Signal GNSS receiver in the field collecting points, routes, etc. Exact known (surveyed) coordinates differ from GNSS coordinates at this location = exact amount of error!

32. WAAS • The Wide Area Augmentation System (WAAS) is a differential GNSS system that is being constructed to support GNSS accuracy in aircraft. • WAAS also provides additional accuracy “on the ground” • The GNSS receivers that we are using are WAAS compatible

33. WAAS Most (but not all) GNSS receivers are WAAS compatible. 95% of GNSS receivers on the market today are WAAS compatible The GARMIN Venture HC is WAAS compatible

34. Other Tricks of the Trade:Averaging • Averaging: A GNSS receiver can collect points continuously for 15-30 seconds. The receiver can then average all these locations together • This only works when you are standing still!! • Note that not all GNSS receivers have an averaging capability GNSS Collected Points GNSS Averaged Position “True” location

35. It is better for your receiver to get a fix on “distributed” satellites, then poorly distributed satellites. Other Tricks of the Trade: Satellite Distribution “Positional Dilution of Precision” Good Satellite Distribution Poor Satellite Distribution

36. GNSS planning software... http://www.trimble.com/planningsoftware.shtml

37. GNSS Satellite Visibility: BlacksburgJuly 12, 2012

38. Try and stay away from buildings and other structures when using a GNSS receiver Satellites may not be visible… This can introduce error… Other Tricks of the Trade: MultiPath Errors

39. Other Tricks of the Trade: Tracking Satellites GNSS has worldwide coverage… HOWEVER… You can lose satellite coverage (or received degraded signals) in areas with dense foliage, in “urban canyons”, etc. You may also lose satellite coverage (or receive degraded signals) in deep valleys or gorges.

40. Accuracy • How accurate is a $150 GNSS? • That’s the million dollar question…

41. How accurate is a $150 GNSS?(It depends…) Acknowledgements: Dr. Phillip Rasmussen, Utah Geospatial Extension Specialist

42. Brand “A” Day 1 Brand “A” Day 2 Brand “A” Day 3 Brand “A” Day 4 Brand “A” Day 5 Brand “B” Day 1 Brand “B” Day 2 Brand “B” Day 3 Brand “B” Day 4 Brand “B” Day 5

44. GNSS Data Collection • Waypoints • Tracks • Routes • Find/GOTO • And more..!

45. What can you do with a GNSS? • Collect and store points (positions) These are called WayPoints. Field corners, insect infestation areas, crop damage, individual trees, trail heads, creek crossings, point source pollution, camping sites, and don’t forget “your car”! • Download the points onto your computer and integrate them with other mapping programs

46. Corner2 Point3 Latitude: 37° 16’ 18” Longitude: W80° 28’ 45” Elevation: 2108 feet 001 Waypoints

47. What can you do with a GNSS? • Collect and store the path that you have walked / driven • These paths are called TRACKS. • Calculate the distance of a track (i.e. perimeter around a field) • Calculate AREA measurements within a TRACK (after walking around a field or parking lot...) • Save and Download TRACKS onto your computer.

48. Tracks(just start walking…) What

49. Latitude: 37° 16’ 18” Longitude: W80° 28’ 45” Elevation: 2108 feet Time: 13:22.15 Date: 05/08/2009 Tracks (just start walking…) • Each track point has important information associated with it... • “Virtual bread crumbs” • Track points can be collected: • Based on a time period (every 10 seconds) • Based on distance (every 20 feet) • Or a combination of time and distance (every 10 secs. or 20 feet, whichever comes first).

50. Tracks • You can “track your way back...”* • You can use the track data to estimate area / perimeter* • You can use the time stamp in the trackfile to “georeference (or geotag)” photographs!* * We’ll do this later!