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An Introduction to GPS John McGee Ph.D. Geospatial Extension Specialist Virginias Geospatial Extension Program A Partnership with VSGC and VCE www.cnr.vt.edu/gep.

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Presentation on theme: "An Introduction to GPS John McGee Ph.D. Geospatial Extension Specialist Virginias Geospatial Extension Program A Partnership with VSGC and VCE www.cnr.vt.edu/gep."— Presentation transcript:

1 An Introduction to GPS John McGee Ph.D. Geospatial Extension Specialist Virginias Geospatial Extension Program A Partnership with VSGC and VCE

2 Outline Why GPS? Why GPS? What is GPS? What is GPS? How GPS Works. How GPS Works. What you need to know about GPS. What you need to know about GPS. What can you do with GPS? What can you do with GPS? Applications of GPS. Applications of GPS.

3 Why GPS? Many features have addresses and landmarks that are associated with a destination. Many features have addresses and landmarks that are associated with a destination.

4 Why GPS? Many features have addresses and landmarks to get you to a destination. Many features have addresses and landmarks to get you to a destination. However, there are many features that do not have addresses… However, there are many features that do not have addresses…

5 Why GPS? Many features have addresses and landmarks to get you to a destination Many features have addresses and landmarks to get you to a destination However, there are many features that do not have addresses… However, there are many features that do not have addresses… There are many MAJOR cities that do not even have STREET NAMES! There are many MAJOR cities that do not even have STREET NAMES!

6 Why GPS? Many features have addresses and landmarks to get you to a destination Many features have addresses and landmarks to get you to a destination However, there are many features that do not have addresses… However, there are many features that do not have addresses… There are many cities that do not even have STREET NAMES! There are many cities that do not even have STREET NAMES! And then there is the open ocean and sky… And then there is the open ocean and sky…

7 Why GPS? Location, Location, Location andINFORMATION!!!

8 Pre-GPS Navigation is critical!!! Navigation is critical!!! Historical Navigational tools have limits: Historical Navigational tools have limits: -The Sextant – contingent on weather

9 Pre-GPS Navigation is critical Navigation is critical Historical Navigational tools have limits: Historical Navigational tools have limits: -The Sextant – contingent on weather -Radionavigation (Lowrance): only works near land…

10 Pre-GPS Navigation is critical Navigation is critical Historical Navigational tools have limits: Historical Navigational tools have limits: -The Sextant – doesnt work if it is cloudy -Lowrance – radionavigation: only worked near land… The military had its own reasons for determining location… The military had its own reasons for determining location… -Identify targets -Friendly fire issues -smart bombs

11 What is GPS? GPS is not a single UNIT! GPS is not a single UNIT! GPS = Global Positioning SYSTEM GPS = Global Positioning SYSTEM GPS was developed by the Department of Defense GPS was developed by the Department of Defense Funding for the GPS was contingent on making the system available to the public. Funding for the GPS was contingent on making the system available to the public.

12 GPS is a SYSTEM There are three major components in this system: 1.Satellites 2.Ground Control Stations 3.GPS Receivers (or units)

13 Satellites There are 24 satellites (and 3 spares). There are 24 satellites (and 3 spares). The DOD knows the EXACT location of each of the satellites at any given moment. The DOD knows the EXACT location of each of the satellites at any given moment. These satellites have VERY accurate clocks on board. These satellites have VERY accurate clocks on board. The satellites continuously send radio signals towards earth. The satellites continuously send radio signals towards earth. The radio signals contain several pieces of information, including the satellite id#, a time stamp, and the satellites true position in space. The radio signals contain several pieces of information, including the satellite id#, a time stamp, and the satellites true position in space.

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

15 GPS Receivers GPS units are referred to as receivers. GPS units are referred to as receivers. They receive information (radio signals) from satellites. They receive information (radio signals) from satellites. These radio signals contain These radio signals contain important information… Signal containing: Time stamp… Satellite ID # Exact position of satellite

16 GPS Receivers The Receiver knows exactly when the signal leaves the satellite (time stamp) and when the signal arrives at the receiver. The Receiver knows exactly when the signal leaves the satellite (time stamp) and when the signal arrives at the receiver. The Receiver is therefore able to calculate its distance from the satellite. The Receiver is therefore able to calculate its distance from the satellite. -Distance = time x velocity -Distance = time x 299,792,458 m/s The receiver knows the exact position (location) of the satellite (via the signal). The receiver knows the exact position (location) of the satellite (via the signal). The receiver is therefore able to determine its exact distance from the satellite. The receiver is therefore able to determine its exact distance from the satellite.

17 How GPS Works Signal with Time stamp… Satellite ID # Satellite position information

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 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 If the GPS 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 A fourth GPS is required to determine the exact location and elevation.

25 What you need to know about GPS Signal Accuracy Issues Signal Accuracy Issues Selective Availability Selective Availability Tricks of the Trade Tricks of the Trade Current Applications of GPS Current Applications of GPS Future applications of GPS Future applications of GPS

26 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. This is only available to the military and some selected public officials. Very precise, not degraded. Very precise, not degraded. C-code: (Civilian Code). Less precise Less precise Degraded (by scrambling the signal) especially in times of conflict Degraded (by scrambling the signal) especially in times of conflict

27 Selective Availability (SA) For national security reasons, the military sometimes degrades the C- code signal. For national security reasons, the military sometimes degrades the C- code signal. These errors are random These errors are random Errors be as high as +300 feet Errors be as high as +300 feet

28 Selective Availability SA errors can put you on the wrong side of a stream, or even a different city block or street! SA errors can put you on the wrong side of a stream, or even a different city block or street! 300 feet is a lot of real estate!!! 300 feet is a lot of real estate!!! GPS Location Real location

29 Selective Availability It is possible to correct for Selective Availability. It is possible to correct for Selective Availability. This process is called Differential Correction This process is called Differential Correction Heres how it works… Heres how it works…

30 Differential Correction There are already established base stations around Virginia There are already established base stations around Virginia Surveyors have determined the precise location of these base stations already. Surveyors have determined the precise location of these base stations already. Each base station has a GPS receiver, which collects incoming (scrambled) signals. Each base station has a GPS receiver, which collects incoming (scrambled) signals. The true (surveyed) location is then compared to the GPS coordinates. The true (surveyed) location is then compared to the GPS coordinates. The correction values are then sent to other GPS receivers in the field. The correction values are then sent to other GPS receivers in the field.

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

32 Other Tricks of the Trade: Averaging Averaging: A GPS receiver can collect points continuously for seconds. The receiver can then average all these locations together Averaging: A GPS receiver can collect points continuously for seconds. The receiver can then average all these locations together This only works when you are standing still!! This only works when you are standing still!! GPS Collected Points GPS Averaged Point True location

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

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

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

36 Other Tricks of the Trade: WAAS The Wide Area Augmentation System (WAAS) is a differential GPS system that is being constructed to support GPS accuracy in aircraft. The Wide Area Augmentation System (WAAS) is a differential GPS system that is being constructed to support GPS accuracy in aircraft. WAAS is supported by a number of satellites that emit signals to standard GPS units. WAAS is supported by a number of satellites that emit signals to standard GPS units.

37 WAAS

38 How accurate is a $100 GPS? Thats the million dollar question… Thats the million dollar question…

39 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

40

41 What can you do with a GPS? Collect and store points Collect and store points (WAYPOINTS) (WAYPOINTS) Trail heads, creek crossings, mountain tops, camping ground, data collection points (for research), etc. Trail heads, creek crossings, mountain tops, camping ground, data collection points (for research), etc. Download the points onto your computer and integrate them with other mapping programs Download the points onto your computer and integrate them with other mapping programs

42 What can you do with a GPS? Collect and store routes (a series of WAYPOINTS) Collect and store routes (a series of WAYPOINTS) A route is a path between two (or more) Waypoints. The GPS guides you from point A to point B.

43 What can you do with a GPS? The GOTO function The GOTO function Using the GOTO function, the GPS will guide you to a predefined Waypoint (you choose which one…) using a compass and pointer You can program the GPS to beep when you are within a certain distance of the defined Waypoint

44 Tide Tables Tide Tables Many of the marine GPSs have built in tide tables. They will provide tide information and ranges for any date and any place… Many of the marine GPSs have built in tide tables. They will provide tide information and ranges for any date and any place… What can you do with a GPS?

45 Speed Speed GPSs calculate your ground speed as you walk, run, drive or fly GPSs calculate your ground speed as you walk, run, drive or fly What can you do with a GPS?

46 Elevation Elevation In addition to providing you with your latitude and longitude, GPS provides you with altitude information. In addition to providing you with your latitude and longitude, GPS provides you with altitude information. What can you do with a GPS?

47 Current Applications of GPS Public Safety Public Safety Environmental resource agents Environmental resource agents Aviation Aviation Military Military Local planning Local planning Surveying Surveying Recreation Recreation Business Business

48 The Future of GPS Acknowledgements: Keith Clarke

49 The Future is here. The applications are endless…

50 Questions? John McGee


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