Lecture 4 - Global Positioning Systems (GPS)
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Lecture 4 - Global Positioning Systems (GPS)

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  • dilution of precision (DOP), a factor that multiplies the uncertainty associated with satellite and receiver clocks, the atmosphere, satellite orbits, and the environmental conditions that lead to multipath errors

Lecture 4 - Global Positioning Systems (GPS) Lecture 4 - Global Positioning Systems (GPS) Presentation Transcript

  • Global Positioning Systems (GPS) Introduction to GIS – Spring 2013 1 M. Corbalis
  • Uses of GPS Technology• Navigation • Military Marine Troop Accountability Automobile Coordination Aircraft Targeting Selective Availability• Wayfinding • Surveying Disaster Relief Hiking Property Delineation Walking Tours Infrastructure Assessment Eco-burials Planning OpenStreetMap• Precise Timekeeping Introduction to GIS – Spring 2013 2 M. Corbalis
  • What GPS Can’t Do• Track you. Its a one way system. You need special equipment to collect the signals then transmit your location through other means.• Follow you inside or underground. Just your hand can block a GPS signal. Further tracking required dead reckoning, inertial navigation, or different technology.• Show you where to go. Consumer GPSs rely on a special purpose GIS. As you should know, its only as good as the data available. Introduction to GIS – Spring 2013 3 M. Corbalis
  • GPS Systems• GPS systems use a constellation of satellites with known orbits. Each satellite beams an encoded signal towards Earth.• A GPS receiver can read the coded signals to determine the receiver’s location on the earth using triangulation.• Higher end GPS units can use the code and the carrier wave for higher accuracy. (“Code vs. Carrier”) Introduction to GIS – Spring 2013 4 M. Corbalis
  • How Does It Work? Triangulation Introduction to GIS – Spring 2013 5 M. Corbalis
  • How Does It Work?• The encoded message allows a receiver to determine its position on the Earth once 3 satellites (2D) or 4 or more satellites (3D) are in view.• The satellite signal includes: • Psudo-Random Code: precisely timed unique signal • Ephemeris: timely and precise orbital information about the satellite • Almanac: granular information about the entire system. Introduction to GIS – Spring 2013 6 M. Corbalis
  • How Does It Work? * * Pseudo-Random is not random at all. Introduction to GIS – Spring 2013 7 M. Corbalis
  • Types of GPS• Autonomous GPS (“GPS”) relies entirely on the signals received from the satellites to determine the user’s location.• Assisted GPS (“A-GPS”) uses an outside network (cellular, WiFi) to download the Ephemeris and Almanac, shortening the Time To First Fix.• Differential GPS (“DGPS”) uses a static base station GPS to provide additional information about the signal, boosting accuracy. Introduction to GIS – Spring 2013 8 M. Corbalis
  • What Affects a GPS Signal?• Position on the Earth• Health of satellites• Multipath issues• Weather disruption• Vegetation cover• Quality of receiver• Additional signals Introduction to GIS – Spring 2013 9 M. Corbalis
  • Typical Error Contributions (meters)? Standard GPS Differential GPS• Satellite Clocks 1.5 0• Orbit Errors 2.5 0• Ionosphere 5.0 0.4• Troposphere 0.5 0.2• Receiver Noise 0.3 0.3• Multipath 0.6 0.6 Introduction to GIS – Spring 2013 Source: Trimble M. Corbalis 10
  • Uncertainty in GIS Data• The location derived from a GPS is not 100% correct.• Uncertainty influences many aspects of our work.• It’s impossible to make a perfect representation of the world, so uncertainty about it is inevitable. Introduction to GIS – Spring 2013 11 M. Corbalis
  • Sources of Uncertainty• Measurement Error: different observers, measuring instruments• Specification Error: omitted variables• Ambiguity, Vagueness and the Quality of a GIS representation Introduction to GIS – Spring 2013 12 M. Corbalis
  • Levels of Uncertainty Introduction to GIS – Spring 2013 13 M. Corbalis
  • Precision and Accuracy Precise Accurate The term precision is often used to refer to the repeatability of measurements. In both diagrams six measurements have been taken of the same position, represented by the center of the circle. On the left, successive measurements have similar values (they are precise), but show a bias away from the correct value (they are inaccurate). On the right, precision is lower but accuracy is higher. Introduction to GIS – Spring 2013 14 M. Corbalis
  • Positional Accuracy• Consumer GPS receivers have a horizontal positional accuracy of 5m (16ft).• Multipath and scattering impacts accuracy.• Accuracy can be improved using differential GPS.• Stated as Dilution of Precision xDOP (G, P, H, V, T…) Introduction to GIS – Spring 2013 15 M. Corbalis
  • Dilution of PrecisionA factor that multiplies the uncertainty contributed by clocks, the condition of theatmosphere, satellite path issues and multipath errors• Numeric value that corresponds to the degree of error in the current GPS position.• Geometric – satellites are too close together.• Horizontal, vertical, and positional dilutions.• Time dilution – the shift from “actual” time.• Value typically from 1-50.• Lower value, lower dilution, better quality.• GPS Forecast and NANUs Introduction to GIS – Spring 2013 16 M. Corbalis
  • Dilution of Precision DOP Value Rating Description This is the highest possible confidence level1 Ideal to be used for applications demanding the highest possible precision at all times. At this confidence level, positional measurements are considered accurate1-2 Excellent enough to meet all but the most sensitive applications. Represents a level that marks the minimum appropriate for making business decisions.2-5 Good Positional measurements could be used to make reliable in-route navigation suggestions to the user. Positional measurements could be used for calculations, but the fix quality could still be5-10 Moderate improved. A more open view of the sky is recommended. Represents a low confidence level. Positional measurements should be10-20 Fair discarded or used only to indicate a very rough estimate of the current location. At this level, measurements are inaccurate by as much as 300 meters with a 6 meter>20 Poor accurate device (50 DOP × 6 meters) and should be discarded.Source: http://en.wikipedia.org/wiki/Dilution_of_precision_(GPS) Introduction to GIS – Spring 2013 17 M. Corbalis
  • How Does It Work? Coded Signals Code Phase Cycle = 1 microsecond: 3-6 meters of error. Carrier Phase Cycle = 1000 times faster Source: http://www.trimble.com/gps/dgps-advanced2.shtml Introduction to GIS – Spring 2013 18 M. Corbalis
  • Selective Availability• Before 2000, GPS was limited to US Military use.• Random error was introduced into the civilian signal, intentionally reducing precision.• Now SA is turned off, but can be selectively turned on for specific regions of the world. Introduction to GIS – Spring 2013 19 M. Corbalis
  • GPS Tutorial• Trimble has a GPS tutorial online that’s worth a look: http://www.trimble.com/gps_tutorial/whygps.aspx Introduction to GIS – Spring 2013 20 M. Corbalis