1. CRGIS Global Positioning Systems The Basics CRGIS National Park Service

2. CRGIS Applying Global Positioning Systems u GPS provides navigational aides –Locating a single point –Navigating between points u GPS provides the basis for mapping –Tracking changing locational information –Collecting coordinates of features for use in GIS –Collecting information about features for use in GIS

3. CRGIS How GPS Works u GPS works by triangulating your position on the earth, based on satellite signals u Satellites broadcast radio signals u Receivers pick up the signals u Receivers calculate geographic coordinates from the satellite signals

4. CRGIS Satellites u GPS satellites are controlled and operated by the Dept. of Defense, but it is an open system u 24 satellites in orbit dedicated to GPS u 6 satellites are within view of any location at one time, provided that physical terrain, or structures do not block them u Satellites constantly transmit their locational information, and time data

5. CRGIS Receivers u Receiver picks up signals broadcast from satellites in known orbits u Radio signals travel near the speed of light u Receiver calculates how long the signal took to reach the earth u Using velocity of the signal and time, receivers calculate distance to satellite

6. CRGIS Calculating Distance with Speed and Time u Speed x time = distance u Satellite radio transmission consists of a series of dots and dashes in a “pseudo-random” code u All satellites transmit a unique code with a time stamp, synchronized by atomic clocks u Receivers decode each signal to determine which satellite the signal is originating from u Receiver compares time stamps with code to determine the time difference between satellite and ground position

7. CRGIS The Mathematics u Once the first satellite distance is calculated, the receiver has narrowed its location down to a sphere with the radius of that distance.

8. CRGIS The Mathematics u From the second satellite, the receiver can narrow its position to the intersection of the two possible spheres.

9. CRGIS The Mathematics u Adding a third satellite narrows the receiver position down to two possible locations. u The fourth satellite will provide more accuracy, narrowing to a single location.

10. CRGIS Position Calculations u Adding a fourth satellite into the calculations helps calibrate timing of the atomic clocks u The fourth satellite also greatly improves the level of accuracy on your positional data Four satellites = 3-D data collection Accuracy +/- 1 meter Four satellites = 3-D data collection Accuracy +/- 1 meter Three satellites = 2-D data collection Accuracy +/- 200 meters: NOT RECOMMENDEDThree satellites = 2-D data collection Accuracy +/- 200 meters: NOT RECOMMENDED

11. CRGIS Sources of Error u Atmosphere slows down the satellite signals u Multi-pathing -- signals bounce off metal fences, large trees, buildings u Static and interference u Atomic clocks are not perfect u Selective availability

12. CRGIS Selective Availability u Inaccuracy introduced by the US Department of Defense for national security u Signals from the satellites are deliberately mistimed u Results in average error of 30 meters, but can be as high as 200 meters u Planned phase out over next 10 years

13. CRGIS Average Horizontal Error Satellite clocks1.5 meters Orbit errors2.5 meters Ionosphere5 meters Troposphere0.5 meters Receiver noise/static0.3 meters Multipathing0.6 meters Selective Availability30 meters

14. CRGIS Differential Correction u Most sources of error can be eliminated through Differential GPS (differential correction) u Uses two GPS units to correct errors u Works on the principle that two receivers placed relatively close to each other will have the same conditions, and the same errors u Cannot correct local error sources, such as static and multi-pathing u Only way to get +/- 1 meter accuracy

15. CRGIS Differential Correction u Uses two GPS receivers: rover and base u Base unit set up on a known location u Base measures and records errors by calculating the correct timing based on its know location u Base and rover files compared using time tags u Correction factor applied to rover files

16. CRGIS Differential Correction u Usually done in the office, after field work is completed u New technology is making real-time differential correction possible u Base station beacon broadcasts correction data to the receivers u Receivers correct positions immediately u Important for navigation

17. CRGIS Vocabulary u SV: Space Vehicle, a satellite u Ephemeris: Information on position and orbit of SV’s which is broadcast to the satellite u 3-D: GPS data collection using 4 SV’s u 2-D: GPS data collection using 3 SV’s u SA: Selective Availability; deliberate mistiming of satellite signals by DoD

18. CRGIS Vocabulary u Position: Set of x,y,z coordinates collected by the GPS unit u Feature: Specific object or place on the ground to be mapped; a collection of positions. May be a point, line, or area u File: format in which positions and descriptions are stored in the GPS unit and transferred to the PC

19. CRGIS Vocabulary u Data Dictionary: Selected list of features to be mapped u Attribute: Descriptive information collected for features i.e. feature=road, attribute=name of road u Attribute Value: List of possible values to answer the attribute. e.g., attribute = road surface, att. values=paved,unpaved

20. CRGIS Vocabulary: PDOP u Positional Dilution of Precision u Measure of the quality of the GPS calculations u Based on the geometry of the visible satellites u Best geometry is with SV’s spread evenly across the sky u Low PDOP = high accuracy

21. CRGIS Our Equipment GPS receiver, with dome antenna and battery pack Data logger, or hand- held computer for collecting attribute data People to conduct fieldwork and collect locational data People and computers to correct and edit data

22. CRGIS Basic Steps in Collecting and Using GPS Data u Create a data dictionary u Conduct fieldwork to gather locational and attribute data u Differentially correct and edit locational data u Bring your edited data into a GIS for analysis and use with other data sets