Global Positioning Systems The Basics

1. Global Positioning SystemsThe Basics CRGIS National Park Service

2. Applying Global Positioning Systems • GPS provides navigational aides • Locating a single point • Navigating between points • 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. How GPS Works • GPS works by triangulating your position on the earth, based on satellite signals • Satellites broadcast radio signals • Receivers pick up the signals • Receivers calculate geographic coordinates from the satellite signals

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

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

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

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

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

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

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

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

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

13. Average Horizontal Error Satellite clocks 1.5 meters Orbit errors 2.5 meters Ionosphere 5 meters Troposphere 0.5 meters Receiver noise/static 0.3 meters Multipathing 0.6 meters Selective Availability 30 meters

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

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

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

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

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

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

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

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

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