Global Positioning System

1. Global Positioning System Anurag Mishra Deputy Director Forest Survey of India, Dehradun

2. Outline for Today Today, we will review the basics of the GPS system and • Its history • Key components • Functioning • Applications etc.

3. Trying to figure out where you are and where you're going is probably one of man's oldest pastimes

4. Latitude & Longitude 78°01’31.2” E and 30°20’01.6” N 135° 7’45.9” W and 8°37’24.4” S

5. A Little Bit of History • In the past, humans had to go to pretty extreme measures to keep from getting lost. • They erected monumental landmarks, laboriously drafted detailed maps and learned to read the stars in the night sky. • For centuries, only way to navigate was to look at position of sun and stars.

6. Things are much easier today Starting Rs.10,000/- you can get a pocket-sized gadget that will tell you exactly where you are on Earth at any moment. As long as you have a GPS receiver and a clear view of the sky.

7. Why GPS ? Accurate & Precise Efficient, Economical Easy to Operate Portable Navigation Works Everywhere Additional Information

8. What is GPS? • Satellite-based navigation system • Continuously transmits coded information • Precisely identify locations • Measuring distances from the satellites Man-made stars

9. Introduction • Developed by US Department of Defense in 1978 • 24 Satellites in 6 orbits • Situated at an altitude of 20,200 km • Life of Satellite is about 7.5 to 10 years • 12 hours period and orbit is precisely predictable

10. Contd. • Satellite clock: Atomic (Rubidium, Cesium) • Powered by solar energy • There are no subscription fees or setup charges to use GPS • No restriction in using GPS signals • Doesn’t work under dense canopy, covered areas • GPS works in all weather conditions

11. Global Positioning System (GPS) NAVSTAR NAVigationSatellite Timing And Ranging satellites (NATO) GLONASS GLObalNAvigationSatellite System (Russian) Galileo To be operational by 2012 (EU)

12. NAVSTAR • The only fully functional Global Navigational Satellite System • Constellation of at least 24 Medium Earth Orbit Satellites that transmit precise Microwave signals, the system enables a GPS Receiver to determine its Location, speed/direction, and time • The cost of maintaining the system is approximately US$750 million per year, including the replacement of aging satellites, and research and development

13. GPS CONSTELLATION

14. What does a GPS receiver do? Position and coordinates. The distance and direction between any two waypoints What direction you are heading Some models can show you: how fast you are going your altitude a map to help you arrive at a destination

15. How does the GPS work? • Using satellites in the sky, ground stations on earth, and a GPS receiver, the distances between each of these points can be calculated. • The distance is calculated based on the amount of time it takes for a radio signal to travel between these points. • This allows the GPS receiver to know where you are, in terms of latitude and longitude, on the earth.

16. Triangulation A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called triangulation or trilateration. Triangulation in three-dimensional space can be a little tricky, so we'll start with an explanation of simple two-dimensional trilateration.

17. Triangulation

18. 3D Triangulation • Fundamentally, three-dimensional trilateration is not much different from two-dimensional trilateration, but it's a little trickier to visualize. • Imagine the radii from the examples in the last section going off in all directions. So instead of a series of circles, you get a series of spheres.

19. GPS Triangulation If you know you are 10000 miles from satellite A in the sky, you could be anywhere on the surface of a huge, imaginary sphere with a 10000-mile radius. 10000 miles Earth

20. GPS Triangulation (Cont’d) If you also know you are 15000 miles from satellite B, you can overlap the first sphere with another, larger sphere. The spheres intersect in a perfect circle. 15000 miles 10000 miles

21. GPS Triangulation (Cont’d) Perfect circle formed from locating two satellites Possible Locations of GPS Receiver The circle intersection implies that the GPS receiver lies somewhere in a partial ring on the earth.

22. GPS Triangulation (Cont’d) If you know the distance to a third satellite, you get a third sphere, which intersects with this circle at two points.

23. GPS Triangulation (Cont’d) The Earth itself can act as a fourth sphere -- only one of the two possible points will actually be on the surface of the planet, so you can eliminate the one in space. Receivers generally look to four or more satellites, however, to improve accuracy and provide precise altitude information.

24. Calculating Distance Distance = Speed x time

25. Three Segments of the GPS Space Segment User Segment Control Segment GroundAntennas Monitor Stations Master Station

26. The Space Segment • Arranged in the orbits in such a way that at least 4 satellites are always available • Circle earth once every 12 hours • Functions • Receive and store information from ground control segment • Maintain very accurate time • Transmit signal to the earth

27. The Control Segment Master Control Station Monitor Station Ground Antenna US Space Command Cape Canaveral Hawaii Kwajalein Atoll Diego Garcia Ascension Is.

28. The User Segment

29. User Segment • Military. • Search and rescue. • Disaster relief. • Environment, Forestry & Wildlife • Marine, aeronautical and terrestrial navigation. • Remote controlled vehicle and robot guidance. • Satellite positioning and tracking. • Shipping. • Geographic Information Systems (GIS). • Recreation.

30. GPS Receivers Better units have multiple receivers, so they can pick up signals from several satellites simultaneously. Radio waves travel at the speed of light (about 186,000 miles per second, 300,000 km per second in a vacuum). The receiver can figure out how far the signal has traveled by timing how long it took the signal to arrive.

31. Downloading of GPS Data • Data Cable • Mapsource • Pathfinder

32. Standard Positioning System (SPS) • Provided on the GPS L1 frequency. Contains a coarse acquisition (C/A) code and a navigation data message. • The P-code and the L2 frequency is not unavailable to SPS users. • Accuracy • 100 m in horizontal position • 156 m in the vertical component

33. Precise Positioning System (PPS) • Available to authorized military users and users with PPS receivers • This consists of the SPS signal plus the P code on L1 and the carrier phase measurements on L2 • Accuracy • 22 m in horizontal position • 27 m in the vertical component • DGPS is used for higher accuracy

34. Differential GPS • There is no such thing as a Differential GPS • It is the Differential capability • Geodetic GPS

35. Differential GPS Uses the point position derived from either the C/A or P-codes Applies correction to that position. These corrections, difference of determined position and the known position, are generated by a reference receiver, whose position is known and is fed to the instrument. Used by the second receiver to correct its internally generated position.

36. Real Time Differential GPS x+5, y-3 x+30, y+60 x-5, y+3 Receiver DGPS Receiver DGPS Site True coordinates = x+0, y+0 Correction = x-5, y+3 DGPS correction = x+(30-5) and y+(60+3) True coordinates = x+25, y+63

37. Causes of Errors Ionosphere and troposphere delays Signal multipath Orbital errors Number of satellites visible Satellite geometry/shading Intentional degradation of the satellite signal

38. Sources of Signal Interference Earth’s Atmosphere Solid Structures Electro-magnetic Fields Metal

39. Sources of GPS Error • Standard Positioning Service (SPS ): Civilian Users • SourceAmount of Error • Satellite clocks: 1.5 to 3.6 meters • Orbital errors: < 1 meter • Ionosphere: 5.0 to 7.0 meters • Troposphere: 0.5 to 0.7 meters • Receiver noise: 0.3 to 1.5 meters • Multipath: 0.6 to 1.2 meters • Selective Availability • User error: Up to a kilometer or more

40. Introduced Errors in GPS Selective Availability To reduce horizontal positioning capabilities from approximately 20 m to 100m Anti Spoofing Encryption of the ‘P-Code’

41. Receiver Errors are Cumulative! System and other flaws = < 9 meters User error = +- 1 km

42. Ideal Satellite Geometry N E W S

43. Good Satellite Geometry

44. Poor Satellite Geometry

45. Planning a Navigation Route = Waypoint Start

46. Applications in Forestry Location of Plantations Area and Perimeter Areas Assessment of TOF Resources

47. Wildlife Management Wildlife Census, Habitats Direct/Indirect sightings Wildlife offenses Settlements

48. Habitations & Encroachments Forest Villages Encroachments Settlements inside forests Delineation of Areas

49. Boundary Pillars Location of Pillars Bearings Distance between pillars Track between the pillars

50. Use of GPS by FSI Ground truthing Forest Inventory Assessment of TOF Monitoring of FDAs