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  1. 1. GPS Global Positioning System
  2. 2. GPS Basics • What is GPS? • How does it work?
  3. 3. What is GPS • GPS is a satellite-based system, operated and maintained by the U.S. Department of Defense (DoD), that provides accurate location and timing information to people worldwide.
  4. 4. • The system transmits radio signals that can be used by GPS receivers to calculate position, velocity and time anywhere on earth, any time of day or night, in any kind of weather
  5. 5. • The NAVSTAR GPS concept was developed in the early 1970's to meet the U.S. military’s need for improved navigation and positioning. The first satellite was launched in 1978 and Full Operational Capability (FOC) was achieved in April 1995. • The Global Positioning System is a National resource and an international utility for positioning, navigation and timing.
  6. 6. • The NAVSTAR Global Positioning System (GPS) is a constellation of orbiting satellites that provides navigation data to military and civilian users all over the world.
  7. 7. What is GPS? • NAVSTAR GPS (Navigation Satellite Timing and Ranging system) • 24 Satellites orbiting the earth • Positioning, navigation and timing • Operates 24 hrs/day, in all weather • Can be used for any application that requires location information
  8. 8. THREE SEGMENTS OF GPS • Space •Control •User
  9. 9. SPACE SEGMENT • The space segment is the satellite constellation. The first Block I satellite was launched in early 1978. • The 1986 Challenger disaster slowed the GPS constellation development. In February 1989 the first Delta 2 launch took place. The constellation is now fully operational and consists of 24 or more satellites.
  10. 10. CONTROL SEGMENT • The control segment is operated by the U.S. Department of Defense (DoD) which tracks and maintains the satellites. The Department of Transportation (DoT) now has management responsibility, along with DoD.
  11. 11. USER SEGMENT • The user segment consists of both military and civilian users. Military uses of GPS include navigation, reconnaissance, and missile guidance systems. • Civilian use of GPS developed at the same time as military use, and has expanded far beyond anyone's original expectations
  12. 12. GPS Segments User Control Space
  13. 13. Space Segment: GPS Satellites • Power – Sun-seeking solar panels – Ni, Cd batteries • Timing – 4 atomic clocks
  14. 14. • The GPS satellites weigh about 900 kg and are about 5 meters wide with the solar panels fully extended. • They are built to last about 7.5 years, but many have outlasted their original estimated life-span. • The solar panels provide primary power; secondary power is provided by Nicad batteries. On board each satellite are four highly accurate atomic clocks. • As of May 2003, there are 28 usable satellites in place.
  15. 15. Satellite Orbits • 24 satellites in 6 orbital planes • Orbit the earth at approx. 20,200 km (11,000 nautical miles) • Satellites complete an orbit in approximately 12 hours • Satellites rise (and set) approximately 4 minutes earlier each day
  16. 16. • There are four satellites in each orbit plane, and each plane is inclined 55 degrees relative to the equatorial plane (the satellite path crosses the equator at a 55 degree angle). • The high altitude ensures that satellite orbits are stable, precise and predictable, and that the satellites' motion through space is not affected by atmospheric drag. It also ensures satellite coverage over large areas.
  17. 17. Satellite Signals • GPS satellites broadcast messages via radio signals on 2 frequencies – L1: 1575.42 MHz (C/A and P/Y code) – L2: 1227.60 MHz (P/Y code) • Two levels of service – Standard Positioning Service (SPS) – Precise Positioning Service (PPS)
  18. 18. • The radio signals travel at the speed of light: 300,000 km per second (186,000 miles per second). • It takes 6/100ths of a second for a GPS satellite signal to reach earth. These signals are transmitted at a very low wattage (about 300-350 watts in the microwave spectrum).
  19. 19. Coarse Acquisition Code • C/A Code - Coarse/Acquisition Code available for civilian use on L1 provides 300 m resolution. • C/A code (Coarse Acquisition code) is available to civilians as the Standard Positioning Service (SPS).
  20. 20. Precise Code • P Code - Precise Code on L1 and L2 used by the military provides 3m resolution • The Precise Positioning Service (PPS), available only to the military (and other authorized users), provides higher accuracy via the P code.
  21. 21. Satellite Signals, cont. • Radio signals contain – Unique pseudorandom code – Ephemeris – Clock behavior and clock corrections – System time – Status messages – Almanac
  22. 22. Unique pseudorandom code • Each satellite transmits a radio signal containing its unique pseudorandom (appears to be random but is not) code. • This code identifies the satellite and distinguishes it from other satellites
  23. 23. • Ephemeris data : The signal also contains the precise location of the satellite (ephemeris data). • Status messages (usually referring to satellite health).
  24. 24. Almanac Data • An almanac is also provided which gives the approximate location data for each active satellite. • The almanac is automatically downloaded from the satellites to the GPS receiver when the receiver is operating outside.
  25. 25. • It takes about 12 minutes to receive an almanac. The almanac data can be transferred to the office computer and used to display a graphic showing the locations of all satellites. This information can also be used to predict satellite availability for a specific mapping time and date.
  26. 26. Satellite Signals, cont. • Satellite signals require a direct line to GPS receivers • Signals cannot penetrate water, soil, walls or other obstacles
  27. 27. Satellite Almanac • Sent along with position and timing messages • Prediction of all satellite orbits • Needed to run satellite availability software • Valid for about 30 days
  28. 28. • The almanac has information about the orbits of all 24 satellites. • A GPS receiver uses the almanac (for quick acquisition of satellite positions), along with satellite data messages, to precisely establish the position of each satellite it is tracking. • Satellite availability software uses the almanac to make graphs of satellite locations overhead and to calculate the best times to survey in a particular area.
  29. 29. • GPS receivers automatically collect a new almanac each time they are turned on. • It is important to use an up-to-date almanac when viewing satellite availability during mission planning. • Almanac data are valid for about 30 days, but a new almanac should be transferred to satellite availability software as frequently as possible.
  30. 30. GPS ORBITS
  31. 31. Control Segment: US DoD Monitoring Colorado Springs Hawaii Ascension Diego Garcia Kwajalein
  32. 32. U.S. DoD Monitoring • Orbits are precisely measured • Discrepancies between predicted orbits (almanac) and actual orbits are transmitted back to the satellites
  33. 33. User Segment
  34. 34. How Does GPS Work? Calculating a Position • GPS receiver calculates its position by measuring the distance to satellites (satellite ranging)
  35. 35. Measuring Distance to Satellites 1. Measure time for signal to travel from satellite to receiver 2. Speed of light x travel time = distance Speed of light (300,000 km/sec)
  36. 36. Trilateration: 3 Distance Measurements 122 mi 80 mi 127 mi
  37. 37. • Here is an example of trilateration in two dimensions. • Three ranges will locate a point in two-dimensional space. • If we know that our location is 127 miles from Great Falls, then we are somewhere on the red circle.
  38. 38. • If we also know that we are 122 miles from Billings, then our position is somewhere on the purple circle, and, if we are 80 miles from Helena, we are somewhere on the green circle. • Considering the three range measurements together, our position must be where the three circles intersect, or, Bozeman!
  39. 39. Measuring Travel Time of Satellite Signals • How do we find the exact time the signal left the satellite? • Synchronized codes Time difference
  40. 40. • In order to measure the travel time of the satellite signal, we have to know when the signal left the satellite AND when the signal reached the receiver. • Our receiver "knows" when it receives a signal, but how does it know when the signal left the satellite?
  41. 41. • GPS satellites generate a complicated set of digital codes (shown red) • They are "pseudo-random" sequences that actually repeat every millisecond. • The trick is that the GPS satellites and our receivers are synchronized so they're generating the same code at exactly the same time. Time difference
  42. 42. • The time difference is how long the signal took to get from the satellite to the receiver. • In other words, the receiver compares how "late" the received satellite code is, compared to the code generated by the receiver itself. Time difference
  43. 43. One measurement narrows down our position to the surface of a sphere
  44. 44. • 12,000 miles is the radius of a sphere centered on the satellite. • Our position could be anywhere on the surface of that sphere. • The intersection of two spheres is a circle. Now we know that our position is somewhere on that circle
  45. 45. A second measurement narrows down our position to the intersection of two spheres
  46. 46. A third measurement narrows down our position to just two points
  47. 47. • The three spheres intersect at only two points. Usually we can discard one of the two points because one point might be nowhere near the earth or it might be moving at a ridiculous speed. • The computers in GPS receivers have various techniques for distinguishing the correct point from the incorrect one. But there is a reason we need a fourth measurement...
  48. 48. Correcting for Timing Offset • The first three measurements narrow down our position • A fourth measurement is needed to correct for timing offset (difference in synchronization between satellite and receiver clocks) – Satellites use highly accurate atomic clocks – Receivers use accurate quartz clocks
  49. 49. • Timing offset refers to the difference in synchronization between the satellite clock and the receiver clock. • Atomic clocks are far too expensive to put in GPS receivers, so a correction must be applied to compensate for the difference between the satellite and receiver clocks.
  50. 50. • In an ideal situation there would be no timing error. • Let's say we're 4 seconds from satellite A and 6 seconds from satellite B. • Our position is where the 2 circles intersect.
  51. 51. 6 seconds 4 seconds A B
  52. 52. • If the receiver clock is one second fast (it's ahead one second from the satellite clock). • The receiver will "think" the distance from satellite A is 5 seconds and the distance from satellite B is 7 seconds. • And it "thinks" our position is where the two dotted circles intersect.
  53. 53. 6 seconds 4 seconds 5 seconds (wrong time) 7 seconds (wrong time) A B
  54. 54. • Back to the ideal situation with no timing error. • If we have accurate clocks, and we add a third measurement, all three circles intersect at the correct point, because the circles represent the true ranges from the three satellites
  55. 55. 6 seconds 4 seconds 8 seconds A B C
  56. 56. • But with inaccurate clocks, the circles cannot intersect: there is no point that can be 5 seconds from A, 7 seconds from B and 9 seconds from C. • When the receiver gets a series of measurements that cannot intersect at a single point, it finds the adjustment to all measurements that lets the ranges go through one point. In this example, subtracting 1 second from all three measurements makes the circles intersect at a point.
  57. 57. 5 seconds (wrong time) 7 seconds (wrong time) 9 seconds (wrong time) B A C
  58. 58. What’s wrong with this picture?
  59. 59. What’s wrong with this picture?
  60. 60. 5 Things to Take Away Today 1. 3 GPS segments 2. Satellites transmit radio signals containing – Unique pseudorandom code – Ephemeris – Clock behavior and clock corrections – System time – Status messages – Almanac 3. Formula for satellite ranging (D = t ∙ v) 4. 4 satellites to compute an accurate 3-D position (the 4th measurement is needed to correct for timing offset)