3. What is the GPS? The Global Positioning System (GPS) is a space-based satellite navigation system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites satellite navigationlocation Developed by US Department of Defense Official name of GPS is NAVigational Satellite Timing And Ranging Global Positioning System (NAVSTAR GPS)

4. Global Navigation Satellite Systems (GNSS) NAVSTAR – USA GLONASS – Russians Galileo – Europeans

5.  Feasibility studies begun in 1960’s.  Pentagon appropriates funding in 1973.  First satellite launched in 1978.  June 26, 1993 Air Force launched 24th SV  System declared fully operational in April, 1995. The History of GPS

6. GPS Satellite Vehicle Four atomic clocks Three nickel-cadmium batteries Two solar panels – Battery charging – Power generation – 1136 watts S band antenna—satellite control 12 element L band antenna—user communication Block IIF satellite vehicle (fourth generation)

7. GPS Satellite Vehicle Weight – 2370 pounds Height – 16.25 feet Width – 38.025 feet including wing span Design life—10 years Transmitter power -50 watts

8. GPS Signals Signals driven by an atomic clock – Fundamental Frequency at 10.23 MHz Two carrier signals (sine waves): – L 1 carrier : f=1575.43 MHz, ( =19 cm ), generated by 10.23 MHz x 154 – L 2 carrier: f=1227.60 MHz, ( =24 cm), generated by 10.23 MHz x 120 Bits encoded on carrier by phase modulation: – C/A-code (Clear Access / Coarse Acquisition): 1.023 MHz ( =300 m ), 10.23/10 – P-code (Protected / Precise): 10.23 MHz ( = 30 m ) at fundamental frequency – Navigation Message: (system time, “Broadcast” orbits, satellite clock corrections, almanacs, ionospheric information, etc.), 50 bps on both L1 and L2 Codes – CA Code use L1 (civilian code) – P (Y) Code use L1 & L2 (military code)

9. Speed Equation GPS receiver uses speed equation to calculate the distance to satellites Distance = Speed × Duration (Time)

10. Example of speed equation If you don’t know the distance between pitcher and catcher, you can calculate it with the speed and duration of the ball Speed : 150km/h Distance ??? 150km/h ≈ 40m/s If it took 1 second, the distance is about 40m.

11. GPS satellites use radio signal Instead of throwing balls, the GPS satellites send radio wave and GPS receivers catch them. Radio wave fly at the same speed of light. (about 300,000 km/s)

12. Signal from GPS satellites Each satellites continually transmits messages including : the time the message was transmitted presise orbital information (location of itself, ephemeris) rough orbits of all GPS satellites (the almanac)

13. Precise calculation of time

14. SUMMING-UP The key idea of positioning by GPS is ‘Trilateration’ Two factors for trilateration 1. the location of each satellite (at least three satellites) 2. the distance from the satellite Distance = speed of light × duration time Satellites send radio signal including time and location

15. Signal Structure Each satellite transmits its own unique code The P code is encrypted into Y code before tx’on and requires decryption equipment in rx’er. Both P code and C/A code are publicly available,but the P code cannot recovered in a GPS rx’er without a knowledge of Y code decryption algorithm. The P(Y) and C/A code tx’ed by each satellite create direct sequence spread spectrum signals which occupy the same frequency bands.

16. GPS Functionality GPS systems are made up of 3 segments – Space Segment (SS) – Control Segment (CS) – User Segment (US)

17. Control Segment Space Segment User Segment Three Segments of the GPS Monitor Stations Ground Antennas Master Station

18. Space Segment: 24 GPS space vehicles(SVs). Satellites orbit the earth in 12 hrs. 6 orbital planes inclined at 55 degrees with the equator. This constellation provides 5 to 8 SVs from any point on the earth. orbit ~ 11h 58min

20. Space Segment--Information The GPS uses a constellation of 24 satellites that orbit the earth at about 11,000 nautical miles, once every 12 hours. The orbital position is constantly monitored and updated by the ground stations. Each satellite is identified by number and broadcasts a unique signal. The signal travels at the speed of light. Each satellite has a very accurate clock, 0.000000003 seconds 19

21. 20 Space Segment--Satellite Signals Because the GPS receiver calculates its location by trilateration, the task of the receiver is to determine its distance from multiple satellites. The GPS system uses two types of signals to calculate distance. – Code-phase ranging – Carrier-phase ranging

22. Space Segment--Satellite Signals-- Code-Phasing Ranging Each satellite has a unique signal. It continuously broadcasts its signal and also sends out a time stamp every time it starts. The receiver has a copy of each satellite signal and determines the distance by recording the time between when the satellite says it starts its signal and when the signal reaches the receiver. 21

23. 22 Space Segment--Satellite Signals- -Code-Phasing Ranging – cont. Distance is calculated using the velocity equation. Rearranging the equation for distance: If the system knows the velocity of a signal and the time it takes for the signal to travel from the sender to the receiver, the distance between the sender and the receiver can be determined.

24. 23 Distance Example—Code Phase Ranging The signals from the GPS satellites travel at the speed of light--186,000 miles/second. How far apart are the sender and the receiver if the signal travel time was 0.23 seconds? It should be clear that this system requires very accurate measurement of time and synchronization of clocks. These time errors limit the precision of this system.

25. Space Segment—Carrier- Phase Ranging Surveying quality receivers use the underlying carrier frequency. Easy to determine number of cycles. The proportion of a partial cycle is difficult to determine. This is called phase ambiguity. Phase ambiguity error is resolved by comparing multiple signals from multiple receivers. More precise system.

26. Control Segment The CS consists of 3 entities: – Master Control System – Monitor Stations – Ground Antennas

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

28. GPS Monitor Stations

29. Master Control Station The master control station, located at Falcon Air Force Base in Colorado Springs, Colorado, is responsible for overall management of the remote monitoring and transmission sites. GPS ephemeris is the tabulation of computed positions, velocities and derived right ascension and declination of GPS satellites at specific times for eventual upload to GPS satellites. Six monitor stations are located at Falcon Air Force Base in Colorado, Cape Canaveral, Florida, Hawaii, Ascension Island in the Atlantic Ocean, Diego Garcia Atoll in the Indian Ocean, and Kwajalein Island in the South Pacific Ocean.

30. Monitor Stations Each of the monitor stations checks the exact altitude, position, speed, and overall health of the orbiting satellites. The control segment uses measurements collected by the monitor stations to predict the behavior of each satellite's orbit and clock. The prediction data is up-linked, or transmitted, to the satellites for transmission back to the users. The control segment also ensures that the GPS satellite orbits and clocks remain within acceptable limits. A station can track up to 11 satellites at a time.

31. Monitor Stations (continued) This "check-up" is performed twice a day, by each station, as the satellites complete their journeys around the earth. Variations such as those caused by the gravity of the moon, sun and the pressure of solar radiation, are passed along to the master control station.

32. Ground Antennas Ground antennas monitor and track the satellites from horizon to horizon. They also transmit correction information to individual satellites.

33. User Segment: It consists of receivers that decode the signals from the satellites. The receiver performs following tasks: – Selecting one or more satellites – Acquiring GPS signals – Measuring and tracking – Recovering navigation data

34. User Segment: There are two services SPS and PPS The Standard Positioning Service – SPS- is position accuracy based on GPS measurements on single L1 frequency C/A code – C/A ( coarse /acquisition or clear/access) GPs code sequence of 1023 pseudo random bi phase modulation on L1 freq

35. User Segment: The Precise Position Service – PPS is the highest level of dynamic positioning based on the dual freq P-code – The PPS rx’er tracks both P code and C/A code on L1 and L2 frequencies. – The PPS is used mainly by military users. – The P-code is a very long pseudo-random bi phase modulation on the GPS carrier which does not repeat for 267 days – Only authorized users, this consists of SPS signal plus the P code on L1 and L2 and carrier phase measurement on L2

36. Operation Overview A GPS receiver can tell its own position by using the position data of itself, and compares that data with 3 or more GPS satellites. To get the distance to each satellite, the GPS transmits a signal to each satellite. The signal travels at a known speed. The system measures the time delay between the signal transmission and signal reception of the GPS signal. The signals carry information about the satellite’s location. Determines the position of, and distance to, at least three satellites, to reduce error. The receiver computes position using trilateration.

37. Trilateration

38. GPS Trilateration Each satellite knows its position and its distance from the center of the earth. Each satellite constantly broadcasts this information. With this information and the calculated distance, the receiver calculates its position. Just knowing the distance to one satellite doesn’t provide enough information.

39. 38 GPS Trilateration--cont. When the receiver knows its distance from only one satellite, its location could be anywhere on the earths surface that is an equal distance from the satellite. Represented by the circle in the illustration. The receiver must have additional information.

40. 39 GPS Trilateration With signals from two satellites, the receiver can narrow down its location to just two points on the earths surface. Were the two circles intersect.

41. GPS Trilateration-- cont. Knowing its distance from three satellites, the receiver can determine its location because there is only two possible combinations and one of them is out in space. In this example, the receiver is located at b. The more satellite that are used, the greater the potential accuracy of the position location.

42. How the GPS System Works 24 satellites + spares 6 orbital planes 55° inclination Each satellite orbits twice every 24 hours. At least 4 satellites visible any time of day, anywhere in the world.

43. A 2 Dimensional Example Time for the signal to reach GPS receiver is determined. Distance is computed by multiplying by the speed of light. Distance from two satellites defines 2 points (in 2 dimensional space.)

44. A 2 Dimensional Example The distance from a third satellite narrows the location to an “error triangle.”

45. A 2 Dimensional Example Assume the error in each of our measurements is a constant, k. Solve for k, so that the “error triangle” is as small as possible.

46. Now for 3 Dimensions Distance from a single satellite locates a position somewhere on a sphere.

47. Now for 3 Dimensions Two measurements put the location somewhere on a circle at the intersection of the two spheres.

48. Now for 3 Dimensions Three measurements put the location at one of two points at the intersection of the three spheres.

49. Now for 3 Dimensions A fourth measurement selects one of the two points, and provides enough information to solve for the constant error.

50. THEORY OF POSITIONING Trilateration Measuring distance Accuracy of time and location of satellites

51. Trilateration A method for determining the intersections of three sphere surfaces given the centers and radii of the three spheres. TrilaterationTriangulation

52. Simplifying the problem Let’s assume several factors for easy understanding. GPS Satellites are on the ground (3D  2D) We know the exact location of satellites We can calculate the distance from each of the satellites

53. Your location is somewhere on the circumference of the circle. Satellite 1 200km Start with first satellite

54. Your location must be one of the intersection. (Point A or B) Satellite 1 200km Satellite 2 50km B A Second satellite gives two points

55. Satellite 1 200km Satellite 2 50km B A Satellite 3 150km Now you know you are on point A. Third satellite sets the location

56. Real trilateration in 3D space The intersection of two sphere is circle. (not two points as in 2D)

57. Real trilateration in 3D space Knowing the distances from three satellites gives you two points.

58. Principle of working using GPS The basis of GPS technology is precise measurement of time; Use of orbiting satellite position to find location of receiver by method of resection

59. Pseudoranging

60. S 1 (x 1,y 1,z 1 ) S 2 (x 2,y 2,z 2 ) S 3 (x 3,y 3,z 3 ) S 4 (x 4,y 4,z 4 ) GPS Receiver  1 =  {(x 1 -x)2+(y 1 -y)2+(z 1 -z)2} + c.Δt  2 =  {(x 2 -x)2+(y 2 -y)2+(z 2 -z)2} + c.Δt  3 =  {(x 3 -x)2+(y 3 -y)2+(z 3 -z)2} + c.Δt  4 =  {(x 4 -x)2+(y 4 -y)2+(z 4 -z)2} + c.Δt Principle of GPS Positioning (X,Y,Z)

61. What Information Does a GPS Provide?

62. BASIC NAVIGATION INFORMATION PROVIDED BY GPS UNITS... YOUR CURRENT POSITION - COORDINATES (LATITUDE & LONGITUDE, UTM, MGRS, ETC.) - ELEVATION (0R BETWEEN WAYPOINTS) DIRECTION TO SPECIFIED WAYPOINTS DISTANCE TO SPECIFIED WAYPOINTS - (0R BETWEEN WAYPOINTS) YOUR SPEED OF TRAVEL YOUR DIRECTION OF TRAVEL

63. GPS C/A code accuracy The major sources of errors in C/A code accuracy are Selective Availability Satellite clock Ephemeris error Ionospheric & Tropospheric delay Multipath Atmospheric effects Satellite geometry

65. Selective Availability The intentional introduction of errors for civilian users is called Selective Availability SA was terminated on May 1, 2000 When SA was on, civilian users accuracy was ~100 meters Military has capability to degrade signal in certain “theaters of operation” – this is called “spoofing”

66. The internal satellite and receiver clocks have limited accuracy, and they are not precisely synchronized. Since position computations are highly dependent on accurate timing information, small clock errors can cause significant errors in position computations. Clock Limitations In spite of the synchronization of the satellite and receiver clocks, and small amount of inaccuracy in timing remains. This can result in errors up to 1 meter. To keep clock errors to 1 meter or less, the time error must be be limited to 20-30 nanoseconds.

67. Ephemeris Error (Orbital errors) Inaccuracies in reported position of satellite Even though the satellites are positioned in very precise orbits, slight shifts are possible do to the gravitational influences of the sun and moon. Orbit errors can be as high as 2 meters.

68. Multipath means that the same radio signal is received several times through different paths. For instance, a radio wave could leave a satellite and travel directly to the receiver, but it also bounces off a building and arrives at the receiver at a later time. Multipath Errors

69. Ionospheric & Tropospheric delay

73. Atmospheric Delay The satellite signal slows as it passes through the atmosphere. The GPS system uses a built in model that calculate an average amount of delay to partially correct for this type of error

74. 73 Error-Atmospheric Radio signals travel at the speed of light in space, but are slowed down by the atmosphere. The majority of this effect can be eliminated by the receiver. – Lower frequency signals are slowed down more that high frequencies. – The receiver can determine the difference in the arrival time of high and low frequency signals and calculate a correction.

75. GPS Satellite Geometry  Satellite geometry can affect the quality of GPS signals and accuracy of receiver trilateration.  Dilution of Precision (DOP) reflects each satellite’s position relative to the other satellites being accessed by a receiver.  There are five distinct kinds of DOP.  Position Dilution of Precision (PDOP) is the DOP value used most commonly in GPS to determine the quality of a receiver’s position.  It’s usually up to the GPS receiver to pick satellites which provide the best position triangulation.  Some GPS receivers allow DOP to be manipulated by the user.

76. Satellite Geometry/Shading This refer to the relative position of the satellite at any given time.Ideal satellite geometry exists when the satellite are located at wide angle relatives to each other.Poor geometry results when the satellite are located in a line or in a tight grouping

77. How Error is Measured: DOP (Dilution of Precision) HDOP VDOP PDOP The geometry of the satellite constellation can affect the accuracy of the GPS positions. DOP is an indicator of quality of the constellation at any given time. Lower the DOP, the better the geometry of the constellation and the more accurate the GPS positions.

78. MEASURING GPS ACCURACY The geometry of the constellation is evaluated by Dilution Of Precision, or DOP. DOP

79. Error-Satellite Geometry Describes the position of the satellites with each other. The best geometry, and least error, occurs when the satellites are equally distributed. Satellite geometry error occurs when the satellites are concentrated in on quadrant or in a line. The Positional Dilution of Precision (PDOP) is an indication of the quality of the 3D coordinate satellite geometry. – General surveys PDOP’s should be less than 3. Satellite geometry error is not measureable, it tends to enhance other errors. 78

80. Global Navigation Satellite System (GNSS) Terms As the size of the area increases the dilution of precision increases. The dilution of precision is given in multiple measurements. GDOP – Geometric dilution of Precision – A combination of navigational position and time error PDOP – Positional Dilution of Precision – The spatial geometrical quality of the positional solution. HDOP – Horizontal Dilution of Precision – Measure of the quality of the horizontal position. VDOP – Vertical Dilution of Precision – Measure of the quality of the vertical position TDOP – Time Dilution of Precision – Mean error of the time estimation.

81. Dilution of Precision (DOP) Geometric location of the satellites as seen by the receiver The more spread out the satellites are in the sky, the better the satellite geometry PDOP (position dilution of precision) is a combination of VDOP and HDOP The lower the PDOP value, the better the geometric strength PDOP value less than 6 is recommended

82. DOP RatingDescription 1IdealHighest possible. Required for surveys requiring the highest precision. 2 – 3ExcellentPositional measurements are sufficient for all but the most stringent surveys. 4 – 6GoodMinimum level appropriated for business decisions. 7 – 8ModerateSufficient for calculations, but a more open sky view is recommended. 9-20FairPositional information should only be used to indicate rough locations. 20 – 50 PoorMeasurements are +- 150 feet and are probably useless. 81 Values below 2 will produce acceptable results for most surveys. Values over three should not be used.

83. GPS ERROR BUDGET Different errors can cause a deviation of +/- 50 - 100 meters from the actual GPS receiver position which are : ATMOSPHERIC CONDITIONS: Speed of GPS signal is affected by ionosphere & troposphere. Which cause a deviation of 0 to 30 m. from the actual position of receiver.

84. EPHEMERIS ERRORS: The predicted changes in the orbit of a satellite. Which cause a deviation of 0 to 5 m. from the actual position of receiver CLOCK DRIFT: Due to different code generations in satellite and receiver simultaneously. Which cause a deviation of 0 to 1.5 m. from the actual position of receiver

85. MULTIPATH: Bouncing of GPS signal due to a reflecting surface before reaching to receiver antenna. Which cause a deviation of 0 to 1 m. from the actual position of receiver Multipath errors are caused by satellite signals reflecting off of objects. Increase chance of occurrence when around tall buildings.

86. INCREASING ACCURACY OF GPS Differential correction provides accuracy within 1-5 m. Coarse Acquisition receiver provides accuracy within 1-5m. Carrier Phase receivers provides accuracy within 10-30 cm. Dual-Frequency receivers are capable of providing sub- centimeter GPS position accuracy.

87. Differential Correction Technique used to correct some of these errors Referred to as “differential GPS” or DGPS In DGPS, two GPS receivers are used One receiver is located at an accurately surveyed point referred to as the “base station” A correction is calculated by comparing the known location to the location determined by the GPS satellites The correction is then applied to the other receiver’s (known as the “rover”) calculated position

88. DGPS Site x+30, y+60 x+5, y-3 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 x-5, y+3 Real Time Differential GPS DGPS Receiver Receiver

89. ERRORS Sometimes errors are occurred while the data are transmitted from the satellites to the GPS receivers. Ionosphere and troposphere delays — Signal multipath — Receiver clock errors — Orbital errors — Number of satellites visible — Satellite geometry/shading — Intentional degradation of the satellite signal —

90. CORRECTING ERRORS 1. Some errors can be factored out using mathematics and modeling. 2. The configuration of the satellites in the sky can magnify other errors. 3. Differential GPS can eliminate almost all error.

91. Applications  Industry  Agriculture  Mapping & GIS Data Collection  Public safety  Surveying  Telecommunication

92. Application Contd…..  Military  Intelligence & Target Location  Navigation  Weapon Aiming &Guidance  Transportation  Aviation  Fleet Tracking  Marine

93. Application Contd…  Science  Archaeology  Atmospheric Science  Environmental  Geology & Geophysics  Oceanography  Wildlife