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Differential gps (dgps) 09 04-12

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  • 1. GPS – Course 7 Differential GPS (DGPS) SAJIKRISHNAN. K Scientific Officer NATMO09 April 2012
  • 2. TOPICS OF DISCUSSIONHISTORY OF GPSDGPSERROR POSSIBILITIESDOMAINS OF DGPSCALCULATION & POSITION ESTIMATIONCOMPONENTS OF DGPSSURVEY METHODS IN DGPS
  • 3. IMPORTANT DATES IN THE DEVELOPMENT OF GPS
  • 4. GLOBAL POSITIONING SYSTEM IS DEVELOPED AND FUNDED BYUS GOVERNMENT AND MANAGED BY DEPARTMENT OF DEFENCE. 1973 - DECISION TO DEVELOP A SATELLITE NAVIGATION SYSTEM FOR MILITARY 1974 - 1979 CONDUCTED SYSTEM TESTS BY US AIR FORCE AND NAVY 1977- FIRST RECEIVER TEST WAS CONDUCTED WITHOUT PLACING THE SATELLITE IN THE ORBIT. SIGNALS RECEIVED FROM PSEUDO – SATELLITES. 1978 - 85 A TOTAL OF 11 BLOCK I SATELLITES WERE LAUNCHED. 1979 - DECISION TO EXPAND GPS WITH 18 SATELLITES IN SPACE.
  • 5. 1980 - 1982 FINANCIAL CRISIS OCCURS WHEN THE SPONSERS QUESTIONED THE USEFULNESS OF THE SYSTEM.1988 - NUMBER OF SATELLITES WERE INCREASED TO 24.1983 - CIVILIAN USE OF GPS WAS ALLOWED AFTER SOVIET UNION SHOT DOWN KOREAN AIRPLANE THAT GET LOST OVER SOVIET TERRITORY.1986 - GPS PROGRAMME SUFFERED A SET BACK DUE TO THE ACCIDENT OF CHALLENGER WHICH WAS SUPPOSED TO CARRY BLOCK II SATELLITES TO THE ORBIT. THEN DELTA ROCKETS WERE USED FOR THE PURPOSE.
  • 6. 1989 – FIRST BLOCK II SATELLITES WERE INSTALLED AND ACTIVATED1990 – 1991 TEMPORAL DEACTIVATION OF SA DURING GULF WAR1993 – INITIAL OPERATIONAL CAPABILITY (IOC) WAS ANNOUNCED AND DECIDED WORLDWIDE CIVILIAN USE FREE OF COST.1994 – LAST BLOCK II SATELLITES COMPLETE THE SATELLITE CONSTALLATION1995 – FULL OPERATIONAL CAPABILITY WAS ANNONCED
  • 7. 2000 – FINAL DEACTIVATION OF SA TO GIVE POSITIONAL ACCURACY OF20m FROM 100m.2005 – LAUNCHING OF THE IIRM GPS SATELLITE THAT SUPPORTS THE NEW MILITARY M SIGNAL AND THE SECOND CIVIL SIGNAL L2C.•The current system became fully operational June 26, 1993 when the 24th satellite was lunched.
  • 8. Control StationsSchriever Air Force Base (formerly Falcon AFB) in Colorado.HawaiiAscension IslandsDiego GraciaKwajalein Newly added Control Stations after 2005 Washington DC England Equador Argentinia Bahrain Australia
  • 9. Whats the Differential?Until 2000, civilian users had to contendwith Selective Availability (SA). The DoDintentionally introduced random timingerrors in satellite signals to limit theeffectiveness of GPS and its potentialmisuse by adversaries of the UnitedStates. These timing errors could affectthe accuracy of readings by as much as100 meters.
  • 10. With SA removed, a single GPSreceiver from any manufacturer canachieve accuracies of approximately 10meters. To achieve the accuraciesneeded for quality GIS records from oneto two meters up to a few centimetersrequires differential correction of thedata. The majority of data collectedusing GPS for GIS is differentiallycorrected to improve accuracy.
  • 11. Differential GPS (DGPS) is a relativelysimple technique to improve positionalaccuracy and integrity. This technique wasdeveloped in the early 1980s, and it iswidely used in various forms.
  • 12. DGPS is a method of improving theaccuracy of your receiver by addinga local reference station to augmentthe information available from thesatellites. It also improves theintegrity of the whole GPS systemby identifying certain errors.
  • 13. •Differential GPS uses one unit at aknown location and a rover. –The stationary unit compares its calculated GPS location with the actual location and computes the error. –The rover data is adjusted for the error.
  • 14. The underlying premise of differential GPS(DGPS) is that any two receivers that arerelatively close together will experience similaratmospheric errors. DGPS requires that a GPS receiver be set upon a precisely known location. This GPSreceiver is the base or reference station. The base station receiver calculates its positionbased on satellite signals and compares thislocation to the known location.
  • 15. The difference is applied to the GPS data recordedby the second GPS receiver, which is known asthe roving receiver. The corrected information canbe applied to data from the roving receiver in realtime in the field using radio signals or throughpost-processing after data capture using specialprocessing software.
  • 16. Differential correction techniques are used toenhance the quality of location data gatheredusing global positioning system (GPS) receivers. Differential correction can be applied in real-time directly in the field or when post-processingdata in the office. Although both methods arebased on the same underlying principles, eachaccesses different data sources and achievesdifferent levels of accuracy. Combining bothmethods provides flexibility during datacollection and improves data integrity.
  • 17. Real-Time DGPSReal-time DGPS occurs when the basestation calculates and broadcastscorrections for each satellite as it receivesthe data. The correction is received by theroving receiver via a radio signalAs a result, the position displayed andlogged to the data file of the roving GPSreceiver is a differentially correctedposition.
  • 18. Satellite Differential ServicesAnother method for obtaining real-timedifferential correction data in the field is byusing geostationary satellites. This systemobtains corrections from more than onereference station, sends the information toa geostationary satellite for verification.The verified information is sent to theroving GPS receiver to ensure it obtainsGPS positions in real time.
  • 19. Errors in GPS
  • 20. Error possibilities in GPSThe receiver is not synchronized with theatomic clock in the satelliteThe estimate of the position of the satelliteSpeed of light is only constant in vacuum”Multi path errors” : Ghost signals fromreflected radio waves”Selective availability (SA)” :Added noise fromdepartment of defenseNot free sight to many enough satellitesNoise in the receiver
  • 21. SOURCES OF ERROR IN GPS Error Value Ionosphere 4.0 meters Clock 2.1 meters Ephemeris 2.1 meters Troposphere 0.7 meters Receiver 0.5 meters Multipath 1.0 meter Total 10.4 meters
  • 22. GPS ErrorsNoiseBiasesBlunderClock
  • 23. Errors addressed by dgpsEliminates or reduces clock errors, path errors,ephemeris errors and ionospheric effectsIdea: The errors are almost the same for tworeceivers close to each other • Place a fixed receiver on a well defined location. • Compute the error in the position • Estimate from the satellites • Calculate backwards to find the time error • Broadcast it by radio to other receiver
  • 24. Noise ErrorNoise errors are the combined effect of codenoise (around 1 meter) and noise within thereceiver (around 1 meter).
  • 25. BIAS ERRORSelective Availability (SA) SA is the intentional degradation of the SPS signals by a time varying bias. SA is controlled by the DOD to limit accuracy for non-U. S. military and government users. Selective availability is turned off.Ephemeris data errors: 1 meter Satellite orbits are constantly changing. Any error in satellite position will result in an error for the receiver position.
  • 26. SV clock errors uncorrected by ControlSegment can result in one meter error. Tropospheric delays: 1 meter. The troposphere is the lower part (ground level to from 8 to 13 km) of the atmosphere that experiences the changes in temperature, pressure, and humidity associated with weather changes. Complex models of tropospheric delay require estimates or measurements of these parameters.
  • 27. Bias Error--cont.•Unmodeled ionosphere delays: 10 meters. –The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air. The transmitted model can only remove about half of the possible 70 ns of delay leaving a ten meter un- modeled residual.•Multipath: 0.5 meters. –Multipath is caused by reflected signals from surfaces near the receiver that can either interfere with or be mistaken for the signal that follows the straight line path from the satellite.
  • 28. BlunderBlunders can result in errors of hundred ofkilometers. Control segment mistakes due to computer or human error can cause errors from one meter to hundreds of kilometers.User mistakes, including incorrect geodeticdatum selection, can cause errors from 1 tohundreds of meters.Receiver errors from software or hardwarefailures can cause blunder errors of any size.
  • 29. Differential GPS of USCG Maritime Differential GPS (DGPS) Managed by the U.S. Coast Guard (USCG) Employs ground stations along the coastswith known fixed locations. Corrections are transmitted from ground stations at low frequencies (200-500kHz). Requires an additional Differential Beacon Receiver (DBR) and an additional antenna. Accuracy is a function of the distance from the ground station.
  • 30. WAASWide Area Augmentation System (WAAS) Managed by the FAA Communicates with several ground stations. Provides atmospheric corrections. Early warning of GPS failures. Same frequency as GPS Higher data rate 250 Hz Satellites are in geostationary orbits.
  • 31. Wide Area Augmentation System Geostationary GPS Constellation WAAS satellites WAAS Control Station (EastWAAS Control Local Area System (LAAS) Coast)Station (West Coast)
  • 32. Calculating a Position• Measure distance to satellites.• Obtain satellite positions.• Perform triangulation calculations. (Trilateration)• Adjust local clock bias.
  • 33. Measuring Distance Distance• Distance = Velocity * Time• Velocity is that of a radio wave.• Time is the travel time of the signal.• Measure the travel time.• Receiver generates the same codes as the satellite (PRN codes).• Measure delay between incoming codes and self generated codes.• D = Speed of light * measured delay.
  • 34. Obtain Satellite positions.• Orbital data (Ephemeris) is embedded in the satellite data message.• Ephemeris data contains parameters that describe the elliptical path of the satellite.• Receiver uses this data to calculate the position of the satellite. (X,Y,Z)
  • 35. Perform triangulation calculations. Triangulation in 2D• If location of point A is known, and the distance to point A is known, desired position lies somewhere on a circle.• Could be anywhere along circle A
  • 36. Triangulation in 2D• Distance to two points are known.• Desired position is in one of two locations.
  • 37. • Distance to three points are known.• Position is known!
  • 38. Triangulation in 3D• Distance to two points is known.
  • 39. • Calculating a Position Review• Measure distance to satellites.• Use pseudo ranges• Obtain satellite positions.• Decoded ephemeris from satellite message.• Perform triangulation calculations.• Need at least 3 satellites for triangulation.• Adjust local clock bias to find position.• Need 4th satellite to adjust bias.
  • 40. SURVEY METHODS IN DGPS
  • 41. S1 S2 Master StationS4 S3 Schematic diagram of Rapid Static Method
  • 42. S1Master station S5 S2 S4 S3 Schematic diagram of Traverse method
  • 43. Master station S1S3 S2 Trilateration method
  • 44. DATA DISPLAY IN GPSOnce the GPS receiver has located its position it is usually displayed in one of two common formats: – Latitude and longitude – Universal transverse mercator (UTM).
  • 45. LATITUDES AND LONGITUDESLatitudes and longitudes are angles.Both use the center of the earth asthe vertex, and both utilize theequator, but they use a differentzero reference.
  • 46. LATITUDELatitude gives the location of a place on theEarth north or south of the Equator.Latitude is an angular measurement indegrees (marked with °) ranging from 0° atthe Equator to 90° at the poles (90° N for theNorth Pole or 90° S for the South Pole)The earth’s circumference is approximately 24,859.82miles around the poles. So Each degree of latitude ≈ 69 miles
  • 47. Latitude cont.
  • 48. LongitudeLongitude describes the location of a placeon earth east or west of a north-south linecalled the Prime Meridian. – Longitude is given as an angular measurement ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. – In 1884, the International Meridian Conference adopted the Greenwich meridian as the universal prime meridian or zero point of longitude.
  • 49. Longiude cont.
  • 50. Longitude--cont. The circumference of the earth at the equator is approximately 24,901.55 miles. So Each degree of longitude ≈ 69 milesA longitude of 134o west would be 9,246 west of the prime meridian.
  • 51. Longitude--cont. • There is an important difference between latitude and longitude. • The circumference of the earth declines as the latitude increase away from the equator. • This means the miles per degree of longitude changes with the latitude. • This makes determining the distance between two points identified by longitude more difficult.
  • 52. COMPONENTS OF DGPSMASTER RECEIVER AND ROVERSMASTER RECEIVER IS KEPT ATKNOWN POINT.RELATIVE POSITION OF ROVERS AREFIXED WITH RESPECT TO THEMASTER RECEIVER
  • 53. The main components of GPSreceivers are: Antenna with pre-amplifier Sensor to sense the data Memory and display panel Keyboard Precision oscillator (clock)-quartz Power supply – Ni-Cd - 12v battery Computer with supporting software for data download and processing.
  • 54. Antenna detects the electromagneticwaves arriving form satellite, converts thewave energy into electric current, amplifythe signal strength and hands the signalover to receive electronics. Several antennas are available Monopole or dipole Helix Spiral helix Microstrip Choke ring
  • 55. S1 S2 Master StationS4 S3 Schematic diagram of Rapid Static Method
  • 56. S1Master station S5 S2 S4 S3 Schematic diagram of Traverse method
  • 57. Master station S1S3 S2 Trilateration method
  • 58. GPSDifferent Hardware available Trimble dual frequency 5000series Ashtech single frequency Leica (Garmin) Links point
  • 59. GPSSoftware available for downloadingand processing the data ESRI Arc Pad Links Point GPS Pathfinder Thales Navigation Trimble Data Transfer Trimble Geomatics Office