Differential gps (dgps) 09 04-12
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The document discusses the history and development of GPS and differential GPS (DGPS). It explains that DGPS uses a reference station at a known location to calculate errors in GPS positioning and apply corrections in real-time or post-processing to improve accuracy. The document outlines various DGPS systems, sources of GPS error, DGPS methods like rapid static and traverse, and components of GPS receivers.
Differential gps (dgps) 09 04-12
- 1. GPS – Course 7 Differential GPS (DGPS) SAJIKRISHNAN. K Scientific Officer NATMO 09 April 2012
- 2. TOPICS OF DISCUSSION HISTORY OF GPS DGPS ERROR POSSIBILITIES DOMAINS OF DGPS CALCULATION & POSITION ESTIMATION COMPONENTS OF DGPS SURVEY METHODS IN DGPS
- 3. IMPORTANT DATES IN THE DEVELOPMENT OF GPS
- 4. GLOBAL POSITIONING SYSTEM IS DEVELOPED AND FUNDED BY US 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 ACTIVATED 1990 – 1991 TEMPORAL DEACTIVATION OF SA DURING GULF WAR 1993 – INITIAL OPERATIONAL CAPABILITY (IOC) WAS ANNOUNCED AND DECIDED WORLDWIDE CIVILIAN USE FREE OF COST. 1994 – LAST BLOCK II SATELLITES COMPLETE THE SATELLITE CONSTALLATION 1995 – 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.
- 9. Control Stations Schriever Air Force Base (formerly Falcon AFB) in Colorado. Hawaii Ascension Islands Diego Gracia Kwajalein Newly added Control Stations after 2005 Washington DC England Equador Argentinia Bahrain Australia
- 11. What's the Differential? Until 2000, civilian users had to contend with Selective Availability (SA). The DoD intentionally introduced random timing errors in satellite signals to limit the effectiveness of GPS and its potential misuse by adversaries of the United States. These timing errors could affect the accuracy of readings by as much as 100 meters.
- 12. With SA removed, a single GPS receiver from any manufacturer can achieve accuracies of approximately 10 meters. To achieve the accuracies needed for quality GIS records from one to two meters up to a few centimeters requires differential correction of the data. The majority of data collected using GPS for GIS is differentially corrected to improve accuracy.
- 13. Differential GPS (DGPS) is a relatively simple technique to improve positional accuracy and integrity. This technique was developed in the early 1980s, and it is widely used in various forms.
- 14. DGPS is a method of improving the accuracy of your receiver by adding a local reference station to augment the information available from the satellites. It also improves the integrity of the whole GPS system by identifying certain errors.
- 15. •Differential GPS uses one unit at a known 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.
- 16. The underlying premise of differential GPS (DGPS) is that any two receivers that are relatively close together will experience similar atmospheric errors. DGPS requires that a GPS receiver be set up on a precisely known location. This GPS receiver is the base or reference station. The base station receiver calculates its position based on satellite signals and compares this location to the known location.
- 17. The difference is applied to the GPS data recorded by the second GPS receiver, which is known as the roving receiver. The corrected information can be applied to data from the roving receiver in real time in the field using radio signals or through post-processing after data capture using special processing software.
- 18. Differential correction techniques are used to enhance the quality of location data gathered using global positioning system (GPS) receivers. Differential correction can be applied in real- time directly in the field or when post-processing data in the office. Although both methods are based on the same underlying principles, each accesses different data sources and achieves different levels of accuracy. Combining both methods provides flexibility during data collection and improves data integrity.
- 19. Real-Time DGPS Real-time DGPS occurs when the base station calculates and broadcasts corrections for each satellite as it receives the data. The correction is received by the roving receiver via a radio signal As a result, the position displayed and logged to the data file of the roving GPS receiver is a differentially corrected position.
- 20. Satellite Differential Services Another method for obtaining real-time differential correction data in the field is by using geostationary satellites. This system obtains corrections from more than one reference station, sends the information to a geostationary satellite for verification. The verified information is sent to the roving GPS receiver to ensure it obtains GPS positions in real time.
- 21. Errors in GPS
- 22. Error possibilities in GPS The receiver is not synchronized with the atomic clock in the satellite The estimate of the position of the satellite Speed of light is only constant in vacuum ”Multi path errors” : Ghost signals from reflected radio waves ”Selective availability (SA)” :Added noise from department of defense Not free sight to many enough satellites Noise in the receiver
- 23. 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
- 26. Errors addressed by dgps Eliminates or reduces clock errors, path errors, ephemeris errors and ionospheric effects Idea: The errors are almost the same for two receivers 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
- 27. Noise Error Noise errors are the combined effect of code noise (around 1 meter) and noise within the receiver (around 1 meter).
- 28. BIAS ERROR Selective 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.
- 29. SV clock errors uncorrected by Control Segment 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.
- 30. 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.
- 31. Blunder Blunders can result in errors of hundred of kilometers. Control segment mistakes due to computer or human error can cause errors from one meter to hundreds of kilometers. User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to hundreds of meters. Receiver errors from software or hardware failures can cause blunder errors of any size.
- 32. Differential GPS of USCG Maritime Differential GPS (DGPS) Managed by the U.S. Coast Guard (USCG) Employs ground stations along the coasts with 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.
- 33. WAAS Wide 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.
- 35. Wide Area Augmentation System Geostationary GPS Constellation WAAS satellites WAAS Control Station (East WAAS Control Local Area System (LAAS) Coast) Station (West Coast)
- 36. Calculating a Position • Measure distance to satellites. • Obtain satellite positions. • Perform triangulation calculations. (Trilateration) • Adjust local clock bias.
- 37. 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.
- 39. 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)
- 40. 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
- 41. Triangulation in 2D • Distance to two points are known. • Desired position is in one of two locations.
- 42. • Distance to three points are known. • Position is known!
- 43. Triangulation in 3D • Distance to two points is known.
- 45. • 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.
- 46. SURVEY METHODS IN DGPS
- 47. S1 S2 Master Station S4 S3 Schematic diagram of Rapid Static Method
- 48. S1 Master station S5 S2 S4 S3 Schematic diagram of Traverse method
- 49. Master station S1 S3 S2 Trilateration method
- 51. DATA DISPLAY IN GPS Once the GPS receiver has located its position it is usually displayed in one of two common formats: – Latitude and longitude – Universal transverse mercator (UTM).
- 52. LATITUDES AND LONGITUDES Latitudes and longitudes are angles. Both use the center of the earth as the vertex, and both utilize the equator, but they use a different zero reference.
- 53. LATITUDE Latitude gives the location of a place on the Earth north or south of the Equator. Latitude is an angular measurement in degrees (marked with °) ranging from 0° at the Equator to 90° at the poles (90° N for the North Pole or 90° S for the South Pole) The earth’s circumference is approximately 24,859.82 miles around the poles. So Each degree of latitude ≈ 69 miles
- 54. Latitude cont.
- 55. Longitude Longitude describes the location of a place on earth east or west of a north-south line called 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.
- 56. Longiude cont.
- 57. Longitude--cont. The circumference of the earth at the equator is approximately 24,901.55 miles. So Each degree of longitude ≈ 69 miles A longitude of 134o west would be 9,246 west of the prime meridian.
- 58. 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.
- 59. COMPONENTS OF DGPS MASTER RECEIVER AND ROVERS MASTER RECEIVER IS KEPT AT KNOWN POINT. RELATIVE POSITION OF ROVERS ARE FIXED WITH RESPECT TO THE MASTER RECEIVER
- 60. The main components of GPS receivers 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.
- 61. Antenna detects the electromagnetic waves arriving form satellite, converts the wave energy into electric current, amplify the signal strength and hands the signal over to receive electronics. Several antennas are available Monopole or dipole Helix Spiral helix Microstrip Choke ring
- 62. S1 S2 Master Station S4 S3 Schematic diagram of Rapid Static Method
- 63. S1 Master station S5 S2 S4 S3 Schematic diagram of Traverse method
- 64. Master station S1 S3 S2 Trilateration method
- 65. GPS Different Hardware available Trimble dual frequency 5000 series Ashtech single frequency Leica (Garmin) Links point
- 66. GPS Software available for downloading and processing the data ESRI Arc Pad Links Point GPS Pathfinder Thales Navigation Trimble Data Transfer Trimble Geomatics Office