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GPS

GPS. GLONASS. Global Navigation Satellite Systems GNSS. Galileo. X,Y,Z. X,Y,Z. X,Y,Z. X,Y,Z. Measuring distance. ?. The princple of determining your position using GPS. an estimation in space. X,Y,Z. Lat 55 O N. EQUATOR. Lat 55 O S. GPS satellite orbits.

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GPS

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  1. GPS GLONASS Global Navigation Satellite Systems GNSS Galileo

  2. X,Y,Z X,Y,Z X,Y,Z X,Y,Z Measuring distance ? The princple of determining your position using GPS an estimation in space X,Y,Z

  3. Lat 55O N EQUATOR Lat 55O S GPS satellite orbits • 24 satellites (27 today) • 20 200 km height • 11h 58 min orbital time • Inclination 55O

  4. GPS satellites pass in zenit along an east-westerly line just south of Sweden (Lat 55O N) Lat 55O N

  5. Satellite orbits at Lake Vänern

  6. Satellite orbits at the North pole

  7. Satellite orbits at the Equator

  8. GPS signals L1: 19 cm C/A-kod P(Y)-kod L2: 24 cm P(Y)-kod Satellite message Orbital data Clock information

  9. Code measurements Measuring distance to satellites Signal from satellite Signal generated in receiver Time difference

  10. If you know the Time.... S=v*t and Speed.... then you know the Distance

  11. A position in three dimensions requires four satellites in practice, due to clock errors Let's start with two dimensions

  12. Pseudorange Clock error re-estimated to distance Code measurements

  13. The z-coordinate and the fourth satellite Let's go on with three dimensions

  14. Positioning in 3 dimensions Coordinates in three dimensions require four satellites

  15. ”Signal for disturbance” Selective Availability Impact of SA, 01 May, 2000

  16. ”Signal for disturbance” Selective Availability No impact of SA, 03 May, 2000 “The images compare the accuracy of GPS with and without selective availability (SA). Each plot shows the positional scatter of 24 hours of data (0000 to 2359 UTC) taken at one of the Continuously Operating Reference Stations (CORS) operated by the NCAD Corp. at Erlanger, Kentucky. On May 2, 2000, SA was set to zero. The plots show that SA causes 95% of the points to fall within a radius of 45.0 meters. Without SA, 95% of the points fall within a radius of 6.3 meters.”

  17. Test of a simple GPS receiver N Point registrations during one hour units in metres

  18. Code measurements Fast Cheap Insensitive for signal interruptions Inaccurate

  19. Absolute positioning One single receiver (code measurments) Relative measurements At least two simultaneous receivers (code and carrier phase measurements) Two different methods for positioning

  20. To achieve high accuracy, relative measurment is required X D D Y Z D

  21. Carrier phase measurements Solving the ambiguity value one period or wave length L1 = 19 cm L2 = 24 cm

  22. Ambiguity value

  23. Solving the ambiguity value, fast static measurements 1.0 m A certain amount of data is needed for solving the ambiguity value 0.01 m 1 min 5 min

  24. Impact of the atmosphere Satellite geometry Signal screening Multipathing - bouncing signals Problems to consider

  25. Retardation of signals Jonosfär 200 km Troposfär 50 km

  26. Elevation angle • Avoid low angles • If carrier phase > 15O • Increased sun activity may require larger angles • Nighttime measurements?

  27. Impact of the atmosphere • Risks with short observation time • L1, L2 och L3-solution • Two frequencies can eliminate impact (L3)

  28. Satellite configuration • Satellite location in relation to the receiver is important • Large spread is desired, both in bearing and elevation Large spread, low DOP-value

  29. Satellite configuration DOP is a measure on the configuration quality Dilution Of Precision (DOPs) GDOP-Geometric DOP Includes Lat, Long, Ht & Time Offset • PDOP-Positional DOP Includes Lat, Long, Ht • VDOP-Vertical DOP Includes Height Only • HDOP- Horizontal DOP Includes Lat, Long Only Low spread, high DOP-value

  30. Screening of GPS signal Signal interruptions can be very disturbing

  31. Bouncing signals • - multipathing Multipahing creates false signals

  32. Aspects to consider Free horizon At least 4 satellites Satellite configuration (DOP-value) Risk for multipathing Elevation angle (carrier phase) Sufficient log-time (carrier phase)

  33. Solving ambiguity values in real time data XYZ Radio modem

  34. Solving ambiguity values, RTK GPS 1.0 m Increased demand on satellite configuration for fast solution 0.02 m 0.25 min 0.5 min

  35. Radio- transmitter Reference station Correction data Differential GPS (DGPS)

  36. Satellit ID Distance correction Error speed Example of RTCM-corrections

  37. Radio transmitter FM-band SWEPOS Correction data DGPS via EPOS correction service

  38. SWEPOS A network of fixed reference stations

  39. EGNOS EGNOS is short for European Geostationary Navigation Overlay Service (”European system for navigation through geostationary overlap” – in practice a satellite based system for improvment of GPS). In USA, it is common for instance among farmers to use the American analogue WAAS. Farmers in Europe will use EGNOS in various precision farming activities. It can also be a tool for improved control of the union’s regulations.

  40. EGNOS - area of coverage

  41. EGNOS stations

  42. Commercial DGPS services Ground based Satellite distribution Equipment required GPS-receiver For instance RDS receiver Subscription

  43. DGPS Simple to use Cheap Cover large areas Insensitive against signal interruptions Problem with DGPS-coverage

  44. Carrier phase measurements in real time within a network of reference stations High accuracy within large areas Only rover unit neede Network RTK Network RTK stations in southern Sweden

  45. Single station-RTK / Network-RTK + = Reference station = Area of coverage

  46. RTK-data + estimated model for ionosphere, tropospher, orbit parametres etc. Network - RTK Virtual reference station

  47. Network-RTK • Long distance between reference stations • Allways quality-controlled data transmitted • User only needs one GPS-receiver • Seamless coverage area • Data communication between rover and estimation server currently expensive (GSM) • Poor GSM coverage

  48. Controlling machinery with high accuracy Accurately follow a predetermined track Excavate according to designed model Possibilities with RTK

  49. Cost 40 k€ Accuracy 0.2 k€ 10 m cm mm Cost vs. accuracy Methods and expected accuracy Absolut positioning, C/A-code <10 m Absolut positioning, P-code < 5 m DGPS through beacon < 2 m DGPS with local reference < 0.5 m Relative carrier phase < 0.02 m Relative carrier phase with advanced estimation technique  mm-nivå

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