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Thales GeoSolutions

Thales GeoSolutions. Introduction to GPS. OBJECTIVES. By the end of this session you will be able to: List the 3 segments of the GPS system Explain how a range to a GPS satellite is obtained List the errors of the GPS system

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Thales GeoSolutions

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  1. Thales GeoSolutions Introduction to GPS

  2. OBJECTIVES By the end of this session you will be able to: • List the 3 segments of the GPS system • Explain how a range to a GPS satellite is obtained • List the errors of the GPS system • State the given accuracy of the GPS Standard Positioning Service (SPS) • Describe the characteristics of GPS signals • Name & describe the three segments of the GPS system

  3. WHAT IS GPS? Global Positioning System GPS provides us with: • Worldwide, continuous, high accuracy, three dimensional position and velocity • Precise time transfer

  4. GPS ACCURACY Standard Positioning System (civilian) Single-frequency (L1) performance: • Position accuracy standard • Global 95% horizontal error – 13 meters • Global 95% vertical error – 22 meters • (L1) user performance given accuracy standard conditions • Global 95% horizontal error – 33 meters • Global 95% vertical error – 73 meters Tests conducted June 2000

  5. GPS ACCURACY GPS single-FREQUECY (L1) all-in-view user performance: • Horizontal error statistics • (95%) global average – 8.3 meters horizontal error • (95%) worst site – 19.7 meters horizontal error • Vertical error statistics • (95%) global average – 16.8 meters vertical error • (95%) worst site – 44.0 meters vertical error Tests conducted June 2000

  6. GPS ACCURACY Cumulative global distribution of 95% accuracy performance: • 95% of the earth had better then 16.4 meters 95% single-frequency horizontal performance • 95% of the earth had better then 29 meters 95% single-frequency vertical performance Tests conducted June 2000

  7. THE HISTORY OF GPS • 1957 USSR LAUNCHES SPUTNIK • 1964 USNSS TRANSIT SYSTEM OPERATIONAL • 1973 US DOD NAVSTAR GPS DEVELOPMENT BEGINS • 1985GPS USED FOR OFFSHORE TASKS • 1986STARFIX DGPS SERVICE LAUNCHED • 1989SKYFIX GLOBAL DGPS SERVICE OPERATIONAL • 1993 FULL GPS 2-D COVERAGE • 1994 FULL GPS 3-D COVERAGE • 1995-2000GLONAS MERGE, INMARSAT NAV. • 2000 SELECTIVE AVAILABILITY TURNED OFF

  8. GPS NAVIGATION

  9. ADVANTAGES OF GPS • For military users: • User passive, jam resistant, selective access, low detectability, nuclear hardening, mil spec equipment. • Receivers for all user classes - fighter aircraft, ships, tanks, bombers, soldiers, etc. • For civilian users: • Low cost continuous positioning for general use • Very high accuracy for industrial / commercial users.

  10. NAVSTAR GPS CHARACTERISTICS • 21 operational satellites & 3 active spares • 12 HOURS, 20200km, NEAR CIRCULAR ORBITS • Dual frequency: 1575.42 MHz & 1227.6 MHz • TWO CODES: • C/A COARSE/ACQUISITION (civil users) • P PRECISE (military & authorized users) • Accuracy of c/a code: (1.S.D.) • 10-15m (no SA) • 50m (S.A.Applied) • Carrier phase applications possible for high precision - stationary/low dynamic

  11. GPS SEGMENTS • Control segment • Monitor stations track all satellites • Master control station injects updated orbit and clock data into satellites • Space segment • 21 satellites plus 3 active spares in 6 orbit planes • 12 hour 20,000 KILOMETRE orbits • At least 4 satellites visible to any user at all times • User segment • User set receive ranging signals from the satellites • Compute position and velocity • Many different types of receivers • Civil and military users

  12. THE GPS SEGMENTS

  13. GPS SYSTEM CONFIGURATION

  14. GPS SEGMENTS • User segment • User set receive ranging signals from the satellites • Compute position and velocity • Many different types of receivers • Civil and military users

  15. GPS CONTROL STATIONS

  16. GPS CONSTELLATION 21 SATELLITES WITH 3 OPERATIONAL SPARES 6 ORBITAL PLANES, 55 DEGREE INCLINATIONS 20,200 KILOMETRE, 12 HOUR ORBITS

  17. THE GPS SATELLITE

  18. THE GPS BLOCK 1 SATELLITE

  19. GPS Block IIR SPACE VEHICLE

  20. GPS SYSTEM - SPACE VEHICLES

  21. GPS SATELLITE INSTALLATION

  22. THE MS 750 GPS RECIEVER

  23. GPS SIGNALS (Observables) • Signal L1 L2 • Carrier Frequency 1575.42 1227.60 • Wavelength 19 cm 24 cm • Codes C/A, P (P1) P (P2) • NAVDATA (ephemeris, clock corrections, etc)

  24. GPS SIGNALS • The p-code is clocked at 10.23 MHz and is 7 days long (apparent wavelength = 30 Meters) • The c/a-code is clocked at 1.023 MHz and is 1ms long (apparent wavelength = 300 meters) • The c/a code is used to acquire a satellite • The c/a code provides the means to acquire the p-code • Under normal circumstances, the p-code is encrypted and referred to as the y-code • The navigation message can be decoded once either code is acquired and tracked

  25. GPS SIGNALS C/A CODE • 1023 bit binary sequence with a period of 1 millisecond P CODE • 2.34*1014 bit binary sequence with a period of 38 weeks NAVIGATION MESSAGE • Binary format of 1500 bits transmitted at 50hz includes orbital, clock and ionospheric information

  26. THE GPS NAVIGATION MESSAGE

  27. EPHEMERIS TYPES PRECISE • Observed data from tracking stations available after approximately 10 days over the Internet BROADCAST • Available from the Navigation Message. Full Kepler elements ALMANAC • Available from the Navigation Message. Reduced Kepler elements, used for acquisition and predictions

  28. THE C/A CODE AMBIGUITY

  29. GPS ERROR SOURCES • Satellite clock offset from GPS time • Satellite ephemeris • Atmospheric delays (Ionosheric and Tropospheric) • Receiver clock offset from GPS time • Receiver Multipath • Receiver measurement noise • Satellite geometry (-DOP) • Selective Availability (SA) – Switched off May 2000

  30. Thales GeoSolutions Introduction to Differential GPS

  31. OBJECTIVES By the end of this session you will be able to: • Explain why Differential corrections are needed • Explain the operation of a differential correction system • Explain why time is critical in the application of differential corrections • Explain how dual-frequency DGPS operations overcome Ionospheric delay • Explain how SDGPS differs from DGPS • Explain the techniques used in SDGPS & SkyFix-XP

  32. DIFFERENTIAL GPS • Differential GPS is the use of a correction signal to improve the accuracy of the Standard GPS • DGPS utilizes GPS receivers at a fixed reference station and at a mobile vehicle, vessel or user • The GPS determined position of a reference station is compared to it’s surveyed geodetic position from which a correction is derived • Some DGPS systems use the error in fix position, while others use individual satellite range errors to calculate the correction • The correction message is broadcast via a radio or satellite link

  33. DIFFERENTIAL GPS PRINCIPLES

  34. RTCM SC-104 MESSAGE TYPESRadio Technical Commission for Maritime Services

  35. CONSIDERATIONS FOR DGPS CORRECTIONS Speed • With SA on correction latency must be under 10 seconds and with SA off must be under 50 seconds Accuracy • Must be free of errors Range • Must be capable of transmitting corrections from the reference station to the work area Distance • Distance from reference station effects accuracy

  36. INMARSAT COMMUNICATION SATELLITES

  37. SKYFIX USES THE INMARSAT SYSTEM

  38. CORRECTIONS ARE MONITORED 24/7

  39. SKYFIX - NETWORK • Operational since 1989 • Global DGPS system using Inmarsat for broadcast links • Also available over high-power SpotBeam links • World-wide network of >80 reference stations • Two control centers at Aberdeen and Singapore provide 24 hour monitoring and quality control • Fully redundant equipment and links

  40. SKYFIX CO-ORDINATIONS • Existing control • GPS carrier phase data • Broadcast Ephemeris • Local/ITRF/DMA control used • Accuracy considered better than 2 meter using single reference station and better then 1-meter using multiple reference station network solution

  41. SKYFIX CO-ORDINATION • CURRENT AND FUTURE CONTROL • LOCAL GPS DATA REPROCESSED WITH IGS • PRECISE EPHEMERIS (JPL) • ADJUSTMENTS CONSTRAINED TO IGS EPOCH 1992.5 CO-ORDINATES • ACCURACY CONSIDERED TO BE BETTER THAN 5cm

  42. SKYFIX - REFERENCE STATIONS • Dual equipment, redundancy of communications • Typically 12 channel Trimble 4000 DS receivers with RS 4000 software • High level of remote access and control capabilities (change of parameters, initialization of logging etc) • Both sets of RTCM data returned to control center • Multipath audit every 6 months • Coordinated relative to ITRF91 - WGS84 (G730) • 24 hour local support at each station

  43. BASIC REFERENCE STATION LAYOUT

  44. RIMS A 19 inches rack RIMS A 19 inches rack RIMS A THALES SKYFIX INFRASTRUCTURE GPS Satellites High Power and Low Power satellite links VSAT Satellites • 2 MCC facilities • Aberdeen and Singapore • 2 remote MCC facilities at Perth and Reston • Monitor and Control • Archive • Maintain and Plan • Manage external entities • Interface MultiFix III • LES facilities • Uplink SkyFix messages • Including SkyFix Premier Messages X25NETWORK • 80+ Reference Stations • Dual-Frequency Stations • Single Frequency • Generate, process and transmit messages

  45. THE IONOSPHERE – A SUMMARY Two distinct problems: 1. Differential Ionospheric delay errors • Propagate into the navigation solution causing position biases 2. Scintillation effects • Loss of DGPS corrections from satellite DGPS links • Intermittent tracking of GPS satellites

  46. SOLAR DISTURBANCE AND THE IONOSPHERE

  47. WHERE: AREAS AFFECTED Operational ‘HotSpots’ Scintillation Severe Disturbance Severe Disturbance Severe Disturbance Geomagnetic Boundaries

  48. GLOBAL IONOSPHERIC TEC MAP

  49. SOLAR ACTIVITY CYCLE

  50. SKYFIX PREMIER • Introduced to mitigate ionospheric disturbances • Use dual-frequency GPS reference stations • SkyFix premier messages (type 55’s) • Dual-frequency GPS user setup • Thales’ MultiFix III or MultiFix IV software • Calculated Iono-free DGPS positions: unbiased and consistent performance around the clock

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