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How does the GNSS receiver works ?

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How does the GNSS receiver works ? Join the OpenGNSS community on www.OpenGNSS.org to participate to an open source GNSS receiver development

How does the GNSS receiver works ? Join the OpenGNSS community on www.OpenGNSS.org to participate to an open source GNSS receiver development

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  • 1. www.OpenGNSS.org OpenGNSS www.OpenGNSS.org proposes you this course free of charge OpenGNSS is a project cofinanced by HELILEO (www. HELILEO.com), TECNALIA (www. TECNALIA.es), the Regional Council of Aquitaine and the Basque government
  • 2. www.OpenGNSS.org How does a GNSS receiver work ?
  • 3. www.OpenGNSS.org Summary I. General Principle II. Solving the problem of navigation III. Error correction IV. Notions of integrity 3
  • 4. www.OpenGNSS.org Introduction (1/2) • GNSS Receivers : GPS, GLONASS, Galileo • Application domains : guidance, data synchronization, geodesy • Position computing thanks to satellite signal reception time Errors to consider: • Clock bias • Environment (troposphere, ionosphere, multipath) • Dynamic (movement / satellite receiver: Doppler effect) • Noise 4
  • 5. www.OpenGNSS.org Introduction (2/2) Functioning scheme 5
  • 6. www.OpenGNSS.org I. General principle a) Pseudo-range computation b) Process based upon code measurement c) Process based upon phase measurement 6
  • 7. www.OpenGNSS.org a) Pseudo-range computation (1/2) Receipt channel scheme Receipt channel: Distance - Correlator -Tracking loop Received signal - Data decoding Velocity - Distances computation - Velocity computation Navigation data Satellite code 7
  • 8. www.OpenGNSS.org a) Pseudo-range computation (2/2) Where c is the speed of the wave in the space when the wave is received when the wave is emitted You get then the "pseudo-range" + the clock offset errors 8
  • 9. www.OpenGNSS.org b) Process based upon code measurement (1/3) • Each satellite uses a pseudo random noise code (PRN code) associated to its signal • Periodical signal which value is 0 or 1 • Same signal generated by the receiver Received signal Correlator output Local signal • Use of a correlator • Correlation peak= dt (clock bias) 9
  • 10. www.OpenGNSS.org b) Process based upon code measurement (2/3) 10
  • 11. www.OpenGNSS.org b) Process based upon code measurement (3/3) Code pursuit • Use of correlators (Early, on time, late) • Differentiation=feedback control loop • Ambiguity on measurement, simple but not very precise and surrounded by noise • Phase measurement=> precision more accurate 11
  • 12. www.OpenGNSS.org c) Process based upon phase measurement •Measurement of a difference of phase between local signal and emitted signal (ambiguity increased) •Possibility to combine measurement of code and phase in order to combine precision / non ambiguity •Difference of phase obtained by a phase lock loop (PLL) 12
  • 13. www.OpenGNSS.org II. Solving the problem of navigation a) How the problem is expressed b) Method of resolution using the mean squares c) Method of resolution using a Kalman filter 13
  • 14. www.OpenGNSS.org a) How the problem is expressed 14
  • 15. www.OpenGNSS.org b) Method of resolution using the mean squares (1/3) 15
  • 16. www.OpenGNSS.org b) Method of resolution using the mean squares (2/3) 16
  • 17. www.OpenGNSS.org b) Method of resolution using the mean squares (3/3) 17
  • 18. www.OpenGNSS.org c) Method of resolution using a Kalman filter (1/2) •Kalman filter : Less dependent on the configuration of satellites • Use the available observations + a parameters estimation model 18
  • 19. www.OpenGNSS.org c) Method of resolution using a Kalman filter (2/2) 19
  • 20. www.OpenGNSS.org III. Errors correction a) Main causes of error b) Reducible errors by modelling or calculation c) Reducible errors by differentiation 20
  • 21. www.OpenGNSS.org a) Main causes of error (1/2) • Errors reducing the precision : -Satellite’s errors (Doppler effect, ephemeris) -Environment’s errors (ionosphere, troposphere, multi-path) -Receiver’s errors (antenna, electronic circuits) 21
  • 22. www.OpenGNSS.org a) Main causes of error (2/2) •Plan to use a corrective model for every source of error (depends on the receivers model) • New expression of the pseudo-range : 22
  • 23. www.OpenGNSS.org b) Reducible errors by modeling or calculation (1/3) • Ionosphere (50 to 100km altitude) : Environment ionized by solar radiations, the speed of the wave is modified 23
  • 24. www.OpenGNSS.org b) Reducible errors by modeling or calculation (2/3) • Other method more accurate by calculation, thanks to a dual frequencies receiver • Pseudo-range measurement with 2 frequencies (typicaly L1 and L2, both GPS frequencies) 24
  • 25. www.OpenGNSS.org b) Reducible errors by modeling or calculation (3/ 3) • Tropospheric error (10 to 20km altitude) reducibles by modelisation •Variation of speed led by the angular curvature of the wave when crossing the troposphere because of the change of refractive index(depending on altitude, temperature, pression and humidity) •Amplitude of the delay depending on the angle according to which the receiver gets the satellite (small delay when satellite in the top, important delay when satellite on the horizon) •Use of a corrective model or a balloon which sounds temperature, humidity, and pressure (especially used in geodesy) 25
  • 26. www.OpenGNSS.org c) Reducible errors by differentiation (1/ 2) • Differential techniques => relative location • Need of a second receiver (static, in a shelter for instance) • Errors observed by both receivers identical: elimination by differentiation • Technics available : - Simple differentiation (1 satellite) - Double differentiation (2 satellites) - Triple differentiation (2 satellites but several mesures in time) Simple differentiation 26
  • 27. www.OpenGNSS.org c) Reducible errors by differentiation (2/ 2) • Example : simple differentiation • We find out the correction to be applied to the pseudo-range 27
  • 28. www.OpenGNSS.org IV. Notions of integrity a) The DOP factor b) Integrity problem c) RAIM algorithms 28
  • 29. www.OpenGNSS.org a) The DOP factor (1/3) •The DOP factor (Dilution Of Precision) : Represents the uncertainty on the position and allows to select the satellites which will supply to the receiver the best results •The error of position depends on the error of measure and on the geometry of the constellation of satellites 29
  • 30. www.OpenGNSS.org a) The DOP factor (2/3) 30
  • 31. www.OpenGNSS.org a) The DOP factor (3/3) 31
  • 32. www.OpenGNSS.org b) Integrity problem • Procedure of exclusion, detects the abnormalities • 4 sources of errors: clock satellite, ephemeris, solar radiations, monitoring station • Integrity check up in 2 stages : detection of the breakdown, then exclusion from the failing measures 32
  • 33. www.OpenGNSS.org c) RAIM algorithms (1/2) • RAIM (Receiver Autonomous Integrity Monitoring) : Use the redundancy of information from satellites to detect a breakdown • 3 possible algorithms : Method consisting in separation of the solutions, the method of residues, the comparison method of the pseudo-range 33
  • 34. www.OpenGNSS.org c) RAIM algorithms (2/2) 34
  • 35. www.OpenGNSS.org Conclusion •Complex system, very sensitive to the disturbances • Many parameters to be taken into account • Possibility of being coupled with the others system of navigation, according to the required use (SBAS/ABAS/GBAS) 35
  • 36. www.OpenGNSS.org Conclusion Thank you and let us meet on www.OpenGNSS.org to participate in the development OpenSource of a GNSS receiver 36