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

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
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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
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Introduction (2/2)
Functioning scheme
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I. General principle
a) Pseudo-range computation
b) Process based upon code measurement
c) Process based upon phase measurement
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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
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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
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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)
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b) Process based upon code measurement (2/3)
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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
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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)
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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
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a) How the problem is expressed
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b) Method of resolution using the mean squares (1/3)
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b) Method of resolution using the mean squares (2/3)
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b) Method of resolution using the mean squares (3/3)
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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
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c) Method of resolution using a Kalman filter (2/2)
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20. www.OpenGNSS.org III. Errors correction
a) Main causes of error
b) Reducible errors by modelling or calculation
c) Reducible errors by differentiation
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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)
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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 receiver's model)
• New expression of the pseudo-range :
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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
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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)
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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)
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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
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c) Reducible errors by differentiation (2/ 2)
• Example : simple differentiation
• We find out the correction to be applied to the pseudo-range
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28. www.OpenGNSS.org IV. Notions of integrity
a) The DOP factor
b) Integrity problem
c) RAIM algorithms
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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
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30. www.OpenGNSS.org a) The DOP factor (2/3)
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31. www.OpenGNSS.org a) The DOP factor (3/3)
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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
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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
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34. www.OpenGNSS.org c) RAIM algorithms (2/2)
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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)
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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
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