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Ce Wiki manque cruellement d'illustration... (miguel)
## Welcome to the SmartLoc WIKI :)
[Want to know more about SmartLoc?](What is SmartLoc?)
[Why this wiki?](Why this wiki)
[Want to know more about SmartLoc?](What%20is%20SmartLoc?)
[Why this wiki?](Why%20this%20wiki)
[What are GNSS?](What are GNSS?)
[What are GNSS?](What%20are%20GNSS?)
[What is GEOLOC?](What is GEOLOC?)
[What is GEOLOC?](What%20is%20GEOLOC?)
[What is GEOLOC PVT?](What is GEOLOCPVT?)
[What is GEOLOC PVT?](What%20is%20GEOLOC%20PVT?)
# PART.1 : GNSS
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When processing the signal received from satellites, there are so-called "natural" factors that **limit the accuracy of GNSS.** We can mention: refraction in the ionosphere, refraction in the troposphere, positioning accuracy of GNSS satellites and multipath phenomena, etc.
[Ionospheric and tropospheric error](Ionospheric and troposphericerror)
[Ionospheric and tropospheric error](Ionospheric%20and%20tropospheric%20error)
The ionosphere is an envelope of charged particles (ions) that surrounds the Earth at an altitude of about 20 km. The carrier wave of the GNSS signal must penetrate this layer on its way. The fact that this layer is not neutral in terms of its charge causes a disturbance in the speed of the propagating electromagnetic wave. The time taken by the GNSS wave is modified by an unknown amount of time, called the ionospheric delay. The evaluation of the distance between the satellite and the station will be distorted. Similarly, the propagation time of the GNSS wave is affected by the water vapour content of the lower layer of the atmosphere (0 to 10 km altitude): the troposphere.
[Orbital errors](Orbitalerrors)
[Orbital errors](Orbital%20errors)
The position of the satellites is calculated on the ground and is made public by the constellation control centres several times a day. This information is called the ephemeris. They will be used for position calculations. But it is obvious that if there is an error in the position of the transmitting satellite, this error will be reflected directly in the position displayed by the receiver.
[Multipath Signaling](MultipathSignaling)
[Multipath Signaling](Multipath%20Signaling)
These phenomena are among the most difficult to understand. Any reflective object placed in the vicinity of the GPS station's antenna can reflect part of the signal from the satellite back to the antenna. The signal will therefore be deteriorated, more difficult to interpret and therefore a source of large errors. Moreover, this kind of phenomenon is very frequent in the life of any user, especially in the city where the path that the signal makes is rarely clear.
The satellites of the GNSS system are equipped with several atomic clocks; however, their high accuracy has a limit. The corrections for the offset of the clocks are transmitted in the "navigation message". These corrections are calculated by reference ground stations and are used to compensate for the continuous deviation of the clocks. The uncertainty in the receiver clock, which is much less accurate since it is not an atomic clock, is considered an additional unknown, hence the need for at least four satellites.
[Bonus : Why do you need 4 satellites to locate someone?](Why do you need 4 satellites to locatesomeone?)
[Bonus : Why do you need 4 satellites to locate someone?](Why%20do%20you%20need%204%20satellites%20to%20locate%20someone?)
