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Clock synchronization with GNSS involves comparing the time kept by a GNSS receiver with the time signal sent by the GNSS satellites. The receiver uses the time difference between the two to calculate its position and velocity. The receiver also includes a clock that can be synchronized with the satellite time signal to improve its accuracy. This is done by comparing the time offset between the satellite and receiver clocks, and adjusting the receiver clock to match the satellite clock.
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[](https://www.bodet-time.com/time-servers/articles-and-resources/1595-why-is-time-synchronization-so-important.html)
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Clock synchronization with GNSS involves **comparing the time kept by a GNSS receiver with the time signal sent by the GNSS satellites.** The receiver uses **the time difference between the two to calculate its position and velocity.** The receiver also includes a clock that can be synchronized with the satellite time signal to improve its accuracy. This is done by comparing the time offset between the satellite and receiver clocks, and adjusting the receiver clock to match the satellite clock.
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# **1st synchronization problem: the distance calculation is very sensitive to clock errors**
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Indeed, if we calculate the time a signal makes to go from the satellites to the receptor, in an ideal world, we only would have to use the fact that .
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Indeed, if we calculate the time a signal makes to go from the satellites to the receptor, in an ideal world, we only would have to use the fact that .
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With :
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-  the distance between the satellite and the user,
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-  the speed of light,
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-  the time.
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-  the distance between the satellite and the user,
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-  the speed of light,
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-  the time.
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As time is multiplied by the speed of light, distance is very sensible to time mistake.
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Without any other mistake being taken into account, transmission time between the satellite and the user can be written  with  the propagation time,  the bias of the receptor,  the bias of the satellite.
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Without any other mistake being taken into account, transmission time between the satellite and the user can be written  with  the propagation time,  the bias of the receptor,  the bias of the satellite.
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And the pseudo-distance between the satellite and the receptor 
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And the pseudo-distance between the satellite and the receptor 
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With :
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- 
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- 
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The satellites clock is very precise and permanently controlled by the “ground segment”. We know its bias with the accuracy of less than a nanosecond. Concerning the receptors clocks, they are much less accurate. Phones clock are often quartz clock, and their accuracy is unknown.
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Therefore, the localisation problem has 4 unknown parameters: 3 geometrical parameters ,  and  and time .
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Therefore, the localisation problem has 4 unknown parameters: 3 geometrical parameters ,  and  and time .
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# **2nd synchronization problem: the problem with the frequency**
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The same problem affects the measurements made thanks to the Doppler Effect. The clock drift is a phenomenon that describes the precision of a clock through time. This drift depends on the quality of the clock : the drift of an atomic clock will be more predictable and smaller than the drift of a quartz clock.
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We get from the derivation of the previous equality that : 
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We get from the derivation of the previous equality that : 
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with  the clock drift of the receptor,  the clock drift of the satellite.
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with  the clock drift of the receptor,  the clock drift of the satellite.
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 is controlled and considered insignificant for the measurements.
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 is controlled and considered insignificant for the measurements.
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Therefore, the localisation problem has 4 unknown parameters : the speed following the 3 axis of the reference frame, and the clock drift of the receptor.
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