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The distance to a satellite is determined by measuring how long it takes for the satellite signal to reach the receiver.
The principle of code measurement
The time between transmission and reception of the GNSS signal is relatively easy to determine when using code measurements. The satellites and GNSS receivers are intended to generate the same code at the same time. By shifting the local code in the receiver until it corresponds to the received code, a time difference is obtained. The distance between receiver and satellite can therefore be calculated as the speed of light multiplied by the measured time difference.
By measuring against three satellites at the same time, the position of the GNSS receiver can in principle be determined. Since the GNSS receiver's clock is relatively uncertain, its error must also be determined, which means that measurements must be made against at least four satellites.
Simpler GNSS receivers, e.g. in car navigation systems and smartphones, use code measurement.
How accurate is distance measurement with GNSS?
A useful rule of thumb is that the signal resolution is about one percent of the wavelength, which is true for both code and carrier phase. Below that limit, the signal copies cannot be matched reliably. As an example, we can take the so-called C/A code on the L1 frequency of GPS satellites. The C/A code has a chip length of approximately 300 meters and the signal resolution - or "accuracy" - in measuring the distance to a satellite is therefore approximately three meters when using code observations.