The pseudorange measurement by code observation is based on a correlation by code observation is based on a correlation analysis between the signal emitted by a satellite and the replica generated by the receiver. Thus, the time lag observed between these two signals gives us the time taken by the wave to travel the distance between its point of reception.
The position of the receiver is then calculated by intersection of spheres, hence the need to have at least four nsatellites in visibility.
Such a calculation requires extremely precise tools : stability of the replica of the code generated by the receiver, resolution of the time measurement ( a few tenths of a nanosecond for the best!) …
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