In fact, if we had a receiver clock that is perfectly synchronized with the system time and ignore atmospheric and multipath errors, i.e. if we would measure true ranges rather than pseudo-ranges, the vertical station positions would be more precise than the horizontal positions (Kuang et al., 1996). In practice, there will hardly ever be a perfectly synchronized receiver clock. But, if the internal oscillator of the receiver is replaced by a stable atomic clock one can attempt to model the receiver clock offset, e.g. by a low-order polynomial, instead of estimating it on an epoch-by-epoch basis. The validity interval of the clock model then depends on the frequency stability of the oscillator.