Unfortunately, there are some drawbacks associated
with the procedure of epoch-wise clock estimation. One
of the consequences is the need to observe at least four satellites
at the same time to compute a position because we
need to solve for three coordinates and one clock offset.
Another effect is the degraded vertical position accuracy
due to the fact that only satellites in the hemisphere above
the horizon are observed. This asymmetry leads to significant
mathematical correlations between the estimates of
the receiver clock corrections, troposphere parameters
and the station height