Leica Geosystems GPS Basics - Manual (Page 12)

12
GPS Basics -1.0.0en How GPS works
4 The problem with GPS is that only
pseudoranges and the time at which the
signal arrived at the receiver can be
determined.
Thus there are four unknowns to deter-
mine; position (X, Y, Z) and time of travel
of the signal. Observing to four satellites
produces four equations which can be
solved, enabling these unknowns to be
determined.
At least four satellites are required to obtain a
position and time in 3 dimensions