In satellite positioning, the central problem is to determine the values (X, Y, Z) of a receiver of satellite signals, i.e. to determine the position of the receiver with the accuracy and precision required. A receiver on or above the planet, also equipped with a clock, receives the message slightly later and reads its own clock. From the time delay observed between the two clock readings, and knowing the speed of radio transmission through the medium between (satellite) sender and receiver, the receiver can compute the distance to the sender. It would appear that the signals of three satellites would be sufficient to determine a positional fix for our receiver. In theory this is true, but in practice it is not.