Consider the scenario shown in Fig. 4.1 consisting of an object and a few reference points R1, R2, R3, R4, and R5. Suppose that the coordinates of the reference points are known. It is possible to estimate the coordinates of the object if the distances \(d_1\) , \(d_2\) , \(d_3\) , \(d_4\) , and \(d_5\) between the object and the reference points are also known. This is the basis of a satellite navigation (or satnav) system. In a satellite navigation system, the reference points are the satellites. The distance between the object and a satellite is determined from the duration the electromagnetic wave takes to travel from the satellite to the object. Each navigation satellite carries one or more atomic clocks keeping very accurate time reference; the time between satellites is also very accurately synchronized. In a three-dimensional space, if the object also carries an atomic clock synchronized to those of the satellites, three satellites are sufficient for geospatial positioning. However, in practice, the object does not have an accurate time reference and so at least four satellites are needed for geospatial positioning.