The possibilities of using the measurement of the ranges to a pair of satellites to give an observer his position have been extensively studied, and expressions giving latitude and longitude have been derived. These since they involve powers of the fourth order are somewhat cumbersome, but given the appropriate computer on his craft an observer at a known distance from the centre of the Earth can obtain track data by measuring the ranges and continuously calculating his position. Alternatively, he may measure the incremental changes in these quantities from their value at the starting point which avoids (if a synchronous time or frequency technique is used) the necessity for continual two-way exchange of signals between the craft and the satellite, but the real-time solution of relatively complicated equations still has to be done.