Abstract: The invention concerns a navigation and tracking system, for monitoring the movement of a roving object which carries one of a pair of receiving stations (D, E) the other receiving station being at a known position. Signals are transmitted by a number of independent transmission sources (A, B, C) equal at least to the number of dimensions in which movement is to be monitored. A link (F) passes a representation of the signal or signals received at one of the receiving stations (E) to the other receiving station (D) and means (R, S, T, U) at the other receiving station compares the signal representations and determines the phase difference or time delay between the received signals and hence the change in phase difference or time delay of the signals as the object moves. The movement of the object relative to the transmission source or sources and hence its position can thus be determined.