Abstract: A navigation and/or tracking system has a base station B and a roving receiver R both of which are capable of receiving signals from a plurality of transmission sources T.times.1,T.times.2. A link signal W,W' is passed between the base station and the roving receiver to enable the relative location of the roving receiver to be determined. The apparatus enables the determination of the position of the roving receiver within a space (such as a tunnel T) shielded from receipt of signals from the transmission sources and has equipment P,Q for producing substitute signals, each signal being locked in phase with a signal from a respective one of the transmission sources. A curvilinear transmission element or elements F,F',F", extend within the shielded space T for generating fields in the shielded space. The equipment P,Q propagates at least two of the substitute signals in opposite directions along the curvilinear transmission element or elements for receipt by the roving receiver R.