A navigation system is described which includes a mobile terminal 100 which has a transmission source receiver 204 for receiving the signals from one or more unsynchronised terrestrial transmission sources 102-105 and a satellite positioning receiver 200 for receiving signals from the satellite or satellites 107-110 of a satellite positioning system. The terminal also has a clock 208. A processor 209 is arranged to acquire a measurement vector having a list of values, each value representing a measurement made by a receiver 200,204, and the terminal clock's bias. It computes a state vector representing the current state of the system, using a previously-determined state vector, the measurement vector, and a dynamic model in order to derive a dynamic navigation solution. The elements of the state vector include (a) in one or more dimensions at least position information of the terminal 100; (b) representations of the bias of the clocks associated with the unsynchronised terrestrial transmission sources 102-105 relative to a given time; (c) the bias of the terminal clock 208; (d) quality indicators indicating the error associated with elements of (a) to (c); and (e) linking indicators indicating the dependency of elements in (a) to (c) on others of those elements.