A target device's location in a radio network is estimated by maintaining a probabilistic model for several sample points that indicate expected distributions of signal values at a given location. The target device observes signal values, wherein the sequence of observations and the respective locations constitute a Hidden Markov Model. A graph models the topology of the positioning environment. The graph indicates several nodes which are permissible locations in the environment and several arcs which are permissible transitions between two nodes. The graph is used to estimate the target device's location based on the probabilistic model and the sequence of observations.