We present a formal method, based on the Logic of Possibility, to fuse uncertain sensory information and to produce an estimate of the position of a mobile robot. The robot navigates in, an office environment, using a topological map, with the assistance of a ''slave'' robot acting as a portable local landmark. Each relevant place in the map is characterized by a set of logical formulae axiomatizing both ''crisp'' knowledge and uncertain information from the sensors. At each time instant during navigation, the necessity degree of each place is calculated using a purely syntactical method based on sequent calculus.