A method and apparatus for assisting the calculation of the position of a receiver device (1200), by observing a transmitted signal having a known structure. The method comprises: comparing (S220) the time of arrival, at a reference position (X1), of a first portion of the signal with the time of arrival at the receiver, at an unknown position (Y1), of a second portion of the signal; obtaining (S230) a local wave propagation model of the signal, the model comprising an estimate of the direction of propagation of the signal in the neighborhood of the reference position and unknown position; and using (S240) the direction of propagation and the result of the comparison to assist in the calculation of the unknown position relative to the reference position.