A method of assisting the calculation of the position of a receiver, by observing a transmitted signal having a known structure, said known structure making it possible to determine the relative times of transmission of different portions of the signal, the method comprising: comparing the time of arrival at the receiver, at a reference position (X 1), of a first portion of the signal with the time of arrival at the receiver, at an unknown position (Y 1), of a second portion of the signal; obtaining a local wave propagation model of the signal, the model comprising an estimate of a direction of propagation of the signal in the neighbourhood of the reference position and unknown position; and using the estimate of the direction of propagation and the result of the comparison to assist in the calculation of the unknown position relative to the reference position, wherein the step of comparing the times of arrival comprises at least one of: measuring a first time difference between them; and solving a system of one or more equations in which the times of arrival are variables, and wherein the step of obtaining the local wave propagation model comprises: a second comparison, between the time of arrival, at a first position (X 1), of a third portion of the signal and the time of arrival, at a second different position (X 2), of a fourth portion of the signal, wherein a displacement vector between these two positions is known; and estimating the direction of propagation of the signal based on the displacement vector and the result of the second comparison. comparing the time of arrival at the receiver, at a reference position (X 1), of a first portion of the signal with the time of arrival at the receiver, at an unknown position (Y 1), of a second portion of the signal; obtaining a local wave propagation model of the signal, the model comprising an estimate of a direction of propagation of the signal in the neighbourhood of the reference position and unknown position; and using the estimate of the direction of propagation and the result of the comparison to assist in the calculation of the unknown position relative to the reference position, wherein the step of comparing the times of arrival comprises at least one of: measuring a first time difference between them; and solving a system of one or more equations in which the times of arrival are variables, the estimate of the direction of propagation and the result of the comparison measuring a first time difference between them; and solving a system of one or more equations in which the times of arrival are variables, and wherein the step of obtaining the local wave propagation model comprises: a second comparison, between the time of arrival, at a first position (X 1), of a third portion of the signal and the time of arrival, at a second different position (X 2), of a fourth portion of the signal, a second comparison, between the time of arrival, at a first position (X 1), of a third portion of the signal and the time of arrival, at a second different position (X 2), of a fourth portion of the signal, wherein a displacement vector between these two positions is known; and estimating the direction of propagation of the signal based on the displacement vector and the result of the second comparison.