A method of determining a filling level of a product contained in a tank, the method comprising generating and transmitting electromagnetic signals; propagating the transmitted electromagnetic signals towards a surface of the product contained in the tank; receiving echo signals resulting from reflections at impedance transitions encountered by the transmitted electromagnetic signals, including a surface echo signal resulting from reflection at a surface of the product; determining a position of a reference impedance transition using a reference echo signal resulting from reflection of the transmitted electromagnetic signals at the reference impedance transition; determining an update level located above the surface, based on the determined position of the reference impedance transition and a known position of the reference impedance transition; determining a disturbance echo profile using at least one of the echo signals resulting from reflection of the transmitted signals at at least one impedance transition located above the update level; and determining the filling level based on the received echo signals and the disturbance echo profile.