A method of determining a filling level of a product in a tank, comprising generating and transmitting a transmit signal; propagating the transmit signal towards a surface of the product along a signal propagation path comprising a first portion having known signal propagation properties adjacent to the transceiver and a second portion adjacent to the first portion; receiving a reflection signal resulting from reflections at impedance transitions encountered by the transmit signal, including the surface of the product; determining a first measurement relation based on a timing relation between the transmit signal and the reflection signal; determining a second measurement relation based on the first measurement relation and the known signal propagation properties; and determining the filling level based on the second measurement relation.