A method of determining a filling level of a product in a tank using a radar level gauge system including a transceiver; a signal propagation device coupled to said transceiver; and processing circuitry coupled to said transceiver for determining said filling level, said method comprising the steps of: generating and transmitting an electromagnetic transmit signal (S T) by said transceiver; propagating said transmit signal (S T) towards a surface of said product along a signal propagation path including said signal propagation device, wherein said signal propagation path comprises a first portion having known signal propagation properties, and a second portion, said first portion being located between said transceiver and said second portion; providing a computational model of said signal propagation path; receiving, by said transceiver, an electromagnetic reflection signal (S R) resulting from reflections at impedance transitions along said signal propagation path, including said surface of the product; determining, based on a timing relation between said transmit signal (S T) and said reflection signal (S R), a first measurement relation (S M1) between energy reflected by said impedance transitions along the first and second portions of said signal propagation path and travel time of said reflection signal (S R) along the first and second portions of said signal propagation path; determining a second measurement relation (S M2) between energy reflected by a first sub-set of said impedance transitions along the second portion of said signal propagation path and travel time of said reflection signal (S R) along the second portion of said signal propagation path based on said first measurement relation (S M1) and said known signal propagation properties of the first portion of said signal propagation path, using the computational model of said signal propagation path; and determining said filling level based on said second measurement relation (S M2). generating and transmitting an electromagnetic transmit signal (S T) by said transceiver; propagating said transmit signal (S T) towards a surface of said product along a signal propagation path including said signal propagation device, wherein said signal propagation path comprises a first portion having known signal propagation properties, and a second portion, said first portion being located between said transceiver and said second portion; providing a computational model of said signal propagation path; receiving, by said transceiver, an electromagnetic reflection signal (S R) resulting from reflections at impedance transitions along said signal propagation path, including said surface of the product; determining, based on a timing relation between said transmit signal (S T) and said reflection signal (S R), a first measurement relation (S M1) between energy reflected by said impedance transitions along the first and second portions of said signal propagation path and travel time of said reflection signal (S R) along the first and second portions of said signal propagation path; determining a second measurement relation (S M2) between energy reflected by a first sub-set of said impedance transitions along the second portion of said signal propagation path and travel time of said reflection signal (S R) along the second portion of said signal propagation path based on said first measurement relation (S M1) and said known signal propagation properties of the first portion of said signal propagation path, using the computational model of said signal propagation path; and determining said filling level based on said second measurement relation (S M2).