A method of determining a filling level (L) 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 (L), 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 (L) 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 (L) based on said second measurement relation (S M2 ). <EOS>