A method of determining a filling level of a product in a tank using a radar level gauge system , the tank having a tank wall , and a tank atmosphere above a surface of the product in the tank, wherein the method comprises: generating and transmitting an electromagnetic first transmit signal (S T1 ); propagating the first transmit signal through the tank atmosphere towards a corner reflector formed by the surface of the product and the tank wall where the surface of the product meets the tank wall, the corner reflector being at a known horizontal distance (B) from the radar level gauge system; receiving an electromagnetic first reflection signal (S R1 ) resulting from reflection of the first transmit signal (S T1 ) at the corner reflector ; generating and transmitting an electromagnetic second transmit signal (S T2 ); propagating the second transmit signal through the tank atmosphere vertically towards the surface of the product; receiving an electromagnetic second reflection signal (S R2 ) resulting from reflection of the second transmit signal (S T2 ) at the surface of the product; determining a signal propagation speed compensation factor based on a first timing relation between the first transmit signal (S T1 ) and the first reflection signal (S R1 ), a second timing relation between the second transmit signal (S T2 ) and the second reflection signal (S R2 ), and the known horizontal distance (B) between the radar level gauge system and the corner reflector ; and determining the filling level (L) of the product in the tank based on the second timing relation between the second transmit signal and the second reflection signal, and the signal propagation speed compensation factor. generating and transmitting an electromagnetic first transmit signal (S T1 ); propagating the first transmit signal through the tank atmosphere towards a corner reflector formed by the surface of the product and the tank wall where the surface of the product meets the tank wall, the corner reflector being at a known horizontal distance (B) from the radar level gauge system; receiving an electromagnetic first reflection signal (S R1 ) resulting from reflection of the first transmit signal (S T1 ) at the corner reflector ; generating and transmitting an electromagnetic second transmit signal (S T2 ); propagating the second transmit signal through the tank atmosphere vertically towards the surface of the product; receiving an electromagnetic second reflection signal (S R2 ) resulting from reflection of the second transmit signal (S T2 ) at the surface of the product; determining a signal propagation speed compensation factor based on a first timing relation between the first transmit signal (S T1 ) and the first reflection signal (S R1 ), a second timing relation between the second transmit signal (S T2 ) and the second reflection signal (S R2 ), and the known horizontal distance (B) between the radar level gauge system and the corner reflector ; and determining the filling level (L) of the product in the tank based on the second timing relation between the second transmit signal and the second reflection signal, and the signal propagation speed compensation factor. <EOS>