A filling level determination system, for determining a filling level of a product in a tank, comprising a measurement unit for arrangement at a measurement position, the measurement unit including a transceiver; and an antenna arrangement coupled to the transceiver for radiating an electromagnetic transmit signal generated by the transceiver from the measurement position towards a target position on the tank wall, and for returning an electromagnetic first reflection signal resulting from reflection of the transmit signal at the target position back towards the transceiver; and processing circuitry coupled to the transceiver of the measurement unit and being configured to determine the filling level based on tank deformation data indicative of a known relation between the level of the product in the tank and deformation of the tank at the first target position, and a timing relation between the first transmit signal and the first reflection signal.