A filling level determination system, for determining a filling level of a surface of a product in a tank having a tank wall separating an inside of the tank from an outside of the tank, the filling level determination system comprising: a first measurement unit for arrangement at a first measurement position on the outside of the tank, the first measurement unit including a transceiver for generating, transmitting and receiving electromagnetic signals; and an antenna arrangement coupled to the transceiver for radiating an electromagnetic first transmit signal generated by the transceiver from the first measurement position towards a first target position on the tank wall, and for returning an electromagnetic first reflection signal resulting from reflection of the first transmit signal at the first target position back towards the transceiver; memory for storing tank deformation data indicative of a known relation between the level of the surface of the product in the tank and deformation of the tank at the first target position, and reference data indicative of a reference distance between the first measurement position and the first target position for a known level of the surface of the product in the tank; and processing circuitry coupled to the transceiver of the first measurement unit and to the memory, the processing circuitry being configured to: acquire the tank deformation data and the reference data from the memory; and determine the filling level based on the tank deformation data, the reference data, and a timing relation between the first transmit signal and the first reflection signal. a first measurement unit for arrangement at a first measurement position on the outside of the tank, the first measurement unit including a transceiver for generating, transmitting and receiving electromagnetic signals; and an antenna arrangement coupled to the transceiver for radiating an electromagnetic first transmit signal generated by the transceiver from the first measurement position towards a first target position on the tank wall, and for returning an electromagnetic first reflection signal resulting from reflection of the first transmit signal at the first target position back towards the transceiver; memory for storing tank deformation data indicative of a known relation between the level of the surface of the product in the tank and deformation of the tank at the first target position, and reference data indicative of a reference distance between the first measurement position and the first target position for a known level of the surface of the product in the tank; and processing circuitry coupled to the transceiver of the first measurement unit and to the memory, the processing circuitry being configured to: acquire the tank deformation data and the reference data from the memory; and determine the filling level based on the tank deformation data, the reference data, and a timing relation between the first transmit signal and the first reflection signal. acquire the tank deformation data and the reference data from the memory; and determine the filling level based on the tank deformation data, the reference data, and a timing relation between the first transmit signal and the first reflection signal.