Abstract: A radar level gauging system for determining a filling level of a liquid in a tank arranged in a mobile unit is disclosed. The system is comprises a processing circuitry for determining a local and momentary filling level at different times at a certain position in the tank of the container based on received echo signals, and is further adapted to identify variations between said local and momentary filling levels corresponding to surface waves in the tank due to movement of said mobile unit. The result of this identification is used for improving the accuracy when estimating an average filling level of the tank.