A radar level gauge system comprising: a transceiver arrangement; a first probe to guide a first transmit signal through a surrounding medium towards a product in the tank, and to return a first surface echo signal, the first probe being configured to guide the first transmit signal with a first propagation velocity exhibiting a first dependence on a dielectric constant of the surrounding medium; a second probe to guide a second transmit signal through the surrounding medium towards the product, and to return a second surface echo signal, the second probe being configured to guide the second transmit signal with a second propagation velocity exhibiting a second dependence, different from the first dependence, on the dielectric constant of the surrounding medium; and processing circuitry for determining the filling level based on the first surface echo signal, the second surface echo signal, and a known relation between the first dependence and the second dependence.