A method for determining a filling level of a product contained in a tank, by means of a radar level gauge system comprising: a transceiver for generating, transmitting and receiving electromagnetic signals; a probe connected to said transceiver and arranged to guide a transmitted electromagnetic signal (S T) from said transceiver towards and into said product inside the tank, and to return a reflected electromagnetic signal (S R) resulting from reflection of said transmitted electromagnetic signal by a surface of said product back towards said transceiver; and a plurality of reference reflectors (4a-j) each being arranged at a respective known position along the probe and being configured to reflect a portion of said transmitted electromagnetic signal back towards said transceiver, characterised in that: said plurality of reference reflectors comprises at least three reference reflectors; and said method comprises the steps of: identifying, based on received electromagnetic signals reflected by said reference reflectors, a set of reference reflectors (4a-e) located above said surface of the product; selecting first and second reference reflectors comprised in said set of reference reflectors; determining a propagation velocity compensation factor based on a known distance between said first and second reference reflectors and a distance therebetween determined using received electromagnetic signals reflected by said first and second reference reflector, respectively; and determining said filling level based on a received electromagnetic signal reflected by the surface of the product, and said propagation velocity compensation factor. a transceiver for generating, transmitting and receiving electromagnetic signals; a probe connected to said transceiver and arranged to guide a transmitted electromagnetic signal (S T) from said transceiver towards and into said product inside the tank, and to return a reflected electromagnetic signal (S R) resulting from reflection of said transmitted electromagnetic signal by a surface of said product back towards said transceiver; and a plurality of reference reflectors (4a-j) each being arranged at a respective known position along the probe and being configured to reflect a portion of said transmitted electromagnetic signal back towards said transceiver, characterised in that: said plurality of reference reflectors comprises at least three reference reflectors; and said method comprises the steps of: identifying, based on received electromagnetic signals reflected by said reference reflectors, a set of reference reflectors (4a-e) located above said surface of the product; selecting first and second reference reflectors comprised in said set of reference reflectors; determining a propagation velocity compensation factor based on a known distance between said first and second reference reflectors and a distance therebetween determined using received electromagnetic signals reflected by said first and second reference reflector, respectively; and determining said filling level based on a received electromagnetic signal reflected by the surface of the product, and said propagation velocity compensation factor. said plurality of reference reflectors comprises at least three reference reflectors; and said method comprises the steps of: identifying, based on received electromagnetic signals reflected by said reference reflectors, a set of reference reflectors (4a-e) located above said surface of the product; selecting first and second reference reflectors comprised in said set of reference reflectors; determining a propagation velocity compensation factor based on a known distance between said first and second reference reflectors and a distance therebetween determined using received electromagnetic signals reflected by said first and second reference reflector, respectively; and determining said filling level based on a received electromagnetic signal reflected by the surface of the product, and said propagation velocity compensation factor. identifying, based on received electromagnetic signals reflected by said reference reflectors, a set of reference reflectors (4a-e) located above said surface of the product; selecting first and second reference reflectors comprised in said set of reference reflectors; determining a propagation velocity compensation factor based on a known distance between said first and second reference reflectors and a distance therebetween determined using received electromagnetic signals reflected by said first and second reference reflector, respectively; and determining said filling level based on a received electromagnetic signal reflected by the surface of the product, and said propagation velocity compensation factor.