A method and a device for measuring, by means of 
radar, in an enclosed space (1) in which a liquid (2) is stored, the level of a liquid surface (6), where the method involves the steps: a 
radar unit (3) mounted on the roof (4) of the enclosed space transmits a 
microwave signal downwards into the enclosed space (1) through a 
waveguide (7) that communicates with the liquid in the enclosed space; the transmitted 
microwave signal's polarization alters according to a predetermined 
time sequence in such a way that the 
signal is propagated alternatively at least in a first and a second plane of polarization; the signal transmitted the first plane of polarization is reflected by the liquid surface (6) back to the 
radar unit (3); the signal in the second plane of polarization is reflected by at least one reference 
transmitter placed at a known distance from the radar unit; a calculating unit calculates the level of the liquid surface based partly on the 
propagation time for the 
microwave signal, i.e. for the time between the signals emitted and received by the radar unit, partly on the microwave signal's velocity of propagation, which is obtained from measurements in relation to the references.