A measuring technique and method are provided to simultaneously determine the positions of the surfaces of slag and metal bath in metallurgical processes. In the metallurgical process industry, e.g. in converter processes, it is important to measure the thickness of the layer of slag on top of the metal bath as well as determine the actual volume of metal in order to determine the weight of the charge. A large number of various methods have been tried, but no method is known to result in fast and accurate measurements of both surfaces from a distance and without mechanical means. The herein presented invention describe a method whereby radio waves are transmitted over a frequency band perpendicular towards the surfaces and how a receiver is used to compare the phase of the reflected waves relative the transmitted wave. The phase-change is measured for a number of frequency channels and a transform is then used to transform from the frequency space to the time delay space which gives the positions. The method is suitable for measuring overlapping surfaces at large distances from the reference level and which are separated by large distances. The estimated errors of the method are as low as 2 mm. If an interferometer is used as antenna then the data will also be sampled in the aperture plane and the three dimensional structure of the surfaces can be reconstructed.