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The possibility to determine water-vapor profiles by combining observations at 21.0 and 31.4 GHz with observations at frequencies near the 183.3-GHz resonance line has been investigated. It is demonstrated that the accuracy of the retrieved profiles will be improved in the height interval 0 to 2 km. In this height interval the presence of inversion layers is one of the main uncertainties. As an alternative to 183-GHz observations, an active sensor can be used to indicate the presence of an inversion layer. It is shown that the improvement of the accuracy would then be of the same order as obtained with a 183-GHz system.