An apparatus obtains fingerprints that have been collected by at least one mobile device for supporting a positioning of other mobile devices. Each fingerprint comprising results of measurements on radio signals of at least one communication node at a particular location and an indication of the particular location. The apparatus estimates values of parameters defining a radio model for the at least one communication node based on the obtained fingerprints. The apparatus determines a quality of the radio model. The apparatus generates data for a feedback for a user of the at least one mobile device based on the determined quality of the radio model.