In the framework of the preparation of the RadioAstron mission the statistic performance of the radio sources at 22 GHz is considered. The expected density of the radio sources with inverted spectra is estimated. A sample of radio sources from the preliminary RadioAstron catalogue is observed. The comparison of the expected and measured angular densities of the radio sources with the inverted spectra are performed. Optical properties of the objects with the inverted spectra are compared with ones of the complete sample sources of searching surveys. The spectral characteristics of the sample of the radio sources which were measured at two frequencies are presented. The conclusion on the completeness and reliability of the preliminary RadioAstron catalogue are made.