In addition to the advantages of the reverberation chamber (statistically isotropic incident field), this method allows to perform radiated susceptibility tests which are impossible to realize in a typical anechoic chamber. This paper deals with a novel radiated susceptibility test method applied in thermal vacuum chamber to assess the GAIA focal plane assembly immunity in X-band. The TVAC (thermal vacuum chamber) has been used as a reverberation chamber with frequency stirring in order to be in the condition of an oversized cavity.