For many years several measurements of the beam coupling impedance have been performed each year in both longitudinal and transverse planes of the CERN Super Proton Synchrotron to keep track of the evolution of its hardware. Copious types of equipments had to be modified or added in the past to allow the SPS to produce the nominal LHC beam. The next challenge would be the operation of the SPS with higher intensity for the LHC luminosity upgrade, which requires a good knowledge of the machine impedance and in particular of its major contributors. The current understanding of the measurements performed over the last few years is presented in this paper. In particular, this analysis reveals that the longitudinal impedance is reasonably well understood, while the situation is less satisfactory in the transverse planes, where about half of the measured impedance still needs to be identified
