The analysis technique and a part of the results obtained from CAT (clear air turbulence) radar echoes from higher troposphere and lower stratosphere have been presented. The effect of processing distortion caused by the periodogram method using an FFT algorithm on the slowly fading ground clutter echo was discussed. It is shown that a narrow clutter spectrum can spill over the entire frequency range if the data are truncated at a time shorter than its correlation time affecting largely the estimation of the CAT spectrum contribution, especially when the latter is a few tens of db weaker than the former.