For FC backscatter projection, I excited the FC length signal. I basically followed the LLO's FC backscatter estimation in 65120.

The excitation result is shown in the first attachment. The FC length is excited between 4 Hz and 400 Hz and the DARM excess noise is seen between 20 Hz and 200 Hz. In the plot, the FC length signal is not calibrated and the DARM is calibrated to displacement. Note that there is no coherence between the excited FC length signal and excited DARM so this is not a linear coupling.

The FC backscatter noise is estimated using the following formula.

DARM_FC_backscatter = FCL_no_exc * sqrt(DARM_exc^2 - DARM_no_exc^2)/sqrt(FCL_exc^2 - FCL_no_exc^2)

The projection is shown in the second attachment. The projected FC backscatter is below DARM by a factor of more than 40.