Boundary conditions for RCS calculation of a ship using CST Micro Wave Studio

Hi,
I am trying to simulate a ship model (Teleost) as given in this journal (attachment) using CST micro wave studio. I am confused about the boundary condition that is to be used to get the same result. Moreover there is this statement mentioned in the journal as follows
"To calculate the RCS of the model,the grid is reflected in the horizontal plane, and then NUmerical Electromagnetic code (NEC) calculates the cross section of the entire structure. Hence, the results here refer everywhere to the ?net? RCS, not to the equivalent free-space RCS which is made 12 dB smaller in order to account for the gains of antennas operating over a conducting surface."
What do this statement mean?. how to simulate that in CST?
I have the proper dimensions of the model from another paper by the same author
Thank you in advance

I finally got the result as in the paper by mirroring the whole model about the ocean plane. But i don't know whether it is the right way of doing it. If it right then can someone tell me why?. Should i do the same for all other models of ship that i am going to simulate?
Image of the model that gave good result