Then, a fast far-field approximation (FAFFA)  is used to calculate the mutual EM coupling interactions between the targets and the sea surface.
To further accelerate the coupling process between the targets and the sea surface, fast far-field approximation (FAFFA) is applied to calculate the two far coupling groups.
In terms of FAFFA, the interactions are separated into the following three steps: aggregation, translation, and disaggregation in far regions.
Meanwhile, the fast far-field approximation (FAFFA)  for the hybrid method is applied to speed up the electromagnetic coupling interaction process.
Borrowing the idea from the fast far-field approximation (FAFFA)  in the MLFMA, the coupling interaction between KA and MLFMA can be accelerated in the interaction between far-field region groups.
The near-region interaction is solved directly, and the far-region interaction is speeded up by the FAFFA. Suppose there are [C.sub.1] and [C.sub.2] basis functions in each object group and in each rough surface group, respectively, the consumed time in the interaction process can be reduced to o(MN)/([C.sub.1][C.sub.2]), even to o([square root of MN]) if [C.sub.1] ~ [square root of M] and [C.sub.2] ~ [square root of N], M and N denote the number of the unknowns of object and rough surface, respectively.
For the composite model, it can be solved well in a common PC machine using the hybrid method and FAFFA.
With the hybrid algorithm and FAFFA acceleration technique, the results of bi-static d-RCS with (or without) sea background scattering are given in the Fig.