“…Other than target recognition problems with the target in free space, research activities have also focused on subsurface target detection-where the target is located at a particular depth below an interface. The motivation of subsurface target detection first originated from unexploded ordinance (UXO) detection using ground-penetrating radar [56][57][58][59][60] and later detection of tumors inside a breast volume [49,51,55,61] as well as detection of small changes in hip prostheses [62][63][64]. The shaded boxes indicate the NRF is properly retrieved in the extraction process (P, total number of target signatures; Reprinted from [49] with permission from IEEE) Table 3.…”