Trihedral corner reflectors are widely used as calibration targets or imaging radars because of their large radar cross section (RCS) and extremely wide RCS pattern. An important source of uncertainty in the RCS of trihedral sitting on a ground plane is the coherent interaction of the ground plane with the trihedral. At UHF and low microwave frequencies the large physical size of corner reflectors become a limiting factor in regard to difficulties in field deployment and deviation of their RCS from the expected values. In this paper, a general class of corner reflectors with high-aperture efficiency referred to as self-illuminating corner reflectors, is introduced whose coherent interaction with their surrounding terrain is minimized and their total surface area is two-thirds of that of a triangular corner reflector having the same maximum RCS. Analytical expressions based on geometrical optics and a new numerical solution based on near-field physical optics for the RCS of two simple self-illuminating corner reflectors are presented and compared with backscatter measurements. Also the panel geometry for an optimum corner reflector which has the shortest edge length among polygonal self-illuminating corner reflectors is obtained. High-aperture efficiency is achieved at the expense of azimuth and elevation beamwidth. It is shown that the 1-dB RCS beamwidths of the optimal corner reflectors, both in azimuth and elevation directions, are about 16°, which is sufficient for most practical applications. RCS measurements of corner reflectors in the presence of a ground plane show that the RCS of selfilluminating corner reflectors are less affected by the coherent ground interaction.