Trihedral corner reflectors have been widely used as standard point targets for synthetic aperture radar (SAR) calibration. The RCS accuracy of the trihedral corner reflector is vital especially for future high-precision SAR radiometric calibration. In order to reduce the interaction between corner reflector and the ground on which the reflector deployed, and also reduce the edge diffraction through minimizing the panel external edge length, improved trihedral corner reflector was developed and it had smaller edge length than the trihedral corner reflector with the polygonous shape for a given panel area. General expression for the panel area and panel external edge length of arbitrarily self-illuminating corner reflectors was presented by parameter equation firstly. Then the edge of reflector panel was assumed as circular arc and through a numerical approximation approach, improved corner reflector of circular arc panel geometry with smaller edge length was obtained.