This work presents and investigates a novel high-performance antenna design for remote sensing and satellite communications applications that is compact and lightweight. The performance requirements include dual-polarization, wide impedance bandwidth, low cross-polarization, high gain, and high aperture efficiency. To simultaneously achieve all these qualities is challenging and a combination of compatible performance enhancement techniques was selected and derived. A new antenna design, namely an octagonal cavity-backed radiating open prism (OCROP) antenna, was first introduced. The antenna is orthogonally and differentially fed to provide dual-polarization and low cross-polarization. The impedance bandwidth is increased by combining the resonances from the radiating feedlines and the open prism. The corner truncated octagonal ground cavity, instead of a more commonly seen square ground plane or square ground cavity, is used to further suppress the cross-polarization and enhance the gain and aperture efficiency. To show the superiority of the octagonal design, it was compared with a square ground cavity backed antenna. To further improve the gain of this novel antenna, its array and flared configurations are considered, investigated, and compared. The array design offers higher gain and aperture efficiency, while the flared design can provide lower cross-polarization. To achieve higher gain with the flared design, a larger flare angle was chosen which increased cross-polarization. To remedy the problem of the increased cross-polarization, a new iris design concept is proposed and investigated. It shows that this method can successfully suppress the cross-polarization of the flared design. This concept is general and has the potential to be applied to other types of aperture antennas. The design concepts and the simulation results are verified by fabricating and testing two prototypes, i.e., with and without a flare.