This paper presents a digital beamforming (DBF) synthetic aperture radar (SAR) for future spaceborne earth observation systems. The objective of the DBF-SAR system is to achieve a low cost, light weight, low power consumption, dualband (X/Ka) dual-polarized module for next-generation spaceborne SAR system in Europe. The architectures and modules of the proposed DBF-SAR system are designed according to a realistic mission scenario, which is compatible with future small/micro satellites platforms. This system fills an important gap in the conception of future DBF-SAR, facilitating a high level of integration and complexity reduction. The proposed system is considered to be not only the first demonstrator of a receive-only spaceborne DBF system but also the first X/Ka-band dual-polarized SAR system with sharedaperture. This paper presents a description of the proposed instrument hardware and first experimental validations. The concept and design of the DBF multi-static SAR system are discussed and presented first, followed by the design of Subsystems such as digital beamforming networks (DBFN), MMIC and antennas. Simulated and measured results of the sub-Systems are presented, demonstrating that the proposed SAR Instrument architecture is well-suited for the future SAR applications.