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The hardware reconfiguration feature of a software-defined radio (SDR) architecture can support multiple modes of a digital beamformer (DBF) striving for compactness and efficient processing power, which are important issues for microsatellite synthetic aperture radar (SAR) systems. In this letter, based on the SDR architecture, a DBF system, consisting of multiple beam, direction-of-arrival (DOA) estimation, and null-steering operation modes, is realized using a field-programmable gate array (FPGA) processor. Since the hardware reconfiguration has to be processed with minimal delay, the FPGA hardware must be of modularized design. Different modes can share the common module during mode switching. Experimental results verify the performance of DOA, null steering, and mode switching. The processing time of each DBF mode is less than the cross-range pulse repetition interval of the microsatellite SAR system.