A fully synchronized modular multichannel software-defined radio (SDR) testbed has been developed for the rapid prototyping and evaluation of array processing algorithms. Based on multiple universal software radio peripherals, this testbed is low cost, wideband, and highly reconfigurable. The testbed can be used to develop new techniques and algorithms in a variety of areas including, but not limited to, direction finding, source triangulation, and wireless sensor networks. A combination of hardware and software techniques is presented, which is shown to successfully remove the inherent phase and frequency uncertainties that exist between the individual SDR peripherals. The adequacy of the developed techniques is demonstrated through the application of the testbed to super-resolution direction finding algorithms, which rely on accurate phase synchronization.