An open scalable architecture will make it easier and quicker to adapt active phased array radar to new missions and platforms. This will provide radar manufacturers with larger markets, more commonality in radar systems, and a better continuity in radar production lines. The procurement of open scalable active phased array radars becomes feasible for a larger number of customers because a cooperative development becomes easier and cheaper. Active phased array radar with an open scalable architecture should have fixed planar arrays, T/R-modules with switchable polarization, a combination of a linear and a stretch receiver for each of the antenna beams, and a blade server as the hardware for signal and data processing. The software architecture should have a layered hierarchical structure that uses open standards such as MPI for message passing and a serial packet-switched interconnection network such as Infiniband for communication between the application software processes.