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The DARPA ISIS Program is developing and demonstrating technologies to enable a stratospheric airship-based dual-band active-array radar system. The antenna is a UHF and X-band shared aperture with backend digital beamforming. ISIS achieves unprecedented airborne radar performance through use of a large antenna aperture instead of high transmit power. The low-power-density aperture has an areal mass density of only 2 kg/m2. A conceptual future system achieves 360° azimuth coverage using a cylindrical antenna of 36m height and 50.5m diameter. A small demonstration system scheduled for launch in 2013 has a UHF antenna cylinder 8m high and 22m in diameter. A 98m2 rectangular section of this cylinder is populated with active X-band elements. The dual-band array has total receive and overhead power consumption, including off-array components, of only 5W/m2.