This report considers the use of millimeter to submillimeter wavelength space based radars for strategic defense and considers the prospect of developing rf sources to power them. The main application envisioned here is midcourse decoy discrimination for which the radar would have the capability of producing, with modest power requirements, a series of images in real time, at strategic range. These images have resolution from 2-20 meters, depending on the radar configuration. One advantage of a radar over a passive system is that with advanced radar techniques of monopulse and SAR and ISAR, the diffraction limited resolution for a given antenna size can be exceeded by a large amount. A monopulse system generally scales favorably at longer wavelength, while a SAR has more favorable scaling at short wavelength. Wavelengths from 300 μm to 1 mm are good compromises and work well for both radar configurations. One interesting scaling law is that for a SAR at short wavelength, the energy per pixel of image is independent of wavelength. This then gives a tremendous advantage to the most efficient rf source, and fast wave, electron beam driven sources could have significant advantages over CO2 lasers in this respect. The state of fast wave technology is then reviewed, and the prospect of developing a tube to power such a space based radar is evaluated.