Initial measurements and results of a multi-channel, adaptive pre-distortion system for an airborne phased array radar

With the advancement of synthetic aperture radars (SAR) and phased array radars, research is being done with a variety of amplifier/antenna schemes and over a wide range of frequencies. One major problem is spectral regrowth due to AM/AM and AM/PM effects due to memory effects and when operating near saturation. The distorted output of these systems can leak out-of-band and therefore create problems, such as unauthorized broadcast frequencies and unintended beam steer angles. By properly utilizing digital pre-distortion, most, if not all, of these out-of-band components can be eliminated, allowing radars to be operated in much wider bands and closer to the frequency limits. Work is being done at NASA's Goddard Space Flight Center on the Ecological Synthetic Aperture Radar (EcoSAR) to address these issues in this system. When completed, EcoSAR will be used to measure vegetation density from an airborne system in place of the traditional method of taking measurements on the ground and extrapolating. EcoSAR will potentially be operated in populated areas where broadcast frequencies are highly regulated and spectral regrowth will need to be minimized. The university team is partnering with NASA to develop advanced digital radar techniques for EcoSAR that will enable a new class of radar operations that improves science, enhances system performance, facilitates a path for space-borne implementation, and pushes technology beyond the current state-of-the-art, while keeping the costs to execute the project low. The results section of this paper provides details of the progress in this important area of research.