The ucsspm is an open-source clear-sky prediction model, incorporating math algorithms based on latest research by the Environmental and Water Resources Institute of the American Society of Civil Engineers and a few veteran but still valid and publicly available NOAA/NASA computations. It has been around for a while but received a major revision, code refactoring and got bumped to primary project status, since it's becoming an essential tool in predicting Apollo-NG deployment location feasibility.
A long-term test setup collecting simulation and reference measurement data for the odyssey and aquarius has been set up and will be published soon as well. The first results look quite promising - it's definitely on the right track to become a unified, reliable and open clear-sky global solar radiation prediction model. We are looking forward for you, to use, review, verify and enhance it as well.