For several years now, NASA has supported observing system simulation studies to evaluate measurement capabilities and requirements for the ASCENDS mission in the context of advancing our knowledge of carbon flux distributions and their dependence on underlying physical processes. ASCENDS (Active Sensing of Carbon Emissions, Nights, Days, and Seasons) is recommended by the US National Academy of Sciences Decadal Survey (2007) for the next generation of space-based CO2 observing systems, and is currently planned for launch in 2023. Measurements of CO2 from space using active (lidar) sensing techniques have a number of potentially significant advantages in comparison to current and planned passive CO2 instruments, in particular a greatly reduced sensitivity to errors resulting from cloud and aerosol scattering. Several possible lidar instrument approaches have been demonstrated in airborne campaigns and the results indicate that such sensors are technologically feasible in space. The model studies have tested a range of possible space mission implementations and quantified their impact on CO2 flux inference. A variety of forward and inverse model frameworks have been used to reduce the potential dependence of the results on model specifics. Considerations include requirements for precision, relative accuracy, spatial/temporal coverage and resolution, vertical information content, and possible tradeoffs among these parameters. The necessity of including coincident measurements of O2 column, in order to normalize the CO2 column density to dry air mole fraction, has also been considered. In this presentation, we summarize the results of ASCENDS modeling carried out to date by the science requirements definition team, point out some as yet unanswered questions, and provide a glimpse of possible further model testing to demonstrate the role and value of ASCENDS in carbon cycle science.