Unmanned Aircraft Systems (UAS) have the potential to expand the current observing network over remote or dangerous areas, such as the polar regions or inside tropical cyclones. Many different aircraft platforms and instrumentation combinations are under consideration to help fill existing observational gaps. In order to determine the >possible benefits of UAS observations on operational forecasting as >well as to find the optimal configuration of aircraft and observations, >an Observing System Simulation Experiment (OSSE) is under development.  Working jointly with ECMWF, NCEP, NASA, and other institutions, a 13-month Nature Run has been completed at T511 resolution, and synthetic observations are currently being generated. The synthetic observations will be assimilated into the GFS operational model with the GSI assimilation package. By comparing the GFS forecasts with and without assimilation of the synthetic UAS observations to the Nature Run, the impact of the UAS systems will be quantitatively predicted.