Presented at the 12th Specialist Meeting on Microwave Radiometry and Remote Sensing of the EnvironmentAn Observing System Simulation Experiment for the Aquarius/SAC-D mission is being developed for assessing the accuracy of soil moisture retrieval from passive and active L-band remote sensing. The implementation of the OSSE is based on: a 1-km land surface model over the Red-Arkansas River Basin, a backscatter model and a forward microwave emission model to simulate the radiometer and scatterometer observations, a realistic orbital and sensor model to resample the measurements, and an inverse soil moisture retrieval model. The simulation implements zero-order radiative transfer model for emission and Dubois model for backscattering. Retrieval is done by direct inversion. The Aquarius OSSE attempts to capture the influence of different error sources: land surface heterogeneity, instrument noise and retrieval ancillary parameter uncertainty. In order to assess the impact of these error sources on the estimated volumetric soil moisture, a quantitative error analysis is performed through the comparison between of footprint-scale synthetic soil moisture product and high spatial resolution degraded at coarse resolution `true' soil moisture product. The root mean squared errors are evaluated for all the conditions.