Since the discovery of the radiation belts (tori of energetic particles trapped by the Earth magnetic field) significant efforts have been devoted to understanding the physics behind the acceleration and loss of relativistic electrons, which produce significant damage to Earth-orbiting satellites. Our understanding of the dynamics of the radiation belts is still incomplete. Satellite measurements are usually limited to a single location in space, which complicates the data analysis. In this study we propose to develop a 3D-Data Assimilative Radiation Belt Code by combining a database developed at LANL with the UCLA 3D radiation belt code. By blending the model with observations using data assimilation we propose to globally reconstruct the radiation belt fluxes (perform reanalysis) using multiple satellite observations. The 3D code will allow us to use a vast array of available data including pitch-angle distributions, flux profiles at different energies and L-shells (measure of radial distance in terms of Earth radii). Results of the reanalysis will allow us to determine conditions in the solar wind under which radial peaks in phase space density are formed, and consequently conditions under which local acceleration is efficient. Using the reanalysis we will also be able to understand the physics of radiation belts acceleration and loss, and develop a comprehensive model to predict radiation belt fluxes, which are hazardous for satellites and humans in space. The tools developed can also be applied in future to data collected from the multiple NASA Radiation Belt Storm Probe (RBSP) spacecraft. Understanding the physical processes will help in the development of mechanisms for active remediation of the radiation belts, which are important for the civilian and military satellites.