Testing digital circuits before their implementation is critical to design highly reliable systems. This allows the designer to detect faults and measure the effectiveness of the used faulttolerance techniques. Fault injection is used to measure the robustness of fault-tolerant systems during the design process and determine the dependability parameters of the system. This paper proposes a fault injection system to inject both permanent and transient faults at different fault rates that vary with the time to simulate fault rate in space orbits. Simulation results illustrate that the calculated variations are close to the actual on-orbit rates.