This paper presents a case study that aims to develop a hardware design for the pc to the satellite, supported fault-tree design and calculations of dependableness. The on-board pc of a satellite, inserted into the on-board management system, has functions of receiving, process and distribution of commands to the subsystems and payload of the satellite; and exploit, formatting, storage and transmission of measurement from subsystems and payload. The most necessities of associate on-board pc are high dependableness, capability of real time process, resistance to radiation, and minimizing power consumption, volume and mass. Enlarged dependableness is achieved by the fault tolerance technique that relies on adding redundancy to the system, or the utilization of elements with high price of individual dependableness. During this work, hardware redundancy techniques are thought-about. First, a pc is meant and its dependableness is calculated, for a mission of one year period. Mission dependableness goal isn't earned during this calculation. So as to extend dependableness, two kinds of fault tolerant redundant architectures are analyzed, with their various reliabilities calculated for the identical amount of the mission.