Nowadays the design and operation of GNC of small satellites is widely being studied, due to the current and past space missions designed by several industries, research institutes, and space agencies. Some in-orbit demonstrations require heavy and bulky payload that must be embedded with complex set of sensors, actuators, and maintenance systems. Thus, the redundancy of elements is always inferior, so that a mathematical model estimating variables is needed to achieve the same goal with less sensors. At the same time, a good control of the state variables can be helpful in order to reduce fuel consumption, with the consequence of less space occupied by the actuation system or in an increase of the operational life span of the satellite.