Sliding Mode Control (SMC) is well known as a robust control approach and is proven to be able to deal with nonlinear systems. To achieve this capability, the SMC controller input design is divided into two parts: A sliding surface design (continuous control) and a switching function design (discontinuous control). A spacecraft's attitude model is a multi-input and multi-output (MIMO) system and thus control design is difficult for some methodologies, however, in this case a SMC, is straightforward to construct. In this paper, for the continuous part, a reduction of order method (ROOM) is used to construct the sliding surface. For the discontinuous control, three different switching functions are designed and evaluated such as relays with constant gains, relays with state dependent gains and linear feedback with switched gains. The main contribution of this paper is to both analyse and investigate the limitations of these three switching functions at two different points (critical gains and proper gains) on a spacecraft's attitude model. The gains are selected using trial and error techniques as long as these gains meet the sufficiency conditions for the existence of a sliding mode. The discontinuous control is a high-speed switching function that produces chattering in the control input; however, solutions for chattering drawbacks are not discussed here. The best switching function is chosen based on the spacecraft's attitude transient performance requirements.
