The selection of a Fine Attitude Control Actuator (FACA) is a systems engineering decision for small satellites in Low Earth Orbit (LEO), directly impacting mission success by affecting pointing accuracy, stability, and image quality. This thesis addresses the lack of a standardised rationale for selecting one actuator over another, and a quantitative selection methodology, by developing a model-based framework to determine the most suitable FACA for nadir-pointing small satellites ($20–300 \text{ kg}$) performing sub-meter spatial-resolution Earth observation. \\

A 4-DOF modular simulation is developed to model orbital dynamics, dominant disturbance torques present at altitudes of $200$ to $500 \text{ km}$ (atmospheric, gravity-gradient, residual-dipole, and solar-radiation-pressure), and closed-loop PID attitude control. This tool enabled quantitative trade-off analysis between Momentum Wheel (MW) and Reaction Wheel (RW) assemblies, evaluating their pointing accuracy and precision under identical mass and volume constraints.

The results demonstrate that for the defined mission scope, an RW assembly is the most suitable actuator, consistently achieving superior pointing performance. Under perfect sensing conditions, RW systems can achieve sub-meter precision, outperforming MW systems by an order of magnitude for the same mass allocation. While MW systems exhibited lower susceptibility to sensor noise, their best-case precision remained around 2 meters, at the expense of significantly higher pointing-accuracy error (approximately $200 \text{ m}$). The primary trade-off is quantified as the required torque authority, which depends on orbital altitude and satellite cross-section, versus the total mass of the actuator assembly.

The main contribution of this work is a generalised and validated framework for quantitatively predicting closed-loop attitude performance from high-level mission parameters, providing a practical tool for rapid preliminary actuator sizing and selection during early-phase mission design studies.