The rapid growth of Low Earth Orbit (LEO) satellite constellations, driven by the New Space paradigm, demands Guidance, Navigation, and Control (GNC) subsystems that balance high performance with the mass, power, and cost constraints of small satellite platforms. This thesis presents the design, integration, and validation of the GNC subsystem for Sidus Space's LizzieSat-4 (LS-4) and LizzieSat-5 (LS-5), two 100 kg-class satellites targeting a 510 km Sun-Synchronous Orbit with a 5-year mission life. The LS-5 mission hosts an optical communication terminal, which imposes the most stringent pointing requirements in the constellation to date: pointing accuracy below 0.5° (2σ), pointing knowledge and stability below 0.1 mrad, and single-point failure tolerance.