Spacecraft platform instability constitutes one of the most significant limiting factors in hyperacuity pointing and tracking applications, yet the demand for accurate, timely and reliable attitude information is ever increasing. The PhD research project described within this dissertation has served to investigate the solution space for augmenting the DTU μASC stellar reference sensor with a miniature Inertial Reference Unit (IRU), thereby obtaining improved bandwidth, accuracy and overall operational robustness of the fused instrument. Present day attitude determination requirements are met and surpassed by the μASC in the low frequency domain. However, the intrinsic limitation in the photon flux available from starlight necessitates relatively long sensor exposure periods for the μASCs unparalleled performance to be realized, thus introducing an inherently limited time resolution of the instrument, and affecting operations during agile and complex spacecraft attitude maneuvers. As such, there exists a theoretical foundation for augmenting the high frequency performance of the μASC instrument, by harnessing the complementary nature of optical stellar reference and inertial sensor technology. With both sensor types providing measurements of the spacecraft attitude in space, harnessing the extreme accuracy of the μASC throughout the low frequency range and the inherent fidelity of miniature accelerometers in the high frequency domain allows the combined instrument to provide unsurpassed accuracy over the entire span of frequencies applicable to spacecraft attitude control systems. Completing the first steps from theoretical possibility towards a proven concept constitutes the primary focus of the project, having necessitated extensive research and development within several diverse technical areas such as highly miniaturized analog and digital electronics, instrument space qualification, test and validation procedures, sensor fusion techniques and optimized software implementations to reach a successful conclusion. The content of the project thus represents cutting edge aerospace technology due to the extreme performance that must be ascertained on all fronts whilst harnessing only a minimum of resources. Considering the physical limitations imposed by the μASC instrument as well as the next generation of smaller and more agile satellites, the main design drivers of the IRU implementation become critical parameters such as power consumption, volume and mass in addition to system level robustness and operational safety. The nature of the Ph.D. project requires not only cross disciplinary research, but also the application of emerging technologies never before employed in High-Rel space instrumentation systems.