Estimating Space Station alignment for Shuttle docking

The problem of orienting the Space Station docking module to the correct alignment in local level coordinates is addressed for Space Shuttle docking. The docking module is at the end of a chain of structural modules assembled in orbit without alignment survey. Therefore, docking module alignment to the structurally distant navigation base can only be predicted on the basis of nominal alignments augmented with whatever groundbased measurements were made on flight articles, with accuracy based on the probabilistic combination of manufacturing tolerances and the uncertainty of any measurements. This lack of stack alignment knowledge can be remedied by using attitude matching techniques between the Shuttle inertial platform and the Space Station strapdown inertial navigation system, on attitude data recorded during two different rotations of the docked stack. Use of three different Orbiters, several docking modules, and growth in the ISS assembly creates a different stack for each early stage flight, compounding the docking alignment prediction problem. Alignment knowledge accuracy is shown to improve substantially with minimal cost in maneuvering propellant as more combinations of Space Station and Shuttle Orbiter hardware are sampled during the early assembly stages, which significantly improves the robustness of the docking process.