Space trajectory computation at the University of Michigan

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Abstract
Extensive investigation of methods of real-time and fast- time computation of earth orbits and reentry trajectories has been conducted for both analog and digital computers. Integrator drift and multiplier errors in the case of analog computers preclude the use of direct computation in in ertial frames or geocentric coordinate systems. Such co ordinate systems can also lead to serious digital computer problems due to truncation and roundoff errors. Introduc tion of a vehicle-centered rotating coordinate system with variables referred to a nominal orbit permits greatly im proved computer scaling which, in turn, very substantially improves the absolute computational errors. In addition, the use of angular momentum and energy constraints is shown to reduce greatly the effect of integrator drift on the in-plane motion. Analog solutions to orbital and re entry problems have been obtained in real-time, fast-time, and high-speed repetitive operation using the above co ordinate system.

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