In the last decades, low-thrust propulsion has gained an increasing interest by the scientific community, and has been already employed in some mission scenarios. This work proposes a unified guidance and control architecture, termed VTD-NOG & PD-RM, and describes its application to low-thrust orbit transfer from LEO to GEO. The variable time-domain neighboring optimal guidance (VTD-NOG) is a feedback guidance technique based upon minimizing the second differential of the objective function along the perturbed trajectory. This minimization principle leads to deriving all the corrective maneuvers, while avoiding possible singularities that often arise in alternative neighboring optimal guidance schemes. VTD-NOG identifies the trajectory corrections by assuming a thrust direction always aligned with the longitudinal axis, thus generating a discontinuous commanded attitude. A proportional-derivative approach based on rotation matrices (PD-RM) is employed in order to drive the actual spacecraft orientation toward the desired one. Reaction wheels are employed to perform the attitude control action. In the dynamical simulations, oscillating perturbations of the propulsive thrust, errors on the initial conditions, and gravitational perturbations are considered. Extensive Monte Carlo campaigns point out that orbit injection at GEO occurs with very satisfactory accuracy, thus demonstrating that VTD-NOG & PD-RM indeed represents an effective methodology for the application at hand.
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