Orbit and Attitude Control for (non-) Rotating Space-Based Telescopes Utilizing Reflectivity Control Devices

Abstract

Satellite-based imaging has allowed for advancements in knowledge of Earth's environment, the Solar System, and the cosmos that were otherwise not possible using ground-based counterparts. It is no surprise that scientists call for advancements in telescope technology that allow for longer-lasting and larger apertures to improve the quantity and quality of data. An increase in the aperture's diameter and fuel capacity coincides with an increase in satellite size and mass that may be incompatible with current and proposed launch systems. Mission lifetime limitations due to propellant is of particular concern for satellites operating in unstable orbits such as the Sun-Earth Lagrange points. Therefore, there exists a need for novel methods that allow space telescopes to reduce fuel usage and satellite's volume and mass.

To address fuel reduction and increase mission lifetime, reflectivity control devices (RCDs), or devices that are capable of regulating the effective force produced by Solar radiation pressure on a surface, are utilized in conjunction with the dynamics around the Sun-Earth Lagrange Points to provide a method of fuel-free orbit and attitude control. Additionally, RCDs produce lower actuator disturbances compared to traditional spacecraft actuators leading to a reduction in line-of-sight jitter. To address satellite's volume and mass limitations, Rotating Synthetic Apertures (RSA) telescopes are analyzed as a potential technology that enables larger apertures due to the reduction in mirror surface area compared to traditional satellites RSA satellites consist of a thin strip aperture which is rotated about an axis normal to the aperture plane. As the satellite rotates, multiple images are taken that are combined to recover the full image as if it was taken from a circular aperture. This dissertation presents the methodology of achieving fuel-free orbit and attitude control via reflectivity control devices; this enables long-mission lifetime, large-aperture sizing, and low disturbances for non-rotating and rotating space-based apertures.

Although reflectivity control devices have been demonstrated in orbit and extensively studied, no previous method has shown the ability to obtain full six degrees of freedom with RCDs as the satellite's only actuators. An allocation algorithm for utilizing RCDs that can be continuously switched from a specular reflective state to an absorptive state is presented along with an analysis of the impact of satellite attitude on the control envelope of an aggregate RCD configuration. Regarding the operation of space telescopes, the field of regard region, or the region in which a satellite with RCDs can maintain combined orbit and attitude control, is derived for both RSA-like and non-rotating telescope configurations. An optimization scheme for the placement of RCD cells in a given configuration to maximize the control authority over different attitudes is also presented. Additionally presented in this work is the development of a dynamically similar testbed to allow for the testing of pointing control algorithms for rotating synthetic apertures. The testbed serves as a method of testing conceptual RSA satellites at lower orbital regimes where the RCDs are not able to provide control, but still correspond to regions of interest for the operation of RSA satellites for Earth science observation. The derivation of scaling laws for the testbed and hardware-based results for RSA satellites ranging from low Earth orbit to medium Earth orbit are demonstrated.