High-precision pointing and attitude estimation and control algorithms for hardware-constrained spacecraft

Abstract

The overarching objective of this thesis is to develop algorithms for high-precision pointing and attitude estimation and control on hardware-constrained spacecraft. This includes small spacecraft, where tight mass, volume, power, and cost constraints exist, as well as spacecraft where certain hardware has failed. As a case study, the attitude determination and control subsystem (ADCS) for ExoplanetSat will be designed. ExoplanetSat is a three-unit CubeSat (10 x 10 x 34 cm, ~ 4 kg) designed to detect exoplanets around bright Sun-like stars via the transit method. To achieve the photometric precision necessary to detect Earth-sized exoplanets, a pointing precision on the arcsecond level must be achieved. This is an unprecedented level of pointing precision on a spacecraft of this size that will be accomplished through a two-stage control system: reaction wheels for coarse attitude control and a piezo stage for fine pointing control. A linear analysis developed using stochastic linearization techniques is used to analyze the various contributions to pointing error, allowing software improvements to be made, which decrease pointing error by as much as 50%. Simulations show that a pointing precision of 2.3 arcsec (3[sigma]) can be achieved, which is two to four orders of magnitude beyond the current capability of other comparable spacecraft. In addition to performing high-precision pointing, the spacecraft must perform many other ADCS modes. These modes are complicated due to the lack of certain hardware, specifically gyros and coarse Sun sensors covering the entire sky. To deturrble the spacecraft after initial deployment, a novel control algorithm is proposed that will simultaneously detumuble the spacecraft while avoiding angular rate observability singularities, allowing the rate to be properly estimated with a magnetometer alone throughout the detumnbling process. This is done by regulating the amount of kinetic energy in the system relative to the decreasing momentum, which excites nutation in the spacecraft and maintains a full-rank nonlinear observability matrix. To search for the Sun, a guidance and control law is developed that efficiently searches the sky while navigating based on the body magnetic field direction alone and again avoiding observability singularities. During the slews between orbit day and night, the star camera is the main sensor. To dramatically reduce the image processing time, a method of tracking stars with small windows will be developed. To support this star camera mode, a new and efficient window generation technique is developed to find new stars to track as stars fall out of the field of view of the star camera during the slew. While these algorithms will be designed with ExoplanetSat in mind, they can easily be applied to other spacecraft with similar hardware. Finally, a three-degree-of-freedom air bearing testbed was developed to test some of these algorithms on flight-equivalent hardware in a representative environment. Testbed results show the ability to initialize the star camera and operate it in a fast windowed mode, use the reaction wheels to slew to a target attitude, and achieve 12 arcsec (3[sigma]) pointing with the reaction wheels and piezo stage. The simulation, modified to match the environment and parameters of the testbed, correctly predicted the testbed results within 10%, verifying the simulation and increasing confidence in the on-orbit simulation predictions. This successful hardware demonstration increases the technology readiness level (TRL) of this pointing control system to TRL 6.