Christopher M. Pong

High-Precision Pointing and Attitude Determination and Control on ExoplanetSat

ExoplanetSat is a 10 10 34-cm, 5-kg space telescope designed to detect exoplanets around the brightest Sun-like stars via the transit method. Achieving this objective while meeting strict mass, volume, and power constraints necessitates an innovative highprecision pointing and attitude determination and control subsystem (ADCS) design. This paper will present the overall ADCS hardware and software design of ExoplanetSat. The software description will focus on the payload operation during slews (ecient window generation for stars entering the eld of view) as well as the guidance, navigation, and control algorithms for high-precision pointing with two-stage actuation. Analyses and results on the achievable pointing precision using a high-delity simulation will be presented. To demonstrate this two-stage control idea in hardware, a testbed on a spherical air bearing was designed, fabricated, and tested at the Massachusetts Institute of Technology (MIT) Space Systems Laboratory (SSL) in collaboration with Draper Laboratory. This hardwarein-the-loop testbed successfully demonstrated precision pointing, raising this subsystem to a technology readiness level (TRL) of 5. Furthermore, results from this testbed were used to validate the simulation results, thereby increasing the condence in results produced by the simulation.

A Dual-Spinning, Three-Axis-Stabilized CubeSat for Earth Observations

e Microsized Microwave Atmospheric Satellite (Micro) combines two traditional control approaches: a dual spinner and a three-axis gyrostat. Unlike typical dual spinners, the purpose of Micro’s  cubesat bus and spinner assembly is to actuate a  cubesat payload, not to add gyroscopic stiffness. An orthogonal triple reaction wheel assembly will both counter the angular momentum from the payload and rotate the satellite’s bus about its orbit-normal vector to maintain bus alignment with the orbital frame, resulting in a zero-angular-momentum system. e payload spins about the spacecra velocity axis to scan successive swaths of the Earth. However, the cubesat form factor restricts the velocity axis to be along the spacecraminor axis of inertia. is orientation leaves the spacecra at a gravity-gradient-unstable equilibrium. Further, imperfect cancellation of the payload’s angular momentum induces nutation behavior. An extended Kalman lter is implemented on a -bit  microcontroller to combine gyroscope, limb sensor, and magnetometer data to provide attitude estimation accuracy of approximately  arcminutes. Simulations show that the reaction wheels can consistently maintain pointing to within  arcminutes for orbits above  kilometers with the payload rotating at . hertz.

Reduced-Attitude Boresight Guidance and Control on Spacecraft for Pointing, Tracking, and Searching

A guidance and a control law are developed to maneuver the boresight axis of a spacecraft relative to a reference vector, using knowledge of the reference vector and the angular rates of the spacecraft. Therefore, this can be used in cases when the full attitude is not known or when the angle of the spacecraft around the boresight or reference vector does not need to be controlled. A Lyapunov-based analysis is performed to prove asymptotic stability of the system to a reference trajectory. Even in the presence of inertia uncertainty, it is shown that asymptotic stability to a stationary attitude can be guaranteed. To assist with practical gain selection, a linear analysis is performed. Finally, the boresight guidance and control laws are applied to four different scenarios to demonstrate their utility. The first two show examples of pointing in an inertially-fixed direction and tracking a time-varying target. The third and fourth demonstrate searching tasks. A novel example of searching for the Sun by red...

Prototype Optical Sensor for CubeSat Attitude Determination with Air Bearing Validation

State of the art CubeSats such as ExoplanetSat require pointing precision for the science payload on the order of arcseconds. ExoplanetSat uses dual stage control to achieve the pointing requirement. Reaction wheels provide coarse satellite attitude control while a high bandwidth piezoelectric stage performs fine optical stabilization. This paper discusses the development of ExoplanetSat’s prototype optical system which serves dual roles as the payload and attitude determination sensor. A new fast star centroiding algorithm is developed based on centroid window tracking. The tracking algorithm utilizes centroid data from previous image frames to estimate the spacecraft slew rate which provides a prediction of the current centroid locations. An image window is centered at each predicted star location. A center of mass calculation is performed on the image window to determine the centroid location. This proposed algorithm is shown to reduce the computation time by a factor of 10 with a novel air bearing hardware testbed. This paper also develops a high fidelity optical imager model in MATLAB ® Simulink®. The air bearing testbed data provides confidence in the model results so that it can be used to complete future hardware trade studies.