Spacecraft attitudes must be stabilized and controlled for various reasons: to point toward Earth so the high-gain antenna may work, achieve precise pointing for experiments onboard, or guide their orbital path. This integrated field investigates sensors, actuators, and algorithms which help orient vehicles to meet desired attitude requirements.
Kinematics
Kinematic equations describe the movement of vehicles or spacecraft through velocity and acceleration. If certain other information is known about a vehicle’s position, kinematic equations can help predict unknown information regarding its exact position.
Attitude control systems typically employ magnets attached to spacecraft as their basis, providing accurate pointing accuracy while expending limited energy to oscillate around energy minima. To reduce this energy expenditure, adding a damper made of either hysteretic material or viscous fluid can help. Friction in these dampers convert oscillation energy into heat that lowers actuator power requirements significantly.
An advanced system features electrically powered reaction wheels, also called momentum wheels, mounted on three orthogonal axes of a spacecraft. These computer controlled wheels can trade angular momentum with one another by either accelerating or decelerating one wheel on any given axis; additionally, magnetic bearings suspend them to prevent mechanical failures.
Kinetics
Attitude control requires taking measurements from spacecraft sensors and using these in a Kalman filter that statistically combines previous estimates with current sensor readings to form an estimate of where the vehicle currently stands.
The estimated current state is used to update velocity and position control actuators, driving the vehicle toward a new attitude while minimizing angular velocity error with the aid of kinematic equations of motion.
Reorientation maneuvers must respect various mechanical constraints such as maximum available torque of reaction wheel array and orientation constraints such as exclusion zones/inclusion zones defined for mission hardware such as telescopes/cameras that could be damaged from celestial objects as well as antennas that need to point toward Earth’s magnetic field in order to maintain continuous pointing profiles. Different techniques are used to resolve such constrained problems including Non-Linear Optimization Problems and Semi-Definite Programming.
Stability
An attitude control strategy must typically be employed on spacecraft in order to meet various mission objectives such as high-gain antenna pointing for communications, absolute pointing capability for experiments on board and short propulsive maneuvers – however these maneuvers often incur high costs and consume substantial propellant amounts.
An alternative, more cost-effective method involves shifting mass in order to steer a spacecraft in its desired direction – known as mass expulsion control (MEC) or reaction control systems.
As aerodynamic and atmospheric uncertainty may introduce errors to the location of the host spacecraft’s center-of-pressure (CoP), relative to that of a shifting system’s CoM, accuracy in stability analysis analysis is vital in order to account for these uncertainties and ensure reliable results.
This research shows that the MEC method can effectively eliminate disturbances and maintain a stable attitude without resorting to additional actuators like reaction wheels or magnetic torquers. Furthermore, MEC accommodates Euler angle changes induced by mass shifts with acceptable performance.
Control
As spacecraft operate, disturbance forces may produce torque on their vehicles that results in attitude changes that require corrective actions from onboard systems to restore equilibrium and correct direction of motion. Such control systems comprise sensors which measure current state data, actuators to apply desired torques and algorithms which command them based on (1) sensor measurements and (2) desired attitude specifications – an integrated field known as guidance, navigation, and control (GNC).
The course introduces the fundamentals of rotational kinematics and dynamics: Euler rigid body equations; major/minor axis spins, asymptotic stability, energy dissipation. Following that it discusses means for attitude control such as reaction wheels, control moment gyros, magnetic force actuators and thrusters.
It also details the basic operation of Kufasat Attitude Determination and Control System (ADCS), used to fly cube satellites in low Earth orbit. The system features three-axis gravity gradient stabilization for better orientation with respect to Earth, as well as using Kuiper belt observations for orientation purposes. A mission simulation was also developed in order to explore autonomous closed-loop attitude control performance of ADCS.