All About Attitude
Determine, model, and control the attitude of your spacecraft
The study of spacecraft attitude covers the determination, modeling, and control of the evolution of a spacecraft's orientation in space. The discipline of constructing attitude determination and control systems (ADCS) is intimately connected to, though distinct from, that of guidance, navigation, and control (GNC) systems. GNC systems typically model a spacecraft as a point mass and consider only its dynamics. On the other hand, attitude systems model and control the orientation, slew, and (hopefully not!) tumbling of a spacecraft, regardless of its physical position in space.
In a real sense, ADC and GNC subsystems work together to determine and control the kinematic state ("position and orientation") of the spacecraft. GNC is responsible for position, and ADC is responsible for orientation.
This article covers the basics of attitude systems and why they are critical in modern space mission design. Blacknight Space has prepared a collection of articles on attitude systems, and this article also maps that collection.
Understanding Attitude
Attitude is always represented using a rotation in 3D space relating two reference frames. The standard convention is that attitude represents a rotation from the spacecraft's body axes to an inertial reference frame. A commonly used inertial reference frame the J2000 Earth-Centered Inertial (ECI) frame; however, you can use a different inertial frame if you desire.
Attitude can be represented numerically in many ways (Euler angles, rotation matrices, axis-angle vectors, quaternions). See our article, "Representing and Converting Attitude", for more information about attitude representations.
Attitude dynamics are highly nonlinear, and asymmetries in a spacecraft's design can cause attitude to drift even in the absence of perturbations. These perturbations, such as solar radiation pressure, gravity torques, and energy dissipation, can also cause attitude to drift over time. For this reason, modern spacecraft design requires active attitude control to maintain stable attitude over time. Our articles, "Modeling Attitude" and "Controlling Attitude", will give you insight in how to use STK to model perturbations and control your spacecraft's attitude.
Attitude Representations
Attitude can be numerically represented in many ways. We have prepared an article on attitude representations, explaining how STK represents attitude and how to convert between various representations.
Representing and Converting Attitude
Determining Attitude
To keep a spacecraft's attitude stable over time, a controller must know what the attitude is to begin with! This prompts the need for attitude determination systems. We have prepared an article on attitude determination, which covers Wahba methods and the use of the Multiplicative Extended Kalman Filter (MEKF) for attitude determination.
Modeling Attitude
The STK ecosystem supports many methods for modeling the attitude of a spacecraft, from simple static models to highly complex slew maneuvers. Our article on modeling attitude explores these various methods.
Controlling Attitude
As aforementioned, active attitude control is a critical element of modern mission engineering. Learn about how to use STK to model attitude control laws and their effect on your spacecraft's attitude dynamics.