Spacecraft Attitude and Stabilization
Spacecraft attitudes require sensors to track vehicle orientation, actuators to apply the torques necessary to steer it in its desired direction, and algorithms that use sensor measurements and desired attitudes as input to control these actuators based on guidance, navigation and control (GNC). This integrated field is known as guidance, navigation and control (GNC).
Simulink provides a Spacecraft Dynamics block which allows for custom attitude dynamics configuration. In this example, starting from its original position of nadir, the satellite will then move towards two ground locations in sequence.
Attitude is the ‘ideal’ orientation of a spacecraft.
Spacecraft are equipped with various sensors to measure vehicle orientation. This information is fed into an attitude determination subsystem for analysis before being used to control actuators that orient the vehicle in response to commands from the system. This field of study, known as guidance, navigation and control (GNC), encompasses sensor data, actuator control mechanisms, and attitude management as a whole.
To determine the current attitude of a spacecraft, it must take measurements at every momentary of time that measure position and velocity vectors – something known as data fusion or assimilation can help with.
Most systems rely on rate sensors to observe and calculate attitude for spacecraft frames and calculate attitude. Gyroscope devices are common choice, though high accuracy quartz rate sensors have become increasingly prevalent. Star trackers detect stars using precalculated catalogs; Earth and horizon sensors provide coarse attitude estimates; additionally most systems come equipped with emergency modes in case of sensor failure or anomalous spacecraft dynamics.
Attitude is the ‘deviation’ from this.
Spacecraft attitude stabilization and control is necessary for several reasons. First and foremost is communication: an accurate position is required so that a high-gain antenna may point accurately towards Earth for communications, experiments achieve precise pointing for data collection, interpretation, and interpretation, as well as short propulsive maneuvers being executed in the correct direction. Sensor technologies and advanced control algorithms continue to advance these efforts while increasing sensitivity, accuracy and efficiency of spacecraft attitude determination and stabilization.
Attitude determination must be performed within carefully specified performance requirements during the mission duration, not exceeding the maximum allowed spin rate of the launcher and adhering to performance requirements set by NASA. A combination of sensors such as sun sensors, star trackers, gyroscopes and magnetometers must be employed onboard for this process.
Attitude estimates can either be calculated statically based on sensor observations alone, or using a filter like the Kalman filter that statistically combines past and present sensor information to create an ever-updating estimate. This latter approach is preferred as it eliminates redundant duplicate systems while providing emergency backup mechanisms if the main attitude estimation process should fail.
Attitude is the’spin’ of a spacecraft.
Many spacecraft systems need to know which direction their craft is facing in order to operate efficiently, as this knowledge is critical for navigation, Earth observation and pointing of other instruments (like star trackers, sun sensors or gyroscopes) onboard. Spacecraft attitude determination can be a complex process requiring sophisticated sensors technology and control algorithms.
Final results consist of a set of values representing the current spacecraft orientation in space. These values may be described using rotation matrices, Euler angles or quaternions – with quaternions often being preferred as they don’t suffer from gimbal lock and only require four values to fully describe any orientation.
Once attitude information has been derived, it is fed into an onboard control system which uses it to calculate required torques to actuate the spacecraft using either reaction wheels or small thrusters until its desired attitude state has been reached. At that point, mission objectives can be fulfilled successfully by the spacecraft.
Attitude is the’shape’ of a spacecraft.
Attitude stabilization and control for satellites in orbit must be addressed for various purposes, including positioning high-gain antennas to point towards Earth for communications, pointing instruments precisely so they can collect and process data, as well as thermal control.
Attitude determination subsystems utilize sensors and algorithms to estimate the current orientation of spacecraft. Kalman filters provide predictions based on both present and historical sensor information; their parameters (gain weights) may also be precalculated prior to sending out instructions, aiding rapid convergence.
Attitude systems must be capable of adapting to spin rate changes during acquisition of initial inertial lock and providing coarse and fine pointing with precise accuracy targets. Rate sensors for each of the spacecraft axes (such as gyros or solid state devices such as ring laser gyros) may also be utilized, and must also be resilient enough to withstand failure of individual sensors; two popular representations for spacecraft attitude include Rotation Matrices and Quaternions.