Spacecraft attitude determination and control
It is often needed so that the spacecraft high-gain antenna may be accurately pointed to Earth for communications, so that onboard experiments may accomplish precise pointing for accurate collection and subsequent interpretation of data, so that the heating and cooling effects of sunlight and shadow may be used intelligently for thermal control, and also for guidance: short propulsive maneuvers must be executed in the right direction.
Voyager and Galileo, for example, were designed with scan platforms for pointing optical instruments at their targets largely independently of spacecraft orientation.
Knowing where to point a solar panel, or scan platform, or a nozzle — that is, how to articulate it — requires knowledge of the spacecraft's attitude.
(For some applications such as in robotics and computer vision, it is customary to combine position and attitude together into a single description known as Pose.)
Attitude can be described using a variety of methods; however, the most common are Rotation matrices, Quaternions, and Euler angles.
While Euler angles are oftentimes the most straightforward representation to visualize, they can cause problems for highly-maneuverable systems because of a phenomenon known as Gimbal lock.
A rotation matrix, on the other hand, provides a full description of the attitude at the expense of requiring nine values instead of three.
Quaternions offer a decent compromise in that they do not suffer from gimbal lock and only require four values to fully describe the attitude.
If thrusters are used for routine stabilization, optical observations such as imaging must be designed knowing that the spacecraft is always slowly rocking back and forth, and not always exactly predictably.
Reaction wheels provide a much steadier spacecraft from which to make observations, but they add mass to the spacecraft, they have a limited mechanical lifetime, and they require frequent momentum desaturation maneuvers, which can perturb navigation solutions because of accelerations imparted by the use of thrusters.
[citation needed] Attitude control can be obtained by several mechanisms, including: Vernier thrusters are the most common actuators, as they may be used for station keeping as well.
The fuel efficiency of an attitude control system is determined by its specific impulse (proportional to exhaust velocity) and the smallest torque impulse it can provide (which determines how often the thrusters must fire to provide precise control).
To minimize the fuel limitation on mission duration, auxiliary attitude control systems may be used to reduce vehicle rotation to lower levels, such as small ion thrusters that accelerate ionized gases electrically to extreme velocities, using power from solar cells.
Momentum wheels are electric motor driven rotors made to spin in the direction opposite to that required to re-orient the vehicle.
To maintain orientation in three dimensional space a minimum of three reaction wheels must be used,[6] with additional units providing single failure protection.
A CMG is a bit more expensive in terms of cost and mass, because gimbals and their drive motors must be provided.
For this reason, the International Space Station uses a set of four CMGs to provide dual failure tolerance.
[7] These purely passive attitude control systems have limited pointing accuracy, because the spacecraft will oscillate around energy minima.
[8] When a satellite is utilizing aerodynamic passive attitude control, air molecules from the Earth's upper atmosphere strike the satellite in such a way that the center of pressure remains behind the center of mass, similar to how the feathers on an arrow stabilize the arrow.
The algorithms range from very simple, e.g. proportional control, to complex nonlinear estimators or many in-between types, depending on mission requirements.
The appropriate commands to the actuators are obtained based on error signals described as the difference between the measured and desired attitude.
The PID controller which is most common reacts to an error signal (deviation) based on attitude as follows where
A simple implementation of this can be the application of the proportional control for nadir pointing making use of either momentum or reaction wheels as actuators.
Another important and common control algorithm involves the concept of detumbling, which is attenuating the angular momentum of the spacecraft.
Gyroscopes are devices that sense rotation in three-dimensional space without reliance on the observation of external objects.
Classically, a gyroscope consists of a spinning mass, but there are also "ring laser gyros" utilizing coherent light reflected around a closed path.
This can be as simple as some solar cells and shades, or as complex as a steerable telescope, depending on mission requirements.
It is usually an infrared camera; nowadays the main method to detect attitude is the star tracker, but Earth sensors are still integrated in satellites for their low cost and reliability.
Many solutions have been proposed, notably Davenport's q-method, QUEST, TRIAD, and singular value decomposition.
[citation needed] For terrestrial vehicles and spacecraft operating near the Earth, the advent of Satellite navigation systems allows for precise position knowledge to be obtained easily.
