attitude control

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Attitude control is the process of controlling the orientation of an
aerospace Aerospace is a term used to collectively refer to the and . Aerospace activity is very diverse, with a multitude of commercial, industrial and military applications. consists of and . Aerospace organizations research, design, manufacture, ope ...

vehicle with respect to an
inertial frame of reference In classical physics Classical physics is a group of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matt ...
or another entity such as the
celestial sphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ma ...

, certain fields, and nearby objects, etc. Controlling vehicle attitude requires
sensors A sensor is a device that produces an output signal for the purpose of sensing of a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and se ...
to measure vehicle orientation,
actuatorsAn actuator is a component of a machine A machine is a man-made device that uses power to apply forces and control movement to perform an action. Machines can be driven by animals and people A people is a plurality of person A perso ...
to apply the torques needed to orient the vehicle to a desired attitude, and
algorithms of an algorithm ( Euclid's algorithm) for calculating the greatest common divisor (g.c.d.) of two numbers ''a'' and ''b'' in locations named A and B. The algorithm proceeds by successive subtractions in two loops: IF the test B ≥ A yields "yes" ...
to command the actuators based on (1) sensor measurements of the current attitude and (2) specification of a desired attitude. The integrated field that studies the combination of sensors, actuators and algorithms is called
guidance, navigation and control Guidance, navigation and control (abbreviated GNC, GN&C, or G&C and within the context of NASA operations, often pronounced 'Gintsee' or (IPA) ʤɪnsiː) is a branch of engineering Engineering is the use of scientific method, scientific ...
(GNC).

# Aircraft attitude control

An
aircraft An aircraft is a vehicle or machine that is able to fly Flies are insect Insects or Insecta (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Lat ...

's attitude is stabilized in three directions: '' yaw'', nose left or right about an axis running up and down; ''pitch'', nose up or down about an axis running from wing to wing; and ''roll'', rotation about an axis running from nose to tail.
Elevator U-Bahn Rapid transit in Germany consists of four U-Bahn systems and fourteen S-Bahn systems. The U-Bahn or Untergrundbahn (''underground railway'') are conventional rapid transit systems that run mostly underground, while the S-Bahn or ...
s (moving flaps on the horizontal tail) produce pitch, a
rudder A rudder is a primary control surface used to steer a ship A ship is a large watercraft that travels the world's oceans and other sufficiently deep Sea lane, waterways, carrying goods or passengers, or in support of specialized missions, su ...

on the vertical tail produces yaw, and
aileron An aileron (French for "little wing" or "fin") is a hinged flight control surface Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude Attitude may refer to: Phil ...

s (flaps on the wings that move in opposing directions) produce roll.

# Spacecraft attitude control

A spacecraft's attitude must typically be stabilized and controlled for a variety of reasons. It is often needed so that the spacecraft
high-gain antenna A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performance ...
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.

## Types of stabilization

There are two principal approaches to stabilizing attitude control on spacecraft: * is accomplished by setting the spacecraft spinning, using the gyroscopic action of the rotating spacecraft mass as the stabilizing mechanism. Propulsion system thrusters are fired only occasionally to make desired changes in spin rate, or in the spin-stabilized attitude. If desired, the spinning may be stopped through the use of thrusters or by
yo-yo de-spin A yo-yo (also spelled yoyo) is a toy consisting of an axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with t ...
. The
Pioneer 10 ''Pioneer 10'' (originally designated Pioneer F) is an American , launched in 1972 and weighing , that completed the first mission to the . Thereafter, ''Pioneer 10'' became the to achieve the needed to . This project was conducted by the ...

and
Pioneer 11 ''Pioneer 11'' (also known as ''Pioneer G'') is a robotic Robotics is an interdisciplinarity, interdisciplinary field that integrates computer science and engineering. Robotics involves design, construction, operation, and use of robots. T ...
probes in the outer solar system are examples of spin-stabilized spacecraft. * is an alternative method of spacecraft attitude control in which the spacecraft is held fixed in the desired orientation without any rotation. **One method is to use small thrusters to continually nudge the spacecraft back and forth within a
deadband A deadband or dead-band (also known as a dead zone or a neutral zone) is a band of input values in the domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of def ...

of allowed attitude error. Thrusters may also be referred to as mass-expulsion control (MEC) systems, or
reaction control system A reaction control system (RCS) is a spacecraft system that uses thrusters and reaction control wheels to provide attitude control Attitude control is the process of controlling the orientation of an aerospace Aerospace is a term used to ...
s (RCS). The space probes
Voyager 1 ''Voyager 1'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel t ...
and
Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. A part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on a traje ...
employ this method, and have used up about three quarters of their 100 kg of propellant as of July 2015. **Another method for achieving three-axis stabilization is to use electrically powered
reaction wheel A reaction wheel (RW) is a type of flywheel A flywheel is a mechanical device which uses the conservation of angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent o ...
s, also called momentum wheels, which are mounted on three orthogonal axes aboard the spacecraft. They provide a means to trade
angular momentum In , angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent of . It is an important quantity in physics because it is a —the total angular momentum of a closed system remains constant. In three , the ...

back and forth between spacecraft and wheels. To rotate the vehicle on a given axis, the reaction wheel on that axis is accelerated in the opposite direction. To rotate the vehicle back, the wheel is slowed. Excess momentum that builds up in the system due to external torques from, for example, solar photon pressure or
gravity gradient Gravity gradiometry is the study and measurement of variations ( anomalies) in the Earth's gravitational field In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis ...

## Articulation

Many spacecraft have components that require articulation. Voyager program, Voyager and Galileo (spacecraft), Galileo, for example, were designed with scan platforms for pointing optical instruments at their targets largely independently of spacecraft orientation. Many spacecraft, such as Mars orbiters, have solar panels that must track the Sun so they can provide electrical power to the spacecraft. Cassini–Huygens, Cassini's main engine nozzles were steerable. 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. Because a single subsystem keeps track of the spacecraft's attitude, the Sun's location, and Earth's location, it can compute the proper direction to point the appendages. It logically falls to the same subsystem – the Attitude and Articulation Control Subsystem (AACS), then, to manage both attitude and articulation. The name AACS may even be carried over to a spacecraft even if it has no appendages to articulate.

# Geometry

Attitude is part of the description of how an object is placed in the Euclidean space, space it occupies. Attitude and position fully describe how an object is placed in space. (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 (computer vision), Pose.) Attitude can be described using a variety of methods; however, the most common are Rotation matrix, Rotation matrices, Quaternion, 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. The use of a rotation matrix can lead to increased computational expense and they can be more difficult to work with. Quaternions offer a decent compromise in that they do not suffer from gimbal lock and only require four values to fully describe the attitude.

# Sensors

## Relative attitude sensors

Many sensors generate outputs that reflect the rate of change in attitude. These require a known initial attitude, or external information to use them to determine attitude. Many of this class of sensor have some noise, leading to inaccuracies if not corrected by absolute attitude sensors.

### Gyroscopes

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 "Laser ring gyroscope, ring laser gyros" utilizing coherent light reflected around a closed path. Another type of "gyro" is a Hemispherical resonator gyroscope, hemispherical resonator gyro where a crystal cup shaped like a wine glass can be driven into oscillation just as a wine glass "sings" as a finger is rubbed around its rim. The orientation of the oscillation is fixed in inertial space, so measuring the orientation of the oscillation relative to the spacecraft can be used to sense the motion of the spacecraft with respect to inertial space.

### Motion reference units

Motion reference units are a kind of inertial measurement unit with single- or multi-axis motion sensors. They utilize Vibrating structure gyroscope#MEMS Gyroscopes, MEMS gyroscopes. Some multi-axis MRUs are capable of measuring Six degrees of freedom, roll, pitch, yaw and heave. They have applications outside the aeronautical field, such as: * antenna (radio), Antenna motion compensation and stabilization * Dynamic positioning * Active heave compensation, Heave compensation of offshore cranes * High speed craft motion control and damping systems * Hydro acoustic positioning * Motion compensation of single and multibeam echosounders * Ocean wave measurements * Offshore structure motion monitoring * Orientation and attitude measurements on Autonomous underwater vehicles and Remotely operated underwater vehicles * Ship motion monitoring

## Absolute attitude sensors

This class of sensors sense the position or orientation of fields, objects or other phenomena outside the spacecraft.

### Horizon sensor

A ''horizon sensor'' is an optical instrument that detects light from the 'limb' of Earth's atmosphere, i.e., at the horizon. Infrared#Heat, Thermal infrared sensing is often used, which senses the comparative warmth of the atmosphere, compared to the much colder Cosmic microwave background radiation, cosmic background. This sensor provides orientation with respect to Earth about two orthogonal axes. It tends to be less precise than sensors based on stellar observation. Sometimes referred to as an Earth sensor.

### Orbital gyrocompass

Similar to the way that a terrestrial gyrocompass uses a pendulum to sense local gravity and force its gyro into alignment with Earth's spin vector, and therefore point north, an ''orbital gyrocompass'' uses a horizon sensor to sense the direction to Earth's center, and a gyro to sense rotation about an axis normal to the orbit plane. Thus, the horizon sensor provides pitch and roll measurements, and the gyro provides yaw. See Tait-Bryan angles.

### Sun sensor

A ''sun sensor'' is a device that senses the direction to the Sun. This can be as simple as some solar cells and shades, or as complex as a steerable telescope, depending on mission requirements.

### Earth sensor

An ''Earth sensor'' is a device that senses the direction to Earth. 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.

### Star tracker

A ''star tracker'' is an optical device that measures the position(s) of star(s) using photocell(s) or a camera. It uses magnitude of brightness and spectral type to identify and then calculate the relative position of stars around it.

### Magnetometer

A ''magnetometer'' is a device that senses magnetic field strength and, when used in a three-axis triad, magnetic field direction. As a spacecraft navigational aid, sensed field strength and direction is compared to a map of Earth's magnetic field stored in the memory of an on-board or ground-based guidance computer. If spacecraft position is known then attitude can be inferred.

# Attitude determination

Before attitude control can be performed, the current attitude must be determined. Attitude cannot be measured directly by any single measurement, and so must be calculated (or Estimation theory, estimated) from a set of measurements (often using different sensors). This can be done either statically (calculating the attitude using only the measurements currently available), or through the use of a statistical filter (most commonly, the Kalman filter) that statistically combine previous attitude estimates with current sensor measurements to obtain an optimal estimate of the current attitude. For some sensors and applications (such as spacecraft using magnetometers) the precise location must also be known. While pose estimation can be employed, for spacecraft it is usually sufficient to estimate the position (via Orbit determination) separate from the attitude estimation. For terrestrial vehicles and spacecraft operating near the earth, the advent of Satellite navigation systems allows for precise position knowledge to be obtained easily. This problem becomes more complicated for deep space vehicles, or terrestrial vehicles operating in Global Navigation Satellite System (GNSS) denied environments (see Navigation).

## Static attitude estimation methods

Static attitude estimation methods are solutions to Wahba's problem. Many solutions have been proposed, notably Davenport's q-method, QUEST, TRIAD, and singular value decomposition.

## Sequential estimation methods

Kalman filtering can be used to sequentially estimate the attitude, as well as the angular rate. Because attitude dynamics (combination of Euler's equations (rigid body dynamics), rigid body dynamics and attitude kinematics) are non-linear, a linear Kalman filter is not sufficient. Because attitude dynamics is not very non-linear, the Extended Kalman filter is usually sufficient (however Crassidis and Markely demonstrated that the Unscented Kalman filter could be used, and can provide benefits in cases where the initial estimate is poor). Multiple methods have been proposed, however the Multiplicative Extended Kalman Filter (MEKF) is by far the most common approach. This approach utilizes the multiplicative formulation of the error quaternion, which allows for the unity constraint on the quaternion to be better handled. It is also common to use a technique known as dynamic model replacement, where the angular rate is not estimated directly, but rather the measured angular rate from the gyro is used directly to propagate the rotational dynamics forward in time. This is valid for most applications as gyros are typically far more precise than one's knowledge of disturbance torques acting on the system (which is required for precise estimation of the angular rate).

# Control algorithms

Algorithms, Control algorithms are computer programs that receive data from vehicle sensors and derive the appropriate commands to the actuators to rotate the vehicle to the desired attitude. The algorithms range from very simple, e.g. proportional control, to complex nonlinear estimators or many in-between types, depending on mission requirements. Typically, the attitude control algorithms are part of the Computer software, software running on the Computer hardware, hardware, which receives commands from the ground and formats vehicle data telemetry for transmission to a ground station. The attitude control algorithms are written and implemented based on requirement for a particular attitude maneuver. Asides the implementation of passive attitude control such as the gravity-gradient stabilization, most spacecraft make use of active control which exhibits a typical attitude control loop. The design of the control algorithm depends on the actuator to be used for the specific attitude maneuver although using a simple PID controller, proportional–integral–derivative controller (PID controller) satisfies most control needs. The appropriate commands to the actuators are obtained based on error signals described as the difference between the measured and desired attitude. The error signals are commonly measured as euler angles (Φ, θ, Ψ), however an alternative to this could be described in terms of direction cosine matrix or error quaternions. The PID controller which is most common reacts to an error signal (deviation) based on attitude as follows $T_c \left(t\right) = K_\text e\left(t\right) + K_\text \int_0^t e\left(\tau\right) \,d\tau + K_\text \dot\left(t\right),$ where $T_c$ is the control torque, $e$ is the attitude deviation signal, and $K_\text, K_\text, K_\text$ are the PID controller parameters. A simple implementation of this can be the application of the proportional control for Nadir, nadir pointing making use of either momentum or reaction wheels as actuators. Based on the change in momentum of the wheels, the control law can be defined in 3-axes x, y, z as $T_cx = -K_\text q_1 + K_\text ,$ $T_cy = -K_\text q_2 + K_\text ,$ $T_cz = -K_\text q_3 + K_\text ,$ This control algorithm also affects momentum dumping. Another important and common control algorithm involves the concept of detumbling, which is attenuating the angular momentum of the spacecraft. The need to detumble the spacecraft arises from the uncontrollable state after release from the launch vehicle. Most spacecraft in Low Earth orbit, low earth orbit (LEO) makes use of magnetic detumbling concept which utilizes the effect of the earth's magnetic field. The control algorithm is called the B-Dot controller and relies on Magnetic torquer, magnetic coils or torque rods as control actuators. The control law is based on the measurement of the rate of change of body-fixed magnetometer signals. $m = -K\dot$ where $m$ is the commanded magnetic dipole moment of the magnetic torquer and $K$ is the proportional gain and $\dot$ is the rate of change of the Earth's magnetic field.

# Actuators

Attitude control can be obtained by several mechanisms, specifically:

## Thrusters

Vernier thrusters are the most common actuators, as they may be used for station keeping as well. Thrusters must be organized as a system to provide stabilization about all three axes, and at least two thrusters are generally used in each axis to provide torque as a couple (mechanics), couple in order to prevent imparting a translation (geometry), translation to the vehicle. Their limitations are fuel usage, engine wear, and cycles of the control valves. 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). Thrusters must be fired in one direction to start rotation, and again in the opposing direction if a new orientation is to be held. Thruster systems have been used on most manned space vehicles, including Vostok (spacecraft), Vostok, Project Mercury, Mercury, Project Gemini, Gemini, Apollo (spacecraft), Apollo, Soyuz (spacecraft), Soyuz, and the Space Shuttle. 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.

## Spin stabilization

The entire space vehicle itself can be spun up to stabilize the orientation of a single vehicle axis. This method is widely used to stabilize the final stage of a launch vehicle. The entire spacecraft and an attached solid rocket motor are spun up about the rocket's thrust axis, on a "spin table" oriented by the attitude control system of the lower stage on which the spin table is mounted. When final orbit is achieved, the satellite may be de-spun by various means, or left spinning. Spin stabilization of satellites is only applicable to those missions with a primary axis of orientation that need not change dramatically over the lifetime of the satellite and no need for extremely high precision pointing. It is also useful for missions with instruments that must scan the star field or Earth's surface or atmosphere. See spin-stabilized satellite.

## Momentum wheels

These are electric motor driven rotors made to spin in the direction opposite to that required to re-orient the vehicle. Because momentum wheels make up a small fraction of the spacecraft's mass and are computer controlled, they give precise control. Momentum wheels are generally suspended on magnetic bearings to avoid bearing friction and breakdown problems. To maintain orientation in three dimensional space a minimum of three must be used, with additional units providing single failure protection. See Euler angles.

## Control moment gyros

These are rotors spun at constant speed, mounted on gimbals to provide attitude control. Although a CMG provides control about the two axes orthogonal to the gyro spin axis, triaxial control still requires two units. A CMG is a bit more expensive in terms of cost and mass, because gimbals and their drive motors must be provided. The maximum torque (but not the maximum angular momentum change) exerted by a CMG is greater than for a momentum wheel, making it better suited to large spacecraft. A major drawback is the additional complexity, which increases the number of failure points. For this reason, the International Space Station uses a set of four CMGs to provide dual failure tolerance.

## Solar sails

Small solar sails (devices that produce thrust as a reaction force induced by reflecting incident light) may be used to make small attitude control and velocity adjustments. This application can save large amounts of fuel on a long-duration mission by producing control moments without fuel expenditure. For example, Mariner 10 adjusted its attitude using its solar cells and antennas as small solar sails.

In orbit, a spacecraft with one axis much longer than the other two will spontaneously orient so that its long axis points at the planet's center of mass. This system has the virtue of needing no active control system or expenditure of fuel. The effect is caused by a tidal force. The upper end of the vehicle feels less gravitational pull than the lower end. This provides a restoring torque whenever the long axis is not co-linear with the direction of gravity. Unless some means of damping is provided, the spacecraft will oscillate about the local vertical. Sometimes tether propulsion, tethers are used to connect two parts of a satellite, to increase the stabilizing torque. A problem with such tethers is that meteoroids as small as a grain of sand can part them.

## Magnetic torquers

Electromagnetic coil, Coils or (on very small satellites) permanent magnets exert a moment against the local magnetic field. This method works only where there is a magnetic field against which to react. One classic field "coil" is actually in the form of a Electrodynamic tether, conductive tether in a planetary magnetic field. Such a conductive tether can also generate electrical power, at the expense of orbital decay. Conversely, by inducing a counter-current, using solar cell power, the orbit may be raised. Due to massive variability in Earth's magnetic field from an ideal radial field, control laws based on torques coupling to this field will be highly non-linear. Moreover, only two-axis control is available at any given time meaning that a vehicle reorient may be necessary to null all rates.

## Pure passive attitude control

There exist two main passive control types for satellites. The first one uses gravity gradient, and it leads to four stable states with the long axis (axis with smallest moment of inertia) pointing towards Earth. As this system has four stable states, if the satellite has a preferred orientation, e.g. a camera pointed at the planet, some way to flip the satellite and its tether end-for-end is needed. The other passive system orients the satellite along Earth's magnetic field thanks to a magnet.Attitude and Determination Control Systems for the OUFTI nanosatellites
Vincent Francois-Lavet (2010-05-31) These purely passive attitude control systems have limited pointing accuracy, because the spacecraft will oscillate around energy minima. This drawback is overcome by adding damper, which can be hysteretic materials or a viscous damper. The viscous damper is a small can or tank of fluid mounted in the spacecraft, possibly with internal baffles to increase internal friction. Friction within the damper will gradually convert oscillation energy into heat dissipated within the viscous damper.