Attitude control

Attitude control is control of the orientation of a ships, spacecraft, or other flight vehicle, either relative to the celestial sphere or to a gravitating body influencing its flight path.

Controlling vehicle attitude requires sensors, to measure vehicle altitude, actuators to apply the torques needed to re-orient the vehicle to a desired attitude and algorithms , a set of computations that determine how to command the actuators, based on the sensor measurements.

Sensors

Gyroscopes 

Devices that sense rotation in 3-space, without reliance on observation of external objects. Classically, a gyroscope consists of a spinning mass, but it also includes "Laser Gyros" utilizing coherent light reflected around a closed path. Gyros require initialization by some other means, as they can only measure changes in orientation. In addition, all gyro measurements are subject to drift, and can maintain orientation for limited times only, typically tens of hours or less.

Horizon indicator 

An optical instrument that detects light from the 'limb' of the Earth's atmosphere, i.e., at the horizon. It can be a scanning or a staring instrument. Infrared is often used, which can function even on the dark side of the Earth. It provides orientation with respect to the earth about two orthogonal axes. It tends to be less precise than sensors based on stellar observation.

Motion Reference Unit 

Motion Reference Units are single or multiaxis motion sensors. They utilize Micro-Electro-Mechanical-Structure (MEMS) sensor technology. These sensor are revolutionizing inertial sensor technology by bringing together micro-electronics with micro-machining technology, to make complete systems-on-a-chip with high accuracy. Typical applications for Motion Reference Units are:

• Antenna motion compensation and stabilization
• Dynamic positioning
• 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 AUVs and ROVs
• Ship motion monitoring

Orbital Gyrocompassing 

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, i.e. 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 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.

Star tracker 

An optical device measuring the direction to one or more stars, using a photocell or solid-state camera to observe the star. There are 57 bright navigational stars in common use. One of the most used is Sirius (the brightest). However, for more complex missions entire starfield databases are used to identify orientation. Star trackers, which require high sensitivity, may become confused by sunlight reflected from the exhaust gasses emitted by thrusters.

Algorithms

Control Algorithms are the computer programs that receive input data from the vehicle sensors and derive the appropriate torquecommands to the actuators to rotate the vehicle to the desired attitude. The algorithm can be very simple, e.g., proportional control, a complex nonlinear estimator or many in-between types, depending on mission requirements. Typically, the attitude control algorithms are part of the software running on the hardware which receives commands from the ground and formats vehicle data Telemetry for transmission back down.

Actuators

Attitude control can be obtained in several ways, of which use of

Thrusters

is the most common, as thrusters may be used for station keeping as well. Thrusters (often monopropellant rockets), must be organized as a Reaction control system to provide triaxial stabilization. Their limitation is fuel usage. The fuel-efficiency of an attitude control system is determined by its ISP (essentially, the rocket's exhaust velocity) and the smallest torque impulse it can provide. In practice, vehicle spin is reduced to a rate equivalent to this amount. Afterwards, a tiny blip of thrust in one direction, and a few tens of seconds later, an opposing blip of thrust is needed to keep orientation errors within limits. To minimize this fuel limitation on mission duration, auxiliary attitude control systems are used to reduce vehicle rotation to lower levels, notably smaller, lower thrust vernier thrusters that accelerate ionized gasses to extreme velocities electrically, 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 starfield or the Earth's surface or atmosphere.

Momentum wheels 

These are electric motor driven rotors made to spin in the direction opposite to that required to re-orient the vehicle. Since 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 two 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. While 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, since 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. Pioneer 10 is an example of this use.

Gravity Gradient stabilization 

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 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 field 

Coils (also known as Magnetic Torque 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 to react against. One classic field "coil" is actually in the form of a 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.

Pure passive attitude control 

gravity gradient and magnetic field pointing can be combined to form a completely passive attitude control system. Such a simple system has limited pointing accuracy, because the spacecraft will oscillate around energy minima. This drawback is overcome by adding a viscous damper, 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. As this system has two 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 at need.