3-axis stabilized spacecraft

3 axis stabilisation is a design feature of most modern spacecraft whereby the spacecraft utilizes sensor readings to determine how actuators on the spacecraft can be used to keep it in a desired attitude, or orientation in space.

Description

A rigid body in space that is orbiting a gravitational body, and initially fixed relative to an inertial space, will eventually begin to wobble, as it will always be subject to small torques. One possibility to avoid this wobble is to make the spacecraft rotate around a principal axis, a so-called spin-stabilized satellite. An alternative to spin stabilisation is to use an active attitude control system with attitude sensors to detect mispointing and attitude control actuators, mostly momentum wheels or thrusters, to compensate for the outer torques and to keep the spacecraft in the desired attitude. With such a control system the attitude of the spacecraft can be controlled at will and can be kept in an inertially fixed attitude; it can be turned in any way at any time. This type of attitude control is called 3 axis stabilisation and most modern spacecraft use this kind of attitude control.Fact|date=August 2008

To specify the attitude of a 3 axis-stabilized spacecraft, two reference frames are defined:

#one well defined reference coordinate system, for example the International Celestial Reference Frame
#one "spacecraft reference frame" fixed relative the spacecraft body. This is normally the engineering reference frame used for the spacecraft drawings

With these reference frames defined the attitude can be specified by giving the relative orientation of these 2 systems. This can be

*the 3x3 rotation matrix that rotates frame 1 to frame 2
*the euler angles of frame 2 relative to frame 1
*the quaternions of frame 2 relative to frame 1

The matrix corresponding to the first alternative is

:

where :$hat\{x\}\; ,\; hat\{y\}\; ,\; hat\{z\}$

are the unit vectors defining the "spacecraft reference frame" (frame 2 above) and:$hat\{a\}\; ,\; hat\{b\}\; ,\; hat\{c\}$

are the unit vectors defining the reference coordinate system (frame 1 above).

ee also

*Spin-stabilized satellite
*Attitude dynamics and control

References

*cite web| publisher=Caltech's Jet Propulsion Laboratory | title=Attitude and Articulation Control Subsystems (AACS) | url=http://www2.jpl.nasa.gov/basics/bsf11-2.html | accessdate=2008-08-02