Attitude, Determination and Control
The function of the Attitude Determination and Control System is to determine and control the angular orientation, or direction of pointing of the spacecraft. Spacecraft attitude is defined by the angular relationship between two coordinate systems; one is based on the spacecraft geometry, and the other is based on some external reference frame, such as that defined by the rotating Earth.
Two categories of attitude control exist: passive and active. A passively controlled spacecraft is stable, but has no actuators and therefore no way to command a change in its attitude. Examples of passive attitude control include spin stabilization, gravity gradient stabilization, and the employment of permanent magnets to align the spacecraft with the Earth's magnetic field.
Passive control requires no power and no actuators, is simple and very inexpensive to implement, and is appropriate for missions having loose pointing requirements.
A spacecraft with active attitude control has the ability to change its angular orientation using actuators such as thrusters or reaction wheels. This allows the spacecraft to turn to point an antenna, prepare for a burn, or perform any other task that requires a specific orientation.
The choice of attitude control method for the University of Washington’s Dawgstar nanosatellite is driven foremost by the need to fulfill the ION-F primary mission goal, the demonstration of formation flight. Numerous passive and active attitude control techniques were examined for their abilities to meet the stringent pointing requirements levied on the ADCS by the formation flight requirement.