AdvanceSpace

Subsequent to determining spacecraft alignment, attitude error is reduced, nominal orientation is established and position is directed through control algorithms that use both passive and active methods.

Attitude Control

Spin stabilization uses angular momentum developed by spinning the spacecraft about its major principal axis, that axis with the largest moment of inertia. In this configuration, payloads are often despun to allow for sensor pointing. The meeting of spun and despun sections is often mechanically challenging.

Gravity-gradient stabilization is mostly used to keep spacecraft pointed Earthward. Long booms extending away from the spacecraft develop the required inertia tensor. This simple, durable and inexpensive method is too restrictive for most payloads. Also, at high altitudes, the gravity-gradient torques are too small for necessary stabilization.

Nutation damping removes undesired wobble from spinning spacecraft through passive or active means. Passive nutation dampers include tubes of viscous fluids or conducting pendulums that swing within permanent magnetic fields to producing eddy current. Active nutation dampers include thrusters, reaction wheels or electromagnets.

Reactive control provides three-axis stabilization in pitch, roll and yaw. Thrusters expel mass and momentum wheels (bias momentum wheels, reaction wheels) spin to create an angular momentum. Control moment gyros (CMGs) are bias momentum wheels mounted in two-axis gimbals. These methods are more flexible than passive systems, but also cost and weigh more with typically less longevity due to physical wear.