Because of the cost-effectiveness of flying smallsats compared to large flagship spacecraft, there is increasing interest in boosting their capabilities for supporting precision science payloads and sophisticated instrumentation. Unfortunately, a major current drawback with using smallsats is their inability to hold the pointing line-of-sight steady without jittering. Line-of-sight jitter degrades observations made by cameras and other imaging-type instruments, and fundamentally limits the quality of science that can be obtained.
The solution to the problem is to develop technology to specifically address torque disturbances created by reaction wheel imbalances. Such torque disturbances induce line-of-sight motions that are greatly magnified when acting through the reduced vehicle inertias associated with CubeSats and smallsats.
The GEMINI Stability Control (GSC) concept reduces the effect of reaction wheel vibration disturbances and improves pointing stability performance across a wide range of smallsat platforms. The main idea is to use extra reaction wheels (which are relatively inexpensive for smallsats), combined with precision speed control to maintain reaction wheel speeds on the “sweet spot” of their passive vibration isolators to optimize disturbance attenuation. Using two reaction wheels per axis (six reaction wheels total) simplifies the arrangement, and allows torque to be commanded using common mode control (i.e., commanding the wheels to accelerate in the same direction), and speed commanded using differential-mode control (i.e., commanding the wheels to accelerate opposite each other).
Reaction wheels for smallsats are small and relatively inexpensive when compared to their full-scale spacecraft counterparts. Hence multiple reaction wheel configurations are affordable and practical. This implies that multiple reaction wheel configurations associated with the Gemini concept become practical. The Gemini concept uses differential wheel-speed control to enhance vibration suppression, while using common-mode speed control to satisfy the usual need for generating control torque. The two-wheel-per-axis Gemini configuration is especially simple to develop because each axis can be separately tested on a dynamometer before integrating all three axes into the vehicle.