Agile Earth Observation Under Operational Constraints: CMG vs Reaction-Wheel ACS

As small commercial Earth Observation constellations expand, operators strive to collect more data and increase value per spacecraft while maintaining low costs and deployment times. In this context, spacecraft agility is often overlooked in constellation design: how quickly a satellite can rotate and then meet pointing requirements largely sets how many targets it can image per orbit. Most small satellites use reaction wheel clusters for pointing and reorientation, but fast retargeting with wheels can be costly in both time and power, reducing the number of images that can be taken within a single pass.
Control Moment Gyroscopes (CMGs) offer a major improvement for high-agility missions. By producing control torque through momentum redirection, CMGs can provide much higher manoeuvre capability than reaction wheel clusters, enabling shorter manoeuvre times for the same pointing requirement.
This paper presents a comparative case study that measures the mission-level benefit of an agile spacecraft versus a reaction-wheel-actuated satellite for Earth Observation missions. Using a representative push-broom imaging payload and realistic operating constraints, including viewing geometry and cloud coverage, it was assessed how each cluster affects the number of executable collections and the ability to respond to urgent demands. Results show that CMG-enabled agility provides the largest benefit for time-critical imaging and for missions where it is crucial to collect as much information as possible within a limited timeframe. In addition, CMGs enable observation modes that are difficult to achieve with reaction wheels, such as imaging patterns that adapt to a user-defined area of interest. Overall, CMGs offer a practical way to increase per-satellite imaging output and improve responsiveness in time-critical operations.