What is ADCS?
The Attitude Determination and Control System (ADCS) is the satellite subsystem that senses the spacecraft's orientation in space and maintains it at the commanded pointing direction with specified accuracy. Every satellite function depends on correct attitude: communications antennas must point toward the ground, solar arrays toward the Sun, radiators away from the Sun, and imaging sensors toward the target area on Earth. A 0.1° pointing error on a GEO communications antenna with a 0.5° beam width means significant signal degradation.
Attitude determination sensors
Star trackers: The primary precision sensor; photographs a patch of sky and matches star patterns against an onboard catalogue to determine absolute 3-axis attitude with accuracy of 1–5 arc-seconds. Inertial Measurement Units (IMUs/gyroscopes): Measure rotation rates between attitude updates; drift over time, requiring periodic correction from star tracker fixes. Sun sensors: Coarse sensors providing Sun direction to 0.1–1° accuracy; primarily for safe mode attitude determination when fine sensors might be occluded. GPS attitude determination: Using multiple GPS antenna baselines, attitude can be determined to 0.1–0.5° — used on many smallsats in place of star trackers.
Attitude control actuators
Reaction wheels: Electric motors spinning flywheels; by changing wheel speed, angular momentum is transferred to or from the satellite body, rotating it without propellant expenditure. The most common precision control actuator. Control Moment Gyroscopes (CMGs): Gimballed gyroscopes providing high torque for agile attitude manoeuvres — used on ISS and high-agility imaging satellites. Magnetorquers: Electromagnetic coils that interact with Earth's magnetic field to generate modest torques; used to desaturate (momentum-dump) reaction wheels. Thrusters: Used for large attitude manoeuvres and momentum dumping when magnetorquers are insufficient.