Attitude Determination and Control (ADC)
The Attitude Determination and Control (ADC) subsystem—or Attitude Control System (ACS)—determines the spacecraft’s orientation and reorients it as needed. It ensures accurate pointing for payloads (e.g., cameras, antennas) while maintaining stability.
Core Functions
- Determine orientation via sensors (star trackers, sun sensors, IMUs).
- Control orientation using actuators (reaction wheels, thrusters, magnetotorquers).
- Compute necessary maneuvers in on-board software.
- Maintain safe orientation under fault or safing conditions (e.g., sun-point for power).
Key Design Drivers
| Driver | Driven By | Impact |
|---|---|---|
| Pointing Accuracy | Payload requirements | Sensor & actuator selection |
| Pointing Knowledge | Payload requirements | Accuracy of star trackers, etc. |
| Stability | Payload jitter requirements | Structural stiffness, wheel balance |
| Max Slew Rate | Operational needs | Actuator torque sizing |
| Time Between Momentum Unloads | External torques & ops constraints | Reaction wheel capacity vs. thruster usage |
Sensors
Common ADC sensors include:
- Star Trackers: High-accuracy attitude info; heavier and more complex.
- Coarse Sun Sensors: Provide rough sun direction, critical for safing.
- Earth/Horizon Sensors: Often used for Earth-pointing missions.
- Rate Gyros: Detect rotational rates; can drift over time.
On-board vs. Ground Knowledge
- On-board attitude knowledge is often less accurate than post-processed ground solutions.
- Mission design might use ground-based orbit determination for certain payload tasks.
Actuators
- Reaction Wheels: Precise, continuous control; can saturate under external torques.
- Magnetotorquers: Use planetary magnetic field; can unload wheel momentum in LEO.
- Thrusters: Provide high torque for maneuvers; consume propellant.
Autonomy & Safing
- Coarse sensors (e.g., sun sensors) allow the spacecraft to “safe” itself by finding the Sun autonomously.
- Fault-tolerant software keeps spacecraft power-positive, pointing arrays at Sun if higher-level control fails.
Practical Considerations
- Mass and Power: Reaction wheels and star trackers can dominate mass & power budgets for ADC.
- Thermal: Sensors and actuators may need stable temperatures; be mindful of heater placements.
- Mechanical: Mounting sensor/actuator aligns with structural nodes that meet stiffness requirements.
Cross-Links
- See Spacecraft Thermal Control System for sensor temperature requirements.
- See Propulsion for thruster-based unloading or momentum maneuvers.