ADCs, or Attitude Determination and Control Systems, actively determine and control a satellite’s orientation in space to achieve desired positioning. Satellites operate in microgravity without atmospheric drag to stabilize them, and external forces like solar radiation pressure, gravity gradients, and magnetic fields can disrupt their direction. Therefore, ADCs counteract these disturbances, ensuring satellites precisely point to tasks like transmitting signals to Earth or capturing high-resolution images.
Key Components of ADCs
Sensors act as the system’s “eyes,” actively providing data on the satellite’s current orientation.
– Sun Sensors: These devices detect sunlight direction, helping satellites position relative to the sun.
– Earth Sensors: By scanning Earth’s horizon, these sensors align satellites with Earth, crucial for observation missions.
– Star Trackers: High-precision devices identify star patterns, functioning like astronomical GPS. They deliver accuracy up to arcseconds, ideal for deep-space probes.
– Gyroscopes and Inertial Measurement Units (IMUs): These tools measure angular velocity and acceleration, tracking attitude changes over time.
– Magnetometers: They sense Earth’s magnetic field, providing a reference for low Earth orbit (LEO) satellites.
– Reaction Wheels: These rotate via motors to generate torque, enabling fine adjustments without fuel consumption.
– Control Moment Gyroscopes (CMGs): Advanced reaction wheels, they provide higher torque for large satellites or rapid maneuvers.
– Thrusters: Chemical or electric propulsion systems emit gas for coarse corrections or to offload reaction wheel momentum.
– Magnetorquers: Coils interact with Earth’s magnetic field to produce torque, offering energy-efficient control for low-orbit satellites.
The “brain” of ADCs, a flight computer, actively runs control algorithms. These include proportional-integral-derivative (PID) controllers for basic stabilization and Kalman filters to estimate attitude from noisy sensor data. Additionally, modern systems employ model predictive control (MPC) to optimize maneuvers under constraints like power limits. The software ensures fault tolerance with redundancy, using radiation-hardened processors to withstand space’s harsh radiation environment.
ADCs operate in a continuous loop:
Sensing Phase: Sensors collect raw data on position, velocity, and external references.
Estimation Phase: Algorithms process this data to estimate current attitude, often using quaternion mathematics to avoid Euler angle singularities.
Control Phase: Based on desired attitude (from ground commands or autonomous goals), the system calculates required torque.
Actuation Phase: Actuators apply the torque, and the cycle repeats through feedback.
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