Guidance, Navigation, & Control (GNC)
GNC in One Loop (Sense → Estimate → Decide → Act)
GNC is the system that keeps spacecraft pointed correctly, on the right path, and stable under disturbances. In simple terms: it is how spacecraft “knows where it is” and “keeps itself on track.”
- Guidance: compute a desired trajectory or attitude profile.
- Navigation: estimate state (position, velocity, attitude) from sensors.
- Control: command actuators to track the target and reject disturbances.
Attitude Determination & Control (ADCS)
ADCS focuses on orientation in 3D: pointing antennas, instruments, solar arrays, and performing maneuvers.
- Actuators: reaction wheels, control moment gyros, magnetic torquers, thrusters.
- Sensors: star trackers, sun sensors, IMUs/gyros, magnetometers, horizon sensors.
- Avionics: flight software, control loops, redundancy management.
Disturbances and Error Sources
- Environmental torques: gravity gradient, aerodynamic drag (LEO), SRP, magnetic interactions.
- Sensor errors: bias, noise, misalignment, dropouts.
- Actuator limits: saturation, wheel momentum buildup, thruster minimum impulse bit.
Modern Focus (Autonomy and Onboard Intelligence)
- AI-based autonomous navigation: vision-based relative navigation and terrain-relative navigation.
- Hazard avoidance: safe landing and proximity operations in uncertain environments.
- On-board anomaly detection: detect faults early and reconfigure safely.
In simple terms: spacecraft increasingly handle “unexpected situations” without waiting for Earth.