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Guidance, Navigation, and Control (GNC)

Guidance, Navigation, and Control (GNC) is the set of systems that allow a spacecraft to understand its state and influence its motion. It combines three closely related functions:

  • Guidance – defining what the spacecraft should do (e.g. point an antenna at Earth, align a payload, follow a trajectory)
  • Navigation – determining where the spacecraft is and how it is moving (position, velocity, and time)
  • Control – applying forces or torques to achieve and maintain the desired state

Within this broader framework, Attitude Determination and Control Systems (ADCS) focus specifically on the spacecraft’s orientation – how it is rotated in space and how that orientation is measured and controlled.

In CubeSats and small spacecraft, GNC typically includes:

  • Attitude determination using magnetometers, sun sensors, star trackers, and IMUs
  • Attitude control using magnetorquers and reaction wheels
  • Navigation and timing using GNSS or ground-based orbit determination
  • Control algorithms and estimation

Overview

Guidance

Defines the desired spacecraft behavior, such as pointing targets or trajectory objectives.

To be added here:

  • Mission objectives and pointing modes
  • Target tracking (Earth-pointing, Sun-pointing, inertial pointing)
  • Operational modes (safe mode, detumble, nominal ops)
  • Guidance profiles and timelines

Determines the spacecraft’s position, velocity, and time within a chosen reference frame.

To be added here:

  • Orbit determination basics
  • Onboard vs. ground-based navigation
  • GNSS in space (LEO vs. high altitude considerations)
  • Time synchronization and clocks
  • State vectors and reference frames

Reference Frames and Coordinate Systems

Defines how position and attitude are represented and transformed between coordinate systems. This is fundamental to both navigation and attitude determination.

To be added here:

  • ECI, ECEF, body frame, orbital frame (LVLH)
  • Transformations between frames
  • Attitude representations (Euler angles, quaternions, DCMs)
  • Common pitfalls (gimbal lock, frame confusion)

Orbit Representation / TLEs

Describes how spacecraft orbits are represented and propagated over time.

To be added here:

  • State vectors (position and velocity)
  • Propagation and limitations

Passive Stabilization Methods

Uses environmental forces and simple physical principles to stabilize spacecraft attitude without active control.

To be added here:

  • Gravity gradient stabilization
  • Magnetic hysteresis damping
  • Spin stabilization
  • Advantages, limitations, and mission fit

Attitude Sensors

Inertial Measurement Units (IMUs)

Magnetometers

Measure the local magnetic field and provide a reference vector for attitude determination. They are simple and reliable but sensitive to onboard interference and require careful placement and calibration.

To be added here:

  • Calibration and alignment
  • Noise sources and interference
  • Use in attitude determination and control

Parts

Gyroscopes

Measure angular velocity (how fast the spacecraft is rotating). They are essential for short-term attitude propagation but tend to drift over time, so they are usually combined with absolute sensors (e.g. magnetometers, sun sensors, star trackers).

To be added here:

  • MEMS vs. fiber-optic / ring laser gyros (performance vs. cost)
  • Bias, noise, and drift characteristics
  • Calibration and temperature effects
  • Role in sensor fusion (e.g. with magnetometer / star tracker)

Parts

Star Trackers

Provide high-precision absolute attitude by imaging the star field and matching it against a catalog. They are the most accurate ADCS sensors but require more compute, power, and careful optical design.

To be added here:

  • Star identification and pattern matching
  • Optics, sensor, and exposure considerations
  • Sensitivity to stray light and Earth albedo
  • Typical accuracy and update rates
  • Open-source implementations and datasets

Actuators

Reaction Wheels

Active actuators that control attitude by exchanging angular momentum with the spacecraft. They enable precise pointing but introduce complexity and require momentum management.

To be added here:

  • Wheel configurations (3-axis vs. 4-wheel pyramid)
  • Momentum buildup and desaturation (e.g. with magnetorquers)
  • Jitter, vibration, and balancing
  • Failure modes (bearing wear, saturation)
  • Sizing vs. required torque and agility

Magnetorquers

Generate torque by interacting with Earth’s magnetic field. They are simple, robust, and power-efficient, but provide limited control authority and depend on the local magnetic field.

To be added here:

  • Rod vs. coil (including PCB coil) implementations
  • Torque generation (m × B) and axis limitations
  • Use for detumbling (e.g. B-dot control)
  • Interaction with onboard magnetometers
  • Placement and magnetic cleanliness considerations

Control Algorithms

Defines how sensor data is translated into actuator commands to achieve desired behavior.

To be added here:

  • B-dot and detumbling controllers
  • PID and state-space control
  • Attitude vs. orbit control
  • Quaternion vs. Euler representations
  • Controller tuning and stability

Estimation and Sensor Fusion

Combines data from multiple sensors to estimate spacecraft state reliably.

To be added here:

  • Attitude estimation problem overview
  • Kalman and extended Kalman filters
  • Combining IMU, magnetometer, and sun sensor data
  • Common failure modes

Disturbances and Space Environment

External and internal forces that influence spacecraft motion and must be accounted for in control design.

To be added here:

  • Gravity gradient torque
  • Atmospheric drag (LEO)
  • Solar radiation pressure
  • Magnetic field interactions
  • Internal disturbances (currents, moving parts)

Modes of Operation

Defines how the spacecraft transitions between different control and mission states.

To be added here:

  • Detumble mode
  • Safe mode
  • Nominal pointing modes
  • Transition logic between modes
  • Fault detection and recovery

Provides position, velocity, and time, forming the navigation side of GNC and supporting guidance and control decisions.

To be added here:

  • GNSS in LEO and high-dynamic environments
  • COCOM limits and high-altitude receivers
  • Time synchronization and onboard clocks
  • Ground-based orbit determination vs. onboard solutions
  • Integration with guidance and control systems
  • Dual use technology regulations and considerations

ADCS Integration Considerations

Covers practical aspects of integrating sensors and actuators into a spacecraft system.

To be added here:

  • Mechanical alignment and tolerances
  • Magnetic cleanliness
  • Power and thermal coupling
  • Interaction with payload and comms

Testing and Validation

Covers methods for verifying GNC systems before and during flight.

To be added here:

  • Helmholtz cages and magnetic testing
  • Flatsat and HIL testing for ADCS
  • On-orbit commissioning strategies
  • Common pitfalls during bring-up

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