1. The instruments that know which way is up

Arthur C. Clarke’s 2001: A Space Odyssey returns as the recalled work because Discovery must maintain precise alignment with Earth and with its destination without any external reference except the stars, the Sun, and its own inertial sense. A LEO desktop must do the same, but it can also use Earth’s magnetic field and the planet itself as references.

This entry reads about GNC sensors and attitude determination.

2. The sensor set

A spacecraft attitude determination system typically combines several sensors, each with its own strengths and blind spots. A CubeSat Resources GNC overview lists the common types:

  • Inertial Measurement Unit (IMU): measures angular rates and sometimes accelerations. It is fast and always available but drifts over time if integrated to obtain attitude.
  • Star tracker: identifies stars and provides the most accurate absolute attitude reference. It is sensitive to sunlight, Earth albedo, and stray light, and it cannot be used during fast slews.
  • Sun sensor: measures the Sun vector. It is simple and reliable but useless in eclipse or when Earth blocks the Sun.
  • Magnetometer: measures Earth’s magnetic field vector. It is cheap and robust but sensitive to onboard electrical currents and only useful where the field is strong and well-modeled.
  • GNSS receiver: provides position, velocity, and timing, which indirectly support attitude and orbit knowledge.

A HORUS VLEO mission paper describes an ADCS relying on IMU, magnetometer, and star sensor, with polar orbits favored because the spacecraft traverses a wide range of geomagnetic latitudes and improves attitude observability.

3. Sensor fusion

No single sensor is sufficient. The solution is sensor fusion, usually implemented with a Kalman filter or one of its nonlinear variants. A Southampton thesis on Kalman filtering for spacecraft attitude explains that the ordinary linear Kalman filter cannot preserve the quaternion unit-norm constraint, so spacecraft use extended Kalman filters (EKF), multiplicative EKFs, or invariant filters.

An INHA RoSAT CubeSat ADCS paper describes a practical small-satellite implementation using a deployable magnetometer, fine sun sensors, a VectorNav IMU, and a NovAtel GNSS receiver, with data fused to produce attitude and rate estimates.

4. Accuracy trade-offs

Different payloads need different pointing accuracy:

  • A coarse Earth-imaging payload may tolerate several degrees of pointing error.
  • A high-rate communications payload may need sub-degree pointing.
  • A laser communications payload may need arc-second or sub-arc-second pointing, often requiring a fine steering mirror or fast track loop on top of the body ADCS.

The desktop will host mixed payloads, so its GNC architecture must support multiple accuracy regimes, not just one.

5. What this changes

  • The desktop needs a diverse sensor suite: IMU for rate, star tracker for absolute attitude, sun sensor for coarse reference, magnetometer for backup, and GNSS for position and time.
  • Sensor fusion is mandatory; the choice is between established EKFs and newer invariant or geometric filters.
  • Pointing accuracy requirements must be set per payload and per operational mode.
  • The next entry will read about GNC actuators and control.