1. Making do with what you have
Andy Weir’s The Martian is, among other things, a long demonstration that survival often depends on using the sensors you have rather than the sensors you wish you had. Mark Watney improvises with cameras, telemetry, and dead reckoning. The desktop is not stranded on Mars, but the same spirit applies: attitude determination is the art of combining imperfect measurements into a good enough answer.
Entry 260 set the pointing requirements. This entry chooses the sensors that measure attitude.
2. The sensor set
For a desktop in LEO, a practical sensor suite has five members:
- Star tracker: a camera that identifies stars and reports the spacecraft’s attitude relative to the inertial frame. It is the most accurate sensor, typically delivering arcsecond to arcminute accuracy.
- Sun sensor: a photodiode or small camera that measures the Sun’s direction in the body frame. It is simple, robust, and essential for safe mode.
- Gyroscope: a rate sensor that measures angular velocity around the body axes. It is used for high-rate attitude propagation between star tracker updates.
- Magnetometer: a sensor that measures the local magnetic field vector. In LEO the field is well known, so the magnetometer provides a coarse attitude reference and a rate-independent check.
- GPS receiver: provides position and velocity, which indirectly help attitude determination through the orbit propagator and the nadir vector.
These five sensors together are enough for all of the desktop’s pointing modes.
3. Star tracker
The star tracker is the precision instrument. It looks at a patch of sky, identifies the stars in it, and returns a quaternion that describes the body’s orientation in inertial space.
For the desktop, a single star tracker on the zenith face is probably enough. A second tracker on the opposite face adds redundancy but also cost, mass, and thermal complexity. A typical small-sat star tracker weighs 0.2–0.5 kg, consumes 1–3 W, and provides accuracy from a few arcseconds to a few tens of arcseconds.
Constraints:
- The tracker must not look at the Sun, Earth, or Moon; exclusion angles are typically 20–40 degrees.
- It needs a clear field of view without obstructions from solar arrays, antennas, or radiators.
- It is sensitive to stray light from Earth albedo, so a baffle is usually needed.
- It is the most computationally demanding sensor, because star identification requires a star catalogue and matching algorithm.
For the desktop’s moderate pointing requirements, a commercial-off-the-shelf star tracker is the right choice.
4. Sun sensor
The Sun sensor is the humble workhorse. A coarse Sun sensor can be as simple as a photodiode behind a small aperture; a fine Sun sensor uses a quad-cell or a small imager. The coarse version is cheap and reliable. The fine version gives better pointing for sun-tracking arrays or instruments.
For the desktop, a set of coarse Sun sensors distributed across the body is the baseline. They provide Sun direction even in safe mode, when the star tracker may be blinded or the computer may be restarting. They also help prevent the star tracker from accidentally looking at the Sun.
Typical performance: a coarse Sun sensor gives Sun direction to within a few degrees; a fine Sun sensor can reach arcminutes.
5. Gyroscope
A gyroscope measures angular rate. MEMS gyros are small, cheap, and adequate for small spacecraft. Fibre-optic gyros are more accurate but larger and more expensive. Control-moment gyros are actuators, not sensors, and should not be confused with gyroscopes.
The gyro is used between star tracker updates to propagate attitude. It is also used during slews, when the star tracker may produce blurred images, and during safe mode, when other sensors may be unavailable.
For the desktop, a triad of MEMS gyros is sufficient. A typical unit has three axes in one package, weighs 50–200 g, consumes less than 1 W, and has a noise density of a few degrees per hour.
6. Magnetometer
A magnetometer measures the local magnetic field. In LEO the field is strong and well modelled, so the magnetometer provides a coarse attitude reference relative to the Earth-fixed magnetic field vector. It is also useful for measuring the rate of rotation when the gyro is suspect.
Magnetometers are cheap and reliable, but they are easily disturbed by currents in the spacecraft harness and by magnetic materials. They must be mounted away from motors, relays, and high-current busbars. Calibration on the ground is essential.
For the desktop, a three-axis fluxgate magnetometer is a good choice. It adds redundancy and provides a sanity check on the star tracker and gyro.
7. GPS receiver
GPS in LEO is standard. A GPS receiver gives position and velocity, which the attitude software uses to compute the nadir vector, the Sun direction, and the local magnetic field. It does not directly measure attitude, but it provides the orbit context that makes attitude meaningful.
Some receivers can also provide a time pulse that synchronises the spacecraft clock. Time synchronisation matters for the imager, the star tracker, and the downlink scheduler.
For the desktop, a multi-frequency GPS receiver with LEO-capable firmware is the baseline. A patch antenna on the nadir or zenith face is usually enough.
8. Sensor fusion
No single sensor is enough. The star tracker is accurate but slow and easily blinded. The gyro is fast but drifts. The Sun sensor is robust but only sees one direction. The magnetometer is coarse and disturbed by the spacecraft itself. GPS provides orbit context but not attitude.
The attitude determination software fuses these measurements, usually with an extended Kalman filter or a similar estimator. The filter weights each sensor by its expected noise and availability. The result is an attitude estimate that is better than any single sensor could provide.
For the desktop, the filter should run at 1–10 Hz, with gyro propagation at higher rates if needed for stability during slews.
9. A rough sensor budget
| Sensor | Mass | Power | Accuracy | Role |
|---|---|---|---|---|
| Star tracker | 0.2–0.5 kg | 1–3 W | arcsec–arcmin | primary attitude |
| Coarse Sun sensors | 0.05–0.1 kg | 0.1 W | few degrees | safe mode, Sun avoidance |
| MEMS gyro triad | 0.05–0.2 kg | 0.5 W | few °/h | propagation, slews |
| Magnetometer | 0.05–0.2 kg | 0.2 W | few degrees | coarse reference, sanity check |
| GPS receiver + antenna | 0.1–0.3 kg | 1–2 W | position ~m, velocity ~cm/s | orbit context, time |
Total sensor suite: roughly 0.5–1.3 kg and 3–6 W. This is small compared to the propulsion or power attachments, but it enables almost everything else.
What this changes
- The desktop’s ADCS sensor suite is a star tracker, Sun sensors, gyro triad, magnetometer, and GPS receiver.
- Each sensor has a distinct role: precision, robustness, propagation, coarse reference, and orbit context.
- Sensor fusion is required because no single sensor covers all modes and failure cases.
- The total sensor mass and power are modest, but the placement and field-of-view constraints are real.
- The next entry will choose the actuators that apply torque.