ashfordeOU Star Tracker Attitude Determination (space-systems/adcs/star-tracker)
Use when the task is star tracker attitude determination: matching measured star centroids to a star catalog, computing the angular separation between a measured centroid and its catalog star, choosing the boresight and field of view, or deciding between lost in space and tracking mode.
Domain quick reference
- A star tracker is the ADCS attitude reference with the highest pointing accuracy: it images stars, measures their centroid positions on the focal plane, and matches the pattern against an onboard star catalog to recover the instrument attitude.
- Star identification is the matching step: each measured centroid is compared against catalog stars by angular separation theta = acos(dot(u_meas, u_cat)) between unit vectors, and the best match inside the field of view is accepted.
- The boresight is the tracker optical axis, the center of the field of view. The field of view (FOV) is the angular window the sensor sees; a square FOV of half-angle fov/2 accepts catalog stars within that radius of the boresight.
- Lost in space mode identifies stars with no prior attitude by searching the full star catalog; tracking mode uses the prior attitude to predict which catalog stars should appear and matches within a small tracking radius.
- Pointing accuracy is anisotropic: cross-boresight error is set by centroid noise relative to focal length, while roll error about the boresight is larger because star positions on the focal plane barely change under roll.
- Update rate: a star tracker produces attitude at rates from a few Hz to tens of Hz; the ADCS uses the matched-star angular separation as the pointing error bound between updates.
Workflow
- Normalize the catalog star unit vectors and the measured centroid unit vector.
- Compute the angular separation between each measured centroid and every candidate catalog star: acos of the dot product, clamped to [-1, 1] before the acos call.
- Accept the nearest catalog star when its separation is within the FOV half-angle; return the matched star id and the separation as the boresight error.
- Select the mode: tracking when a prior attitude exists and the nearest match is within the tracking radius, otherwise lost in space against the full catalog.
- Report the boresight error in arcseconds and compare the achieved pointing accuracy against the requirement.
Pitfalls
- Comparing unnormalized vectors: the dot product must use unit vectors or the acos separation is meaningless.
- Letting floating point round-off push the dot product outside [-1, 1], which makes acos return nan; clamp first.
- Accepting a match outside the FOV: the nearest catalog star is not a valid identification when its separation exceeds the FOV half-angle.
- Choosing lost in space when a valid prior attitude exists: tracking mode is cheaper and more robust.
- Reporting roll accuracy as if it equaled cross-boresight accuracy.
- Confusing the field of view (angular window, half-angle fov/2) with the boresight (its center direction).
Behavior contract (gate 3)
The star identification, boresight, and mode logic is exercised by the gate 3 contract test: scripts/test_star_tracker.py against scripts/star_tracker_logic.py (stdlib unittest, offline). Run: python3 scripts/test_star_tracker.py
Compliance
- ECSS (European Cooperation for Space Standardization) standards are freely downloadable, copyright ESA; cite the source and paraphrase. This leaf cites ECSS as reference only per standards-map.yaml; the logic here is generic star tracker geometry, not ECSS text.
- compliance: STANDARDS-REF, gated: false.