  Optical navigation on a CubeSat must rely on the best extraction of the
directions of some beacons from on-board images. We present an experiment on
OPS-SAT, a CubeSat of the European Space Agency (ESA), that will characterize
an onboard algorithm to this aim, named Angle-based Correlation (AbC). OPS-SAT
is a 3-unit CubeSat with an Attitude Determination and Control System (ADCS)
and an imager that have proved their reliability with typical performance at
CubeSat scale. We selected a few star-fields that all present enough visible
stars within a 10 ? field of view. When our experiment is run, OPS-SAT is
pointed to the most convenient star-field at that time. There, the star-field
is imaged and subwindows are extracted from the image around the expected
location of each star, based on the attitude-quaternion reported by the ADCS.
The AbC reconstructs the absolute direction of the central body, in principle
unknown, which is the pointed known star in the experiment. The method
intensively uses the quaternion algebra. The beacon location is first
consolidated in the field of view with the AbC. Then, the field of view is
finely positioned against the sky, again with the AbC. A covariance is
associated with the found beacon direction. Our experiment with OPS-SAT manages
the pointing and the imager, and processes the taken images. Then, it downloads
the on-board computed absolute directions and their covariances, to be compared
with the actual directions. After a campaign of intensive use of the
experiment, the statistical performance of the algorithm will be established
and compared to the on-board computed covariances. As a bonus, an assessment of
OPS-SAT's inertial pointing stability will be available. The AbC can
theoretically get rid of the Attitude Control Error (ACE) of the platform and
of the Attitude Knowledge Error (AKE) estimated by the ADCS, and potentially
converge to (...)
