Uncertainty evaluation of vision-based approaches for distance measurement of a tether tip-mass

A deployed electrodynamic tether, provided with a tip-mass, can be used to deorbit a spacecraft at the end of its operative life. During the deployment of the tether from the spacecraft, the distance measurement between the spacecraft and the tip-mass may be useful to control the whole system. In the present work, three vision-based approaches to measure the tip-mass distance and their achievable uncertainties are compared. The first approach employs two cameras (a stereo camera) to evaluate the tip-mass distance using triangulation. The second and the third approaches are based on the size of the tip-mass projection in the image plane of a monocular camera, respectively when the tip-mass is inside (in focus) or outside (out of focus) the depth of field of the vision system. The obtained results highlight the advantage of the stereo approach, but also the promising behavior of the approach based on defocus. The worst approach is always the monocular one based on the in-focus size of the projected tip-mass.