Prior-aided model-free relative pose measurement of spinning spacecraft by rotation decomposition

Relative pose measurement of uncooperative spinning targets enables on-orbit servicing, debris removal, and space situational awareness. Monocular methods recover the full three-degree-of-freedom (3-DOF) rotation from images even though the chaser's own navigation state already fixes much of it, so a low-resolution image re-measures known quantities at a ≈3--$5$~deg error floor. We adopt the principle ``measure only what the chaser does not already know'': for a gimbaled payload tracking the target, the relative attitude separates into an orbital-rotation component fixed by geometric priors (orbit, attitude, gimbal angles) and the target spin, the only DOF requiring the image. On simulated data: (i)~the full-geometry prior reproduces the ground-truth orbital rotation to below 2×10⁻⁴~deg under error-free telemetry, its residual reducing to the target spin rate ($1.1985$~deg/frame); a tested $0.22$~deg/frame noise budget leaves the spin-rate estimate statistically unchanged; (ii)~the spin rate attains a Kalman-filtered mean absolute error of $0.18$--$0.43$~deg/frame ($0.46$--$0.51$ extended); the spin-axis tilt, from equal-weight fusion of three channels, falls from a $4.6$-deg eight-configuration mean on the $47$~deg arcs to $0.6$--$0.7$~deg on the $92$~deg extension of Seq.~2, the $47$~deg-arc tilt propagating to a $9.4%$ full-rate error; (iii)~the position follows from a Clohessy--Wiltshire fit of the line-of-sight history under the near-circular prior, with no range input, to $0.13$--$0.32%$. It is model-free and works at 256 px. The prior does not lower the per-frame composed rotation error (the homography floor) but provides attribution, the unobservable spin axis, and a $1.3$--3.4× drift reduction.