 Maser-disk geometric distance

An orbit model for a resolved maser disk combines angular positions, Doppler velocities, accelerations and sometimes proper motions to infer physical orbital dimensions and distance. For an ideal matched circular radius, $R=v^2/a_{\mathrm{orb}}$ and $d=R/\theta$; velocities and accelerations of unrelated maser spots cannot simply be inserted into that expression. Warps, inclination, eccentricity and feature identification enter real disk fits.^[ref]