Definition of the Euler angles (Ω, ι, ω) giving the orbital orientation and the true anomaly f that gives the current position within the orbit. The static fundamental frame (X, Y, Z) (blue) is adapted to the observables, and the co-rotated Gaussian frame (n, λ, e_z) (orange) is adapted to the symmetry of the (perturbed) two-body problem. See Fig. C.1 for an alternative convention.