Geometry of the precessing jet nozzle model adopted from Qian (2011). Three coordinate systems are used: (X_n, Y_n, Z_n), (X, Y, Z) and (X_p, Y_p, Z_p). The Y_n axis is directed toward the observer and the X_n–Z_n plane represents the sky plane. The (X, Y, Z) system is defined with respect to the (X_n,Y_n,Z_n) system and the precession axis (Z-axis) is defined by parameters (ϵ, ψ). The trajectory of a knot (superluminal component) is defined by parameters (A(Z), Φ) in the (X, Y, Z) system. θ denotes the angle between the knot’s velocity vector S and the direction toward the observer V; η is the initialhalf opening angle of the jet. (see text).