Outcome of the propagation method for a radial and constant velocity V = (V_R = V_in, 0, 0) in RTN coordinates. Panel a shows the radial proton speed recorded by PSP (V_p, PSP(t) in blue) and V_in(t_in) (in black), calculated as averages of V_p, PSP over Δt = 1 h centered on each considered t_in. Panels b and c show the minimum distance d_min between the plasma parcel and Solar Orbiter and the corresponding propagation times τ (in black) and t_out (in orange), respectively. These are all functions of t_in. The vertical lines in panels b and c correspond to the t_in for which d_min is minimum (called d_MIN). All the time origins are set at t₀, corresponding to the radial alignment time of the spacecraft (Eq. (1)).