Illustration of the model, from Voisin (2023). The emitter travels from left to right, emitting in a forward cone (the cone is illustrative, in practice we use a smooth angular profile in this work). Emission frequency varies by Δf as it travels radially by Δr. Dotted lines show the observer’s direction, with γ the angle with respect to the path. The two thick ticks delimit the visible segment beyond which no emission can be seen from within the cone. The characteristic emission frequency, f_c, maps to the radius, r_c, at the centre of the segment. The visible segment being in a rotating frame, it varies with time until it vanishes.