Observation of the return stroke channel is a key approach in understanding return stroke's developing process and mechanism, and therefore to improve the engineering model. A lightning progression feature photic observation system (LiPOS) is used to observe the bottom luminosity from 21 m to 309 m in the artificially triggered lightning. The output-error (OE) model in system identification is applied to establish the upward propagation transfer function between different luminosity pulses. The curves of phase velocity and group velocity between 1 kHz to 1 MHz are obtained. Time-domain analysis shows that the risetime of the leading edge changes from 1.1 μs at the bottom to 1.84 μs at 309 m height, and the dispersion in velocity is the key factor to induce this effect. The frequency-domain analysis shows that the OE model can suppress the measurement noise and give a clear frequency distribution curve. Below 100 kHz, the group velocity curve has a range with strong non-monotonic variation, and the curve becomes flat and reaches 58% of light velocity above 500 kHz. The analyzed results are finally compared to the research work in the reference literature and some discussions are presented. The results may find further applications in evaluating dispersions in return strokes quantitively.