Robin Clark, I think you are misinterpreting the Doppler effect. The numbers show a frequency offset. So when the satellite and the aeroplane are moving towards each other, the frequency is shifted up. If they move away from each other, the frequency is shifted down. However, the vertical component of the relative aircraft-satellite movement will be negligible compared to horizontal component. Just think about the relative magnitudes of descent rate and ground speed.
Mind you, it might still be possible to detect an ascent or descent in the data - after all, frequency can be measured quite accurately.