Doppler effect, as detected by a satellite.
Suppose you are outside and it is very quiet. You hear a train whistle in the distance. You don’t know where the train is, nor can you see it, but you can hear the whistle. The whistle starts out with high pitch, then gets steadily lower. You can the surmise that the train passed by you, and you would know the general direction that the train was travelling.
If you knew the pitch of the whistle when the train is standing still, and you knew how fast the train was travelling, and you had a precise way to measure the pitch of the whistle as you hear it pass you, you could determine the direction the train is travelling.
Back to the plane...
This method assumes that the plane was travelling at a constant speed. As it was shown, the plane changed altitude drastically. It is entirely possible that the plane changed speed. The data analysts HAD to assume an airspeed. If the airspeed changed just a little, they could be off by a lot.
I am not quite so confident thatthey know where it went.
From what I have heard about the data analysis, I would think you would get the same doppler effect and resulting positional data from both a southern route and a mirror northern route toward India and Pakistan. The satellite did not get directional data, so how would they eliminate the mirror image route?
Nice critique of the speed variation vulnerability of the Doppler technique - thanks.