Inmarsat is a "bent pipe" system, meaning that the satellite just amplifies and sends down to the base station everything that it receives. So the operator's RF fanciness is limited only by the noise factor in the satellite's amplifiers.
Ah, that makes a lot more sense, though nevertheless impressive precision on the part of the base station.
However, given the corridors, it still seems like the suggestion is that the Doppler measurements said it was moving rightward or leftward, when it should only say how much the aircraft was getting closer or getting further away. Looking at the map alone, it doesn't seem like that information would help distinguish which corridor it was in.
The satellite was also moving (south or north), and the measured redshift/blueshift could be used to determine if the plane was north or south of the satellite.
Geostationary satellites don't actually stay completely stationary for any length of time without active stationkeeping. They very quickly develop a north-south motion which makes them trace out an analemma, a sort of figure eight on the ground.
I first wrote geosynchronous and approximately geostationary, then decided it was too cumbersome. What sort of magnitude does the analemma have? Also is the effect amplified by it being a "bent pipe"?
Even if it isn't intentional, in scenarios such as measuring a Doppler shift I can see how it can be a useful tool. Almost analogous to microsaccades we make with our eyes.
