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.
