In the thread about PLBs in slot canyons I wrote-

"US Navy NAVSAT of the 1970's used observed doppler shift to provide location information to vessels. The rate shape of the frequency shift curve (it would be an S from high to low like listening to a locomotive horn going past you) reveals the distance from the observer to the ground track of the satellite. (still have to choose which side you are on from it)"

This is the principle that INMARSAT folks applied to the flight tracking. Only in this case the satellite was "stationary" and the airplane beacon was moving.

This is quite a feat of measurement considering that the airplane is mostly flying ACROSS the line of sight (from 22,500 miles EVERYTHING is across the LOS!) and the doppler effect only applies to the component of velocity IN the line of sight.

Found an online calculator- assuming 400kt aircraft speed the biggest doppler shift would be 0.27Hz. That's quite a measurement feat!