Originally Posted by
Hyperveloce
I understand that this D2 doppler is the plane-satellite relative speed (the 3D vector Vplane-Vsat) projected (using a dot product) onto the line of sight, the 3D vector POSplane-POSsat.
Vplane causes the doppler shift D1. As you state correctly D1 is in the order of ~750Hz. That would probably be too much because it would interfere with adjacent frequency channels. That is why big parts of D1 are taken out of the equation by some preemptive compensation the plane's transceiver does. How exactly this compensation works is not yet understood by the general public, but (hopefully) by the inmarsat engineers.