@ Hyperveloce
The Inmarsat graphic is essentially a media release designed to smooth over technical details that would "complicate" matters.
If you refer to SATCOM manufacturer's data, they have very little to say about what goes on in the Satellite Data Unit or the Satellite Beam Steering Unit, and Inmarsat are also very quiet on the subject. The best way to find what is going on is to go through Patent applications for alternatives to the methods in use. For the Patent application to succeed, it must show why it needs to use the methods that it is promoting. So this is where are a lot of the detail can be found.
US Patent #6008758 will provide an insight into the mysterious "D1/D2".
The ADIRU also supplies data to the SBSU/SDU, and if certain parameters are exceeded, say momentary loss of the P Channel occurs, the AES may initiate an "I am here" handshake with the SAT.