Originally Posted by Shadoko
One more guess about Doppler...
If the AES has to compensate for Doppler, it have to do this from its own.
No. The Ground station runs a continuous carrier on the P-channel. The Aircraft Satellite Data Unit compares the "offset" frequency it receives to the known frequency of that channel using a Phase Locked Loop with an Oven Controlled Crystal Oscillator as its reference. This offset frequency has the percentage difference of the Tx/Rx (1.6GHz/1.5GHz) added to it, and is used to adjust the Tx frequency by the same amount, but in the opposite direction.
The aircraft communicates this "housekeeping" data to the Ground station as a "burst offset" signal on the R-channel. The fact that the burst offset seems to be smaller than the expected doppler shift is the issue that is really creating the problems with those on this thread trying to make sense of it.
For good measure, here is a list of the data channels and for what purpose they are used:-
P-Channel: Packet-mode TDM channel used in the forward (outbound) direction (ground-to-aircraft) to carry signaling and packet-mode data. The transmission is continuous from each GES in the satellite network.
R-Channel: Random access (slotted Aloha) channel used in the return (inbound) direction (aircraft-to-ground) to carry signaling and packet-mode data, specifically the initial signals of a transaction (typically request signals).
T-Channel: Reservation TDMA channel used in the return direction only. The receiving GES reserves time slots for transmissions requested by an AES according to message length. The sending AES transmits the messages in the reserved time slots.
C-Channel: Circuit-mode SCPC channel used in both forward and return directions to carry digital voice or data/facsimile traffic. The use of the channel is controlled by assignment and release signaling at the start and end of each call or FAX transmission.