Previous posters have incorrectly looked upon the aircraft's Inmarsat receiver clock as if it was a free-running quartz wall clock. It's not free running, its locked synchronous to the incoming bit stream from the satellite (which in turn locks it's own highly accurate caesium or rhubidium clock to other satellites in the constellation and timing sources on the ground).
Once the receiver is synchronised to the bit stream it can start looking for a unique (nearly) repeating pattern of bits to tell it where a frame starts, once it knows where a frame starts it can start to assemble groups of bits into bytes, which will start to assemble into a packet.....etc...
Last edited by catch21; 22nd Mar 2014 at 12:48.