Originally Posted by Communicator
Based on a basic understanding of the electronics involved (Inmarsat Classic Aero mobile terminal), the fact that this equipment has been around since the 1990s, and discussions of the "BFO" data (e.g. on Duncan Steel's website), it appears that the BFO values are the amount of Doppler-shift pre-compensation applied by the a/c terminal.
Previously in this thread, I described how it was possible to use the offset detected at the AES Rx of the SAT P channel carrier to pre-compensate the AES Tx frequency in the opposite direction, so that the SAT Rx always received the AES on the assigned channel frequency.
It now transpires that the Honeywell SATCOM package installed on 9M-MRO used a software algorithm to calculate the Doppler shift correction to be applied, based on the aircraft position and motion. However, the software assumed that the SAT was actually at its geostationary position of 0° 64.5°E and didn't take into account the eccentric ellipse of +/- 1.63° about the equator performed by the SAT in a siderial day.
Its this eccentric motion which is used by Inmarsat to differentiate whether the aircraft was on a Northern or Southern vector.
So, in this case the BFO is the difference between what the AES calculated, and what it would have calculated if its software used the actual SAT ephemeris values.