Meaning of BFO Data
Kudos to everyone for finally publishing the Inmarsat "raw data". (BTW - Inmarsat is, of course, bound by its clients' instructions. MAS finally authorized the release of the data.)
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.
This implies that the BFO value is measured by the a/c terminal and then transmitted as part of the Satcom data to Inmarsat for diagnostic purposes. This makes eminent sense since Doppler shift is a major reason for degradation and failure of the a/c-satellite-earth station links.
Thus, the BFO value is proportional to the relative velocity of the satellite and the a/c along the LOS (line of sight) axis between them.
Still unclear is the precise correlation between the relative LOS velocity and the amount of the BFO change. What LOS velocity (in knots) requires a a Doppler shift pre-correction of 100 Hz? Why are the amounts (apparently) never negative?
It may be that the frequency offset relates to a base frequency equivalent to 600 MHz, rather than the uplink frequency of 1600 MHz. This could make sense depending on exactly how the satellite deals with Doppler shift encountered on uplink and downlink transmissions.
Last edited by Communicator; 9th Jun 2014 at 21:04.