Originally Posted by Hyperveloce
If I remember correctly, the most probable cause for the large BFO excursions at each sat link logon (18:25 and 00:19) would be that the OXCO was no longer thermally regulated (SATCOM switched off for at least several minutes).
From memory, the SATCOM unit is designed not to go "on line" until the OCXO (Oven Controlled X'tal Oscillator) has reached and stabilized at its designed operating temperature and therefore complies with the INMARSAT specifications.
The more probable explanation for the BFO excursions is that the aircraft was either turning, descending or climbing at those specific times.