Originally Posted by
mm43
However, that is not the end of the story, as it is the aircraft that senses the Doppler shift between it and the satellite and shifts its Tx frequency to compensate. If flying toward the satellite it would shift its Tx frequency downward so that any transmission from the aircraft would arrive at the satellite at the assigned and expected Rx frequency. Likewise when flying away from the Satellite, the Doppler shift would be deceasing and the aircraft would adjust its Tx frequency in the opposite direction, i.e. upwards.
The satellite has nothing to do with the Doppler correction; that is assigned to each individual aircraft, otherwise the satellite would have a very large job on its hands when communicating with many aircraft. What the Satellite does do, is transmit a constant carrier on the P Channel which the aircraft uses to sense the received frequency offset from the assigned channel and then adjusts its Tx channel(s) in the opposite direct to compensate.
Any Doppler shift between the the Ground Station (GES) and the satellite is corrected at the earth side, which allows the separate isolation of Doppler between the Sat and the aircraft (AES).
Now start thinking about where this "burst frequency offset" info is coming from.
mm43, are you sure that this P-channel continuous monitoring of the doppler shift between the A/C and the 3F1 satellite was implemented by the MH370 AES ?
I ask because the Inmarsat annex 1 clearly states (red circling D2) that "D2 is not corrected by system" (meaning both GES and AES) hence it is "the observed frequency offset". Furthermore, if the BFO were only residuals of such a correction, it would be a poor correction (not that we can expect from the P-channel monitoring) of the relative A/C to sat doppler. D3 is easily predictible via the 3F1 ephemeris so I guess that it is compensed at GES level to reduce the doppler shifts (and it is not red circled).
The meaning of D1 which is an A/C induced doppler is not so clear, D1 could either be the A/C contribution to the relative A/C to 3F1 doppler (but the annex 1 also states that D2 combines the satellite & A/C motion so that this comtribution should be encompassed by D2), or it could be the AES correction to D2 (the opposite of the P-channel doppler shifts, or estimated via the IRS data and satellite ephemeris, but Imarsat says that D2 is not corrected).
Can you make sense of this ?