About the -2 BFO: if valid, is not the (3D) direction of the descent playing a big role? Or the value given (13000'/min) is the minimum in the most efficient direction?
It seems the aircraft terminal compensates for the aircraft speed and direction of travel, but not rate of climb/descent. This can be seen in the BFO values between 16.30 and 17:15 where the initial climb rate is in the BFO signal, then the level-off. If a descent rate of 13000'/min is put into the model at 00:19, I get -2Hz at the predicted BFO. But in that extreme situation how the compensation algorithm works is a matter of conjecture.
One more question...:
Is the way the AES compensate from its own speed and direction trully established? And, first, does it compensate always anything? Or only if the BFO value could be too high?
It works for the first few pings with the rate of climb established from the ADS-B data. By extension it seems to be active at all times. The Doppler due to an uncompensated aircraft velocity would be huge, which is not seen in the BFO data at any time.
I have read it compensates:
1. not using a measured Doppler from a signal from 3F1 (logical, if not, no pro rata BFO)
2. relatively to the theoretical position of 3F1,
3. only for horizontal speed and direction,
4. using the ADIRU.
Are all these statements established or just deduced or guessed?
1. There is a mode (it seems) where an aircraft satellite terminal uses a single channel continuously to monitor a pilot frequency from the satellite which has a known frequency. The measured offset is then an exact measure of the Doppler and the terminal can compensate precisely. However, this uses one of a limited number of channels the terminal has available, so the pre-compensation method was used on the terminal design in MH370. This is stated in the Inmarsat notes and an example terminal manual (see below).
2. As stated in the Inmarsat notes, it uses the average position of the satellite, not the exact position. This is adequate for the purpose of keeping the transmit frequency inside the legal limits, but not perfect.
3. Yes, as stated above. This is not documented but can be observed in the MH370 data.
4. The manual for an example Honeywell terminal says that the navigation data can come from the aircraft IRU or from a GPS system attached directly to the terminal. We don't know which applied to 9M-MRO.
SDIM MSC7200 23-20-35 rev 1
regards