Originally Posted by mm43
US Patent #6008758 will provide an insight into the mysterious "D1/D2".
The ADIRU also supplies data to the SBSU/SDU, and if certain parameters are exceeded, say momentary loss of the P Channel occurs, the AES may initiate an "I am here" handshake with the SAT.
In the patent link you give, we can read:
Originally Posted by US Patent #6008758
.../...However, in this situation, conventional AES terminals continue to reserve one channel for receiving P Channel signals for the sole purpose of monitoring Doppler shift. P Channel signals are conveniently used for this purpose since these signals are continuously available.
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In general, customers who need only circuit-mode services are forced to order an AES terminal equipped with one more channel than the number of telephone calls the system must support. The additional channel needed is a costly, wasted resource.
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One method that has been used in an attempt to address this problem is to pre-compensate the transmit frequencies based on a calculated Doppler shift rather than a measured Doppler shift. The method calculates the Doppler shift using the satellite location, the aircraft location, and data which defines flight characteristics (e.g., velocity, ground speed, heading pitch, roll, etc.). Much of the data needed to calculate the Doppler shift comes from the aircraft's Inertial Reference System (IRS). This method has been difficult to implement due to the many different types of IRS systems which are made by many manufacturers and provide inconsistent flight data. Thus, it has been difficult to produce an AES terminal which calculates Doppler shift and is compatible with different aircraft.
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It would also be advantageous to provide a method for measuring Doppler shift in an AES terminal or communication system independent of P Channel inputs..../...
So, if MH370 used the "precompensation method", the wobbling of 3-F1 can't be included and thus the Doppler is not "perfectly" compensated as it could have been if the P channel was continuously monitored, the "total" Doppler measured and then used to transmit a frequency the sat will receive as the right one.
In this case, it could be useful to calculate the Doppler from the fixed points where MH370 might have been located at the times published on the Inmarsat Doppler graph. Could this may also explain why all the values on the graph are positive?
The words "
This method has been difficult to implement due to the many different types of IRS systems which are made by many manufacturers and provide inconsistent flight data" could also explain why the tests made on other flights to improve the data have been made on B-777s.