Originally Posted by
slats11
I'm trying to understand this.
Total (or uncorrected) horizontal velocity is much greater than vertical velocity. But are you suggesting the aircraft corrects for its own ground speed? And so vertical velocity becomes relatively greater when compared to horizontal velocity corrected for ground speed? That is, vertical velocity makes a significant contribution to BFO because horizontal velocity is corrected for ground speed.
That's what they are saying. It corrects for its own ground speed, and the only reason why we have any meaningful data at all is that it corrects based on the assumption that the satellite is stationary, and in reality the satellite wobbles up to 1.5 degrees along the north-south axis. This gives us a compensation error which, added to a bunch of other terms, gives us BFO. See here
http://www.atsb.gov.au/media/5243942..._18aug2014.pdf, page 55 and onwards.
The aircraft can get ground speed and heading from GPS as well as from the inertial reference system. Both sources would agree with each other under normal circumstances. I'm not sure what would happen if circumstances are less than normal (an in-flight power outage would scramble the state of the IRS and necessitate manual realignment, and there may not be anybody in the cockpit to do this.)