@TolTol: The FMC position is based on IRU position, GPS position (if installed), and radio position. As far as I have understood, the FMC mixes the various signals using a Kalman filter (and different weighting factors) to come up with the most probable position for the aircraft. Based on that position, a groundspeed can be computed. Individual IRS groundspeeds are determined by the ADIRUs, but since FMC groundspeed cannot be determined before the various position inputs are integrated, I'd guess that the FMC itself (not some other system) computes the FMC groundspeed.
@relax.jet: So the groundspeed displayed on both NDs is obtained from the FMC, not the individual ADIRUs? I guess that makes sense. The reason I got confused about this is that the pitch and roll attitudes and heading indications for the primary flight instruments of each pilot are served directly by the on-side IRU/ADIRU to provide redundancy. So does this mean that the groundspeed information will disappear from the EHSI/ND in case of an FMS failure? (I think it will, but just checking if I've understood this correctly now.)
Thanks for the replies!
