B737 groundspeed source for nav displays?

[xetroV]

Does anyone know from which source the EHSI's (on a classic) or ND's (on an NG) obtain their groundspeed information?

As far as I know, the aircraft symbol represents the FMC position, the heading is obtained from the captain's and first-officer's IRS (for the left and right displays, assuming a dual-IRS setup and transfer switches in NORMAL), and the TAS values are drawn from their respective Air Data Computers. However, I'm not sure if the groundspeed indications on the left and right EHSIs are obtained from the captain's and f/o's IRS respectively, or that both displays show the FMC groundspeed. I have searched the FCOM, but can't find any concrete information about this signal hook-up.

[TolTol]

Where does the FMC get the ground speed from? The ADIRU's. So my guess is that the ADIRU's supply the GS to the left and right DEU's which then display it on the NAV screen, this is NG talk of course. I'm probably wrong though!:O

[relax.jet]

FMC`s have GS from IRS and GPS (if installed), and this GS is displayed on ND.

[xetroV]

@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! :ok: