FMC position calculation and logic for primary GPS update

[aaa0808]

I have 2 questions

1) How does the FMC calculate the position using GPS, IRS and Radio? It certainly is not calculated based on GPS or IRS alone. So what is the 'weight' of each of them in the calculation?

2) What is the FMC logic which selects the position from one of the GPS sensor units as the primary update to the FMC position? How is it decided which GPS to be used as primary update?

These queries are in reference to B737-800.

[Intruder]

For the 747-400:

1. If GPS is working, it is used for primary position. An IRS(3) (weighted "triple mix" of 3 IRU positions) is also calculated, and used if GPS fails.

2. Left GPS updates feeds FMC. Right GPS feeds right FMC.

[aaa0808]

But if you look at the picture of the POS REF page on the Boeing website at Navigation (http://www.b737.org.uk/navigation.htm) , it shows the FMC position different from the one given by the GPS.
I too earlier had the same understanding as yours but after researching it a bit, I have found that this is not the case. FMC position is indeed derived from all navigation systems and not just GPS. So I am looking for the formula or logic that is used.

[MarkerInbound]

Read below the picture.


The FMC uses GPS position (NG's only) as first priority for FMC position updates, it will even position update on the ground. If GPS is not available, FMC position is biased approximately 80:20 toward radio position and IRS L.