Why is GPS required in case of a SAARU failure on the 777?

[TheBusFlyer]

Hello everyone,

One of our flights got dispatched with a SAARU failure. MEL says at least one GPS must be serviceable and flight can not transit through known jamming areas. Why is this the case?

I presume GPS is being used in some way or another by both ADIRU and SAARU…and by having it inop or jammed would result in a loss of navigation information /integrity?

Any input would be appreciated.

Thanks