If the ADIRU (interial part) fails, the SAARU takes over. The SAARU can provide Attitude and Air data (not position). You have to update HDG within 3 min on the POS init page. Navigation will be available via the FM's that still receive the GPS data, but you will loose the LNAV,VNAV,TK Sel, FPA, TOGA , LOC and GS modes, so you have to use HDG SEL, and VS or FLCH. You will also loose the flap maneuver speeds, and the autobrakes (no inertial data available). The autopilot can be re-engaged after entering the HDG.