@MarkJJ
ny chance the INS would have become primary navigation means due to a loss of GPS? Flying ref ADIRU only?
No INS exists onboard... IRS exists... and incase GPS signals are shielded or unable for any reason... it will automatically revert to IRS as a primary source mode of navigation. but RNAV means of terrestrial aids such as VOR DME will be used to more accurately position fix.