Just last week a crew flying this plane strayed 10 miles into Iran because of the GPS.
I'm not the most succinct wordsmith in the library but I hope this was meant to say that the NAV error was due to the crew's lack of a working GPS.
A failing INS**, without regularly scheduled cross-checks could lead one astray in an insidious manner, but not a GPS. The box is designed to flag it's own outputs upon failure.
**Come to think of it, a serviceable INS could do this too.
