In anticipation of spoofing/jamming on a 737NG flight into Cyprus, we disabled GPS position updating. I expected this to lead to some drift but I was surprised at how quickly the ANP increased. In approximately 45 minutes it reached 7. IRS technology should allow aircraft to cross oceans for hours without external position information and remain within 10 so this seems rather feeble.
I had assumed that the ANP value would not be affected by a bad GPS signal if the GPS updating is turned off. Is this incorrect or is something else at play here?
As a footnote, although the FMC had a large positional uncertainty, it was in fact very accurate in its calculated position on this occasion.