The concept that INS is a good navigator in the short term but slowly drifts (typically 1nm/hr) while GPS is always accurate (how accurate depends on type of augmentation) is true.
But GPS is easily jammable(low signal strength) and no doubt nasty people will do so in the future when we are relying to a greater extent on GPS.
One possible solution may be to revert to INS,when jamming is detected.However ,drift can be reduced by "calibrating" the sources of INS drifts against the GPS results before the jamming starts
Keith