But redundant INS units protect against failure of LR gyros. They don't protect against the case where an extremely slow turn by the plan can induce drift in an LR gyro. In that case all your redundant systems will all drift together.
LR gyros are also limited by physics to a certain limit of precision, and this is the specific mechanism by which they can drift (from what I understand though building larger gyros might help). So flying for a long time.... I don't know to what extent we know how much drift might occur. It would certainly be possible to build in some checks here though since landing in Iran instead of Afghanistan would seem to show some discrepancy between the two systems.
I am just not sure you can be sure that relative position will be as accurate as absolute position over an extended period of airtime.
> But redundant INS units protect against failure of LR gyros. They don't protect against the case where an extremely slow turn by the plan can induce drift in an LR gyro. In that case all your redundant systems will all drift together.
I'm am pretty sure this is very wrong, although I'm not an expert enough to downvote you. Having multiple systems allows one to average out (statistically independent) drifts; it's not just a protection from failure. I don't think there's anything special about a "slow turn" which will cause military-quality INSs to have correlated drifts.
As I understand it with LR gyros, if you rotate them sufficiently slowly so that they move less than half a wavelength in the time the light transits, they re-lock on the new orientation without interference. This is a limit based on quantum physics and it's why I said that larger gyros might reduce the problem.
Well, if so I think this is an issue that designers of military systems have been well aware of for years. Drones don't seem any more at risk than submarines, which have used INSs successfully for decades. Drones seems much less likely to go through large stretches of time without access to GPS than submarines, so I'd be surprised if it's an issue.
I think the real problem is that INSs are simply to bulky and heavy for drones, rather than anything to do with drift.
There was a fratricide in Desert Storm where a bunch of Apaches drifted behind friendly lines. INS is mentioned as a contributing factor. IIRC they were hovering, but a slight wind didn't register with the INS.