Technically, yes it is possible for an autopilot to use dead reckoning for navigation without GPS at all. (we didn't use GPS to navigate to the moon, right?) However, all commercial autopilots that I know of rely on GPS to some degree. The difference is the use of low-cost gyros and accelerometers. They are good enough when augmented with GPS, but not good enough all alone.
Edit: the low-cost sensors can be augmented with sensors other than GPS too, like vision processing. This has been demonstrated but is not yet in widespread usage. I expect to see this become more common due to demonstrated vulnerabilities in GPS.
State-of-the-art inertial navigation technology has approximately the same precision as state-of-the-art GPS. Even modern run-of-the-mill interferometer-based inertial systems are precise enough for most purposes.
The unique value of inertial navigation is that it requires neither receiving nor sending a signal that can be jammed or spoofed, hence why the military uses it for everything.
1960's inertial navigation systems have better resolution than today's consumer GPS. The issue is not the technology, it's the cost, size, and weight. The small, light, and low-cost sensors fueling the current wave of UAVs are not the most accurate technology available, but that doesn't mean they aren't useful.
The concern I would have with inertial navigation over long distances is the possibility of drift. You might have good resolution over short distances, but what about hours and hours? What if your vectors are not quite right, or you get drift in your gyros (and LR gyros are prone to drift in certain circumstances)?
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.
Out of curiosity, how precise are we talking? I assume the top-level military tech is classified, but if I buy off-the-shelf parts, how far can I fly and still land on the runway using pure dead reckoning?