> Hopefully GPS isn't your sole source of information.

Nowadays, in modern airplanes, it often is your only source of positional information when you're in instrument conditions (which is, of course, when it matters most).

> I carry multiple navigational aids

Yeah? Like what?

Compass, Altimeter, topographic maps, notes about the route that I've made. Even a watch can aid navigation if you can calculate your speed and see any terrain features.

If anything doesn't jive with what I expect or what I'm seeing I try to understand why.

Sure, when you're VFR it's pretty easy to notice that your GPS is flying you into a mountain. When you're IFR, not so much.

Maybe there is a market for civilian version of DSMAC?

Why don't planes just carry a good INS? In today's world, it shouldn't be that bad.

Money. INS is still very expensive relative to GPS, and GPS is pretty frickin' reliable 99.99% of the time.

Would commodity six axis sensors have enough precision to do a reasonable job at INS?. Buy a pack of iPhones and use some kind of quorum protocol to reduce the risks associated with one going bad?

> Would commodity six axis sensors have enough precision to do a reasonable job at INS?

I don't know, but my guess would be no. If it were that easy someone would have done it already.

But who knows? Maybe there's an untapped opportunity here. Why don't you buy one of these:

https://vetco.net/products/9-axis-inertial-navigation-module...

and take it with you the next time you fly (or drive) and see how accurate it is?

No, they won't, not at all.