While INS is not simple, it's also not that difficult to implement. As even the cheapest drones that can hover show new MEMS sensors are accurate enough to work with a proper filtering.
I did some prototype INS system as my master's thesis 10 years ago, the code was quick and dirty and even then the accuracy was like 30 meters after an hour of walking around with the device.
I did some prototype INS system as my master's thesis 10 years ago, the code was quick and dirty and even then the accuracy was like 30 meters after an hour of walking around with the device.