[adrian_b]

There are low-drift-rate inertial solutions, but they have high cost and big size (i.e. laser gyroscopes and accelerometers, with atomic clocks).

So I assume that the research effort is directed towards reducing the cost and size.

[themafia]

The problem with that is if you turn off power to the aircraft then the IMU no longer maintains state and you will need to realign it on startup. The procedures for this are broadly incompatible with military missions.