Salute!
I go with FullWings on the accuracy. Sorry, Gauges, but I flew with INS systems 40 years ago that had plenty of mechanical components that needed bearing lube and constand calibrations and such. Think real gyros and tiny motors to keep the basic inertial platform "level" so the accelerometers could provide good movement rates for the nav computer. Our drift rate without GPS or other "real world" update data averaged less than 2 n.m. per hour. Figure less than 4 feet per second, and we could see this drift on our second bomb passes.
So 12 n.m per hour seems very high
I then flew a later jet (ten years later) that had less mechanical components and more sophisticated computer sfwe. It was even better than the orginal I learned on. No GPS updating or even a doppler to help keep the platform level. Drift rate was about 1 n.m. per hour with no GPS help.
The new stuff has an order of magnitude less moving parts and an order of magnitude more sophisticated sfwe and navigation algorithms than I grew up with.
So my bottom line is I am surprised a company is bragging about 12 n.m. per hour nav accuracy..
Gums sends...