"The INS is initially provided with its position and velocity from another source (a human operator, a GPS satellite receiver, etc.), and thereafter computes its own updated position and velocity by integrating information received from the motion sensors. The advantage of an INS is that it requires no external references in order to determine its position, orientation, or velocity once it has been initialized."
While it is true that INS's exist, the cost of a reliable and accurate one is on the same order of magnitude as the CNC Machine itself. Also, errors accumulate [0] over time in an INS (aka "Integration Drift"), such that it becomes wildly inaccurate after a certain critical threshold. Also, almost all INS's require regular calibration and tuning, [1] and are sensitive to vibrational stresses (which is abundantly present in a machine shop). Therefore it seems unlikely that these machines include INS's.
Again, it is true that drift exists, but you're mischaracterizing, from [0]:
"these errors accumulate roughly proportionally to the time since the initial position was input. Therefore the position must be periodically corrected by input from some other type of navigation system."
[0]http://en.wikipedia.org/wiki/Inertial_navigation_system#Erro...
One good earthquake will cause some real problems...
I find it interesting that the GPS is even receiving signals inside of a machine shop. One would think that the interference from the machine itself, not to mention the typically metal building, would attenuate the already weak signals pretty badly.
You misunderstand, I think. The gyro detects movement, and the machine becomes locked until the manufacturer decides to unlock it for you. When deciding whether to re-enable the machine, the manufacturer can verify its new location using whatever means they want.
A gyroscope doesn't do much besides detect if it was moved it can't tell where it was moved so you're basically relying on spoof-able input.