The context here is the design of a navigation system that works without GPS. Once you have a functioning GPS, the INS is not needed at that point. The INS ca

The context here is the design of a navigation system that works without GPS. Once you have a functioning GPS, the INS is not needed at that point. The INS can run in standby mode and be continuously calibrated using the GPS positions. If GPS is lost, the INS is used as a backup. But at that point, you no longer have the GPS velocity. So the normal thing to do is to integrate the acceleration data from your accelerometer, which gives you the velocity (+noise, +drift).