Why are inertial navigation systems periodically corrected using GPS or another independent system?

Inertial navigation systems (INS) are sophisticated dead-reckoning devices that determine a vehicle's position and velocity by integrating acceleration data over time. Because these systems rely on continuous mathematical integration, they are inherently subject to cumulative errors.123

Even minute inaccuracies in the measuring capabilities of accelerometers or the balance of gyroscopes can lead to significant departures from a planned trajectory over time. To maintain precision, these systems must be periodically reinitialized or corrected using an independent navigation source, such as the Global Positioning System (GPS) or celestial references like star trackers. By providing an external "fix" on the vehicle's actual location, these independent systems allow the onboard computer to reset the INS and eliminate the drift that accumulates during operation.13