First seen in Chuck vs. The Broken Heart
The GLG-4000 is a small, exceptionally powerful tracking device designed specifically for use in the mountainous, cave-riddled border regions of Afghanistan. Despite its powerful transmitter, it is still small enough to be implanted inside a human body. The device was used in "Chuck Versus the Broken Heart," where it is also noted it would take approximately six feet of solid steel to dampen the signal.