Stolen from a platoon of soldiers near the outer perimeter around the military base at the centre of the Zone.
When activated it repels the perimeter anomalies and allows safe passage without the disorientation caused by the anomalies.
Activation also sets off a security system with a GPS beacon attached, which prompted an artillery attack the last time it was used.

The backpack comprises four key components.

1. Transmitter. Sends out an anomalous signal in all directions to a range of about 10 metres. This signal is presumably what repels the perimeter anomalies.
2. Shielding. Sends out a signal in all directions to a range of about 50 centimetres. This seems to shield the device from anomalous interference from outside.
3. GPS beacon. Transmits the location of the beacon whenever it is powered up. The GPS also sends out an alert if it cannot detect a specific RFID tag within range, presumably a military ID tag of some kind.
4. Battery and CPU. Runs everything else, seems to operate on a fairly standard OS.