Two American RQ-4 Global Hawk UAVs are being equipped with in-flight refueling equipment. One RQ-4 will be equipped to receive fuel in the air, the other will be equipped to deliver it (via a hose). The actual "wet" (using fuel) aerial refueling tests won't take place until next year, and refueling from manned aircraft will be involved as well. This year, there will be "dry" tests (doing everything except actually deliver fuel).
The objective here is to extend the maximum sortie duration from 30 to 160 (or more) hours. In addition, having RQ-4s be able to refuel each other is necessary if the UAVs are operating in a very hostile environment, including missions where the RQ-4 is flying higher than a manned tanker can reach.