The U.S. Air Force is buying five RQ-4 Global Hawk UAVs equipped for SIGINT (signals intelligence) work. Until now, all RQ-4s have carried high resolution cameras as their primary sensors. SIGINT work uses just antennas of various types, to collect whatever electronic transmissions there are in the vicinity. The inspiration for this RQ-4 variant came from an effort to sell the RQ-4 to Germany, equipped mainly for SIGINT. Demonstrations for the Germans, using an RQ-4 equipped for SIGINT, were quite successful. Like photo reconnaissance, SIGINT can benefit from persistence (staying in the area for a long time.) Moreover, its getting more difficult to get qualified people who will fly in manned SIGINT aircraft for hours at a time, especially if the flying is being done in some out-of-the-way places. With the RQ-4, the human crew can stay on the ground, usually back in the United States. This reach-back type work is proving much more popular with the troops.