If aircraft carriers enable the U.S. to project force, keeping those forces over the battle falls to its aerial-refueling tanker fleet. Automated tanking will be critical to the persistent surveillance and deep-strike capabilities envisioned for unmanned combat aircraft.

So it makes sense that, once it has shown it can operate from a carrier, the Northrop Grumman X-47B unmanned combat air system (UCAS) demonstrator should be tasked with proving autonomous aircraft can refuel in flight from the same tankers, and using the same methods, as manned aircraft.

To that end, the program plans in 2014 to demonstrate that the X-47B can refuel autonomously in flight via U.S. Navy probe-and-drogue and Air Force boom-and-receptacle systems, transferring 3,000 lb. of fuel to the UCAS by each method.

The flight demo will build on work under way, led by the Air Force Research Laboratory (AFRL), to develop technologies and operating concepts (conops) for automated aerial refueling (AAR). Key to the effort is proving that precision-GPS relative navigation (relnav) can enable unmanned aircraft to rendezvous with and connect to essentially unmodified aerial tankers.

In the common architecture being developed by the Air Force and Navy, the unmanned receiver and manned tanker exchange position information from onboard global-positioning/inertial-navigation systems (GPS/INS) via a high-integrity data link. The receiver calculates its location relative to the tanker and flies itself into formation, from where it is directed, by its ground control station (GCS) or the tanker itself, to move through the standard refueling positions used by manned aircraft.

Developing a draft conops enabling tankers to refuel both manned and unmanned aircraft is a key goal, says AFRL's Daniel Schreiter, AAR program manager. Producing an architecture to which the government has data rights, so it can maintain a common refueling system across industry primes to ensure integration of the tanker fleet with future unmanned aircraft is another key goal, he says.

The demonstration system developed by AFRL is based on the Northrop Grumman LN-251 GPS/INS and Rockwell Collins TTNT (tactical targeting network technology) data link. “Our goal is to get an AAR variant of the LN-251 [to] a production-standard unit that can be bought off the shelf,” Schreiter says.

Phase 1 of the AAR program included four rounds of flight testing: first to test GPS operation in formation flight; then to test performance of the TTNT data link, followed by closed-loop automated station-keeping: first following the tanker, then allowing the GCS to direct the aircraft between refueling positions.

These flights used a manned surrogate for the unmanned aircraft, a Learjet operated by Calspan and equipped with a single-channel version of the precision-GPS relnav system. The first of two rounds of flights under Phase 2 have been completed, again using the Learjet but this time with a “production-like” multi-channel relnav system.

Phase 2 flights are addressing the safety and reliability of the system. “We fly with two LN-251s on the tanker and three on the receiver,” says Schreiter. “We are not yet doing voting, just data collection. We are looking at voting schemes on the ground, and at the impact of candidate architectures on reliability.”

Naval Air Systems Command and AFRL “have been working together since early 2000, and in December/January the Navy will take the reins,” says Capt. Jaime Engdahl, Navy UCAS program manager. Work-ups for the AAR flight demo will begin in the fall of 2012, initially using that same Boeing F/A-18D equipped with X-47B avionics that is being used as a surrogate in the work-up to the carrier demonstration.

“We will go back through the whole work-up for the [AAR] software, beginning with surrogate testing,” says Carl Johnson, Northrop Grumman vice president and UCAS program manager. The flight demonstration will use the second X-47B, air vehicle 2, which will be equipped for both probe-and-drogue refueling from a 707 tanker and boom refueling from a KC-135. “One aircraft is fully provisioned, but the full-up system is not in place yet,” he says.

“We will take the exact boxes and data link out of the carrier and put them into a rack on the tanker, port the software from the carrier air traffic control center into the tanker operator's station, and do relnav to the tanker with the same hardware,” Engdahl says. The Navy will use the same dual-redundant TTNT data link as AFRL, but Honeywell's H-764G embedded GPS/INS instead of the LN-251.

While the two services are developing a common approach to automated refueling, the Navy version requires an additional step. In the Air Force system, the unmanned aircraft navigates itself to the center of the boom envelope, where the refueling operator on the tanker takes over and steers the boom into contact with the receiver.

In the Navy probe-and-drogue system version, the unmanned aircraft will navigate itself to where the drogue is expected to be, where an onboard sensor will guide the probe into contact with the refueling basket.