Flights of two F-35 test jets have begun here with a software patch to explore the effectiveness of upgrades designed to improve the “fusion” of the threat picture within multi-aircraft F-35 formations. 

Fusion is a hallmark selling point for the stealthy, single-engine F-35. Prime contractor Lockheed Martin and officials from the Pentagon’s F-35 Joint Program Office (JPO) say data collected through the aircraft’s electro-optical targeting system, radio-frequency electronic warfare system and advanced electronically scanned array radar will be blended for the pilot in a single display by onboard software in an unprecedented manner. 

The result is expected to be a reduced pilot workload and improved awareness of the battlespace, including threats as well as objects that should not be targeted. 

Additionally, the aircraft are designed to work in a network, sharing the same threat picture among pilots in ship formations through use of the new Multifunction Advanced Datalink (MADL). This link is intended to transmit data covertly to allow for operations in protected airspace; Link 16, the current data-sharing standard, by contrast, broadcasts and does not function stealthily. The result should be to allow multiship formations to distribute the workload more quickly and effectively during missions and to more wisely use the limited munitions onboard the aircraft to attack targets. Though often referenced, F-35 data fusion has been rarely described in detail.

Data fusion would seem to be simple, as the use of mobile telephones with fused data applications is widespread. However, for the F-35, the data must be extremely precise—the system cannot confuse a school bus for a mobile missile erector, for example. The threat data relies on thousands of mission files that cue the F-35’s software, indicating what objects are (a surface-to-air missile site, enemy radar, enemy vehicle, etc.).

Doing this on a single jet is a challenge in and of itself, but creating a single, fused picture based on data collected by four jets in different locations with varying views of the battlespace is another challenge altogether. This is the latest of many unprecedented tasks being tackled by the F-35 team. 

Late last year, U.S. Air Force Lt. Gen. Christopher Bogdan, the F-35 program executive officer, said the latest software challenge was to create an accurate “fused” picture across multiship formations.

“Ideally with fusion working at the 100% level, each F-35 has its picture from its own sensors and shares that picture over the data links—all the wingmen in that link also have their own-ship sensor picture as well as the shared info,” says one program official. “Fusion should correlate what wingmen are seeing to what each own-ship sees so that there is one and only one symbol on each airplane’s [display] for each target out there.”

That is the ideal. Test pilots have reported problems when targets on the display have more than one symbol—a sign the system has not “fused” the inputs on that particular target. Or in some cases, not all wingmen are seeing everything the other pilots in their formation are viewing on the displays.

This latest patch—called the “engineering test build” or ETB by testers—is not a full software release. It is akin to an update one might implement on a mobile telephone and is geared specifically to address multiship fusion shortcomings. It was decided among the test team and JPO to move forward with it in parallel with work to certify the 2B software with which the U.S. Marine Corps plans to declare F-35B initial operational capability (IOC) in July. 

“We think before we release this to the Marines, there need to be some improvements,” says Air Force Col. Roderick Cregier, the F-35 program director at Edwards.

The Marine Corps has said it will proceed with the IOC this summer with or without these ETB improvements. But JPO officials hope to have them on those early jets before the milestone, spokesman Joe Dellavedova says. 

The ETB altered about 900 lines of code specifically to ensure that each pilot in a multiship formation sees the exact same threat picture as his wingmen, the testers say. 

Originally, the program called for fielding the jet with its more-robust 3F software. With 3F, pilots can use external weapons stores, the infrared search-and-track function of the Electro-Optical Targeting System and a wider variety of weapons. However, obstacles earlier in development prompted the Pentagon to prioritize the B version, which was optimized for Marine Corps’ short takeoff and vertical landing—and allow for IOC with this baseline software. 

“Block 2 was never intended to be fielded software. So the fact that we can get to a point where we have these discussions of how usable this aircraft is in an operational environment right now is a success,” says Lt. Col. Andrew Allen, director of the integrated F-35 test force at Edwards. “If the Marines are going to use it right now . . . in our expert opinion [we need to know] what needs to be addressed. . . . We want to do better than this. We think we can. We still have time to do it, so let’s go do extra stuff.”

The team created the software patch in 22 days from when the decision was made to proceed, which was in February. Test sorties are planned through the end of March. 

The patch was loaded onto two test aircraft, which flew with it for the first time on March 12. The teams also plan to load the patch onto a third and possibly a fourth F-35. The requirement to determine mission effectiveness is for three aircraft to have the modification, but a fourth could add more data to the final determination, Allen says. 

“Fusion development is an extremely iterative process. It almost crippled the F-22 program. Now you are taking multiple, federated sensors and trying to integrate them,” he says. Allen also participated in the F-22 program. He notes that the F-35 program is “light years ahead” of where the F-22 was in this stage, which is when the twin-engine stealthy jet was experiencing embarrassing avionics shutdowns during flight. This prompted pilots to have to reboot systems multiple times during a sortie.

“What we have done in the program up to this point has been full software releases. There have been no quick [patches]. . . . What we are getting for the first time ever is a little bit more agile. It is a patch [and] if it is good, we will roll it into the next software release,” Cregier says. 

As they continue testing, engineers are assessing just how much of which type of data should be transmitted among the aircraft to create the threat picture. “Inviting other aircraft into your fusion system . . . that is an extremely difficult thing [and] you have to find that sweet spot of how much data do you let in” to the network, Allen explains. 

If needed, the schedule could allow for an additional ETB patch or two, he adds. 

Meanwhile, the team is also conducting early testing on Block 3F software. Thus far, they have begun conducting asymmetric load tests on AF-1. External weapons carriage is a feature added with Block 3F, with which the Navy will conduct IOC by February 2019; it is unavailable for Blocks 2B and 3i for the Marine Corps and Air Force IOCs.

The conversion to 3F from 3i takes about a week, according to David -Nelson, a Lockheed Martin test pilot.

Thus the test fleet is working with the 2B, 3i and 3F software packages at varying levels. This is possible, in part, because of the infusion of five mission systems aircraft—some pulled or on loan from other locations and missions. These were added when the program was restructured in 2011 to mitigate risk in the software work delaying fielding. Originally, the team planned for a single mission systems jet.

Earlier in the program, officials assumed the bulk of mission system testing would take place in ground-based laboratories or in the Combined Avionics Testbed, a flying Boeing 737 modified with an F-35 radar, sensors and leading edges, Cregier says.

Thus far, the F-35 is about 60% through development, which is slated to finish in 2017.

Digital-Extra Meet the F-35 flight-test fleet at Edwards AFB: AviationWeek.com/F35Edwards