The FAA has limited the Defense Department’s access to the radio frequency band used by Link 16—the military’s primary battlefield communication system used for platforms in the air, on land and at sea.

The ubiquitous battlefield network shares the same radio frequency band with several safety-critical systems managed by the FAA, which requires an elaborate deconfliction process to make sure the military system never interferes with civil aviation signals.

• Issue raised by data link cryptography update
• Agency’s certification infrastructure limited
• Exercises, flight-test plans affected

However, a mandated security update to Link 16 that took effect on Jan. 1 has not been certified by the FAA, which has given rise to concerns that the military’s test and certification process for inserting new software into terminals that use the data link is inadequate.

The FAA’s concerns about interference have already limited the U.S. Air Force to only “a few temporary flight authorizations for test and training” using Link 16, an Air Force spokeswoman says. The restrictions mean that military operators’ planning exercises and weapon system tests now face a shifting patchwork of regulatory approvals for the Link 16 network.

Some Link 16 users have already been cut off by FAA denials, while others, such as the Boeing F/A-18E/F Super Hornet and EA-18G Growler, still enjoy a “mitigation approval” to use the network with certain caveats. But most surviving authorizations are temporary and expire in several weeks, with no certainty of follow-on approvals as discussions between the FAA and Defense Department officials continue.

The FAA says it is continuing to work with the Defense Department to “resolve issues related to Link 16, a frequency used by core elements of the national airspace system,” an agency spokeswoman says. Although the Air Force says frequency authorizations are now limited to a “few,” the FAA statement released to Aviation Week downplays the effect of the problem on regular operations.

“The number of uncertified operations, and the power they use for training and exercises, could be limited at times,” the FAA spokeswoman says.

The ongoing restrictions highlight the Pentagon’s increasing reliance on access to a data link that inhabits a region of the electromagnetic spectrum—specifically, 960-1215 MHz—controlled by the FAA as the primary user for several civil aviation signals, including secondary surveillance radars, distance-measuring equipment and the tactical air navigation system. During peacetime, the FAA allows the military to use the same frequency band for Link 16—as long as the signals never interfere with the civilian agency’s primary users.

Although nearly a half-century old, the Link 16 network’s role in military operations is only increasing in importance.

A Link 16 signal does not come with a low probability of detection, but it features an architecture that makes it resistant to jamming. This time-division-multiple-access approach carves the network up into 120 time slots per second. In each slot, one terminal is allowed to transmit, while the rest can only receive. To make it harder for an adversary to get a fix on a jamming technique, transmissions on Link 16 hop among 51 different frequencies within the 960-1215 MHz frequency band.

Other networks, such as Israel’s BNET, offer more bandwidth and throughput, but Link 16 provides near-ubiquity across air, land and sea domains. The Space Development Agency is experimenting with orbital transmitters for Link 16.

“Training should as much as humanly possible try to reflect the reality you think that combat will look like,” says Chris Daugherty, a senior fellow in the defense program at the Center for a New American Security. Link 16 is now a fundamental element of how large military operations are conducted, so training without such a tactical data link is disruptive.

“It would require a fundamental rethinking of where we put our sensors and how we design weapons if you assume that you wouldn’t have that ability,” Dougherty adds.

That said, although the Link 16 network is central to military operations, it is not exclusive. Lockheed Martin F-22s and F-35s each have unique data links that use stealthy waveforms to exchange data within their own formations. The F/A-18E/F Super Hornet is being upgraded with the wideband Tactical Targeting Network Technology data link to share data with Northrop Grumman E-2Ds and surface vessels.

But all of those aircraft still rely on Link 16 as the primary system for communicating with other aircraft types, including Boeing F-15s and Lockheed F-16s. Link 16 also has been adopted by the air and missile defense systems of the Army and Navy, and as a data link for inflight updates to certain weapons, such as the Raytheon GBU-53/B Stormbreaker guided glide bomb.

The Defense Department’s access to the Link 16 frequency band, however, is contingent upon the FAA’s approval. An authorization is required each time Link 16 is used in national airspace. The temporary frequency authorizations, which specify the location, timing, frequencies and pulse details for each approved usage, are stored in a Link 16 Pulse Deconfliction Server maintained by the Joint Staff. Moreover, each Link 16 terminal is equipped with a fail-safe system, dubbed the “interference protection feature,” that automatically shuts down the transmitter if the data link sends out signals at times or frequencies that could interfere with the FAA’s systems.

This elaborate deconfliction system has succeeded in preventing Link 16 from interfering with the frequency’s primary users for over 20 years, says Steve Horsburgh, director of product development at Curtiss-Wright.

But the system has broken down due to a mandated software change rolled out last year for all Link 16 terminals and what industry officials say is a dramatic increase in safety certification data by the FAA.

The software change has been several years in the making. The military’s reliance on Link 16 has fueled concerns about cyber vulnerabilities. In 2013, the House Armed Services Committee publicly raised concerns that “many” of the Pentagon’s tactical data links “are not currently designed or funded to operate against a robust electromagnetic warfare threat.” In response, updating the cryptography on Link 16 became one of the first priorities for the National Security Agency’s Cryptographic Modernization Program. The Defense Department mandated that all Link 16 users had to transition to the new cryptography standard by Jan. 1.

Because the Link 16’s fail-safe system could be affected, any software change to a terminal must be certified. For years, the FAA has entrusted software certification for Link 16 to the Navy-Marine Corps Spectrum Center (NSMC), a small organization based at Fort Meade, Maryland.

The NSMC process allows the vendors for each of the different Link 16 terminals to perform a self-assessment of the effect of any software change on the fail-safe mechanism, according to two industry officials. A team of NSMC engineers then travels to each vendor to witness a series of two verification tests. If the tests show the software complies with the Link 16 deconfliction features, the NSMC passes the paperwork to the FAA for approval.

Over the last year though, the FAA has pulled back from the arrangement. The reasons for the shift are murky. According to an industry official familiar with the situation, the Air Force sent out a letter to Link 16 stakeholders, which identified two different types of terminals that were delivered to military users with noncompliant software. The finding comes as the FAA is adopting sweeping reforms to its processes in the wake of two Boeing 737 MAX crashes, in 2018 and 2019, that exposed flaws in the agency’s policy of delegating authority for certifying the safety of certain systems to manufacturers.

As certification standards evolve, the Defense Department faces a crunch on certification testing capacity at the NSMC, which two industry officials say is not staffed or equipped to handle the extra workload required by the FAA.

“There’s resource limitations at the Naval Marine Corps Spectrum Center,” Horsburgh says. “They were initially ordered [by the FAA] to recertify every terminal and platform type. And they have one testbed to do that from.”

For the long term, the Defense Department and the FAA already have agreed on a permanent solution to Link 16 deconfliction with civil aviation signals. In 2019, the agencies agreed on a frequency remapping plan for all of the systems in the 960-1215 MHz band. The FAA intends to move its systems to the lower end of the band, with the Pentagon’s Link 16 communications confined to the higher end. The shift will reduce the available frequencies for Link 16 to 37 from 51, but the chances of signal interference will disappear.

The frequency mapping mandate, however, remains three years away. In the meantime, the FAA and the Pentagon must come to agreement on a new certification standard for certifying Link 16 software.