WASHINGTON — An independent review of Ligado’s planned deployment of a terrestrial wireless network concluded that it will likely interfere with some GPS signals and with space-based communications services provided by Iridium

The review, conducted by the National Academies of Sciences, Engineering, and Medicine (NASEM), was mandated by Congress in the 2021 National Defense Authorization Act. A committee of experts who worked on the review for over a year released its findings Sept. 9 in a 78-page report.

Congress ordered the study after Ligado Networks in 2020 was authorized by the Federal Communications Commission to develop a 5G cellular service by repurposing a portion of L-band radio spectrum adjacent to that used by GPS and Iridium. The FCC’s order led to a major dustup, as the Defense Department, Iridium and several industry groups pushed to overturn the regulatory approval arguing that Ligado’s network will disrupt services that support national security, civil aviation and other sectors. 

The chairman of the NASEM committee Michael McQuade, of Carnegie Mellon University, said the group looked at the technical issues regarding the FCC’s spectrum allocation but was not asked to assess whether the FCC made a good or bad decision. 

“It is complex, there are significant economic, legal, and regulatory issues at play that have been debated and analyzed for a very long time,” McQuade said Sept. 9 on a webcast hosted by the National Academies. 

On the central issue of whether Ligado’s network will disrupt GPS services, the panel said most receivers will not be affected but the “high precision” systems used by DoD likely will be.

“We do conclude that most commercially produced general navigation timing cellular or certified GPS receivers will not experience significant harmful interference from Ligado emissions as authorized by the FCC,” said McQuade. 

“The receivers that are potentially most vulnerable are the high precision receivers,” he said, In tests, the highest proportion of systems that experienced  “significant harmful interference were the high precision receivers.”

McQuade also noted that the interference issue  would be easily solved in newly designed GPS receivers “that could coexist with the authorized Ligado signals and achieve good performance.” But he cautioned that replacing receivers would be unrealistic for DoD that has fielded millions of GPS-enabled pieces of equipment. 

With regard to Iridium terminals, “our conclusion is that the Iridium terminals will experience harmful interference on their downlink caused by Ligado user terminals operating in the Uplink 1 band,” said McQuade. The interference would happen when Iridium terminals are closer than 732 meters to the Ligado emitters. 

DoD, Ligado react to report

In a statement, DoD said the NASEM study “confirms that Ligado’s system will interfere with DoD GPS receivers, which include high-precision GPS receivers. The study also confirms that Iridium satellite communications will experience harmful interference caused by Ligado user terminals.”

These conclusions are “consistent with DoD’s long-standing view that Ligado’s system will interfere with critical GPS receivers and that it is impractical to mitigate the impact of that interference.”

As to what happens next, DoD said it “looks forward to continuing to work with the National Telecommunications and Information Administration, FCC and Ligado on this complex and important issue.”

The Commerce Department’s National Telecommunications and Information Administration (NTIA) is the interagency organization that oversees the government’s spectrum policies.

Ligado said in a statement it “appreciates the review of the FCC’s April 2020 order by the National Academies of Sciences.” The company’s take on the review is that it “confirms what the FCC found over two years ago. Ligado’s licensed and authorized operations can co-exist with GPS.”

Ligado noted that only a “small percentage of very old and poorly designed GPS devices may require upgrading. Ligado, in tandem with the FCC, established a program two years ago to upgrade or replace federal equipment, and we remain ready to help any agency that comes forward with outdated devices. So far, none have.”

Now that the review is completed, Ligado said, “it is our sincere hope the DoD and the NTIA will stop blocking Ligado’s license authority and focus instead on working with Ligado to resolve potential impacts relating to all DoD systems.”

An industry group that has worked to overturn the FCC order, known as the Keep GPS Working Coalition, said the NASEM report validates the group’s belief that the “FCC essentially authorized terrestrial operations in a satellite band without adequately considering the impact Ligado’s proposed operations would have on countless consumers, farmers, ranchers, pilots, boat owners, surveyors, construction companies and others.”

Iridium said in a statement that the study offers proof that the FCC order “failed to fully consider the risk of harmful interference posed to mission-critical satellite systems. Iridium urges the FCC to take swift action to reverse the order before Ligado starts its technical demonstrations this fall.”

The FCC has not yet commented on the report. In response to congressional pushback, the agency in the past has defended its decision to authorize Ligado’s network as part of a national effort to advance 5G connectivity.  The FCC set conditions, such as requiring that Ligado network design ensures that adjacent band operations, including GPS, are protected from harmful interference.

The NASEM report said mitigation measures are helpful but can’t be applied unilaterally. “It has to be the result of extensive dialogue between a presumed affected party and Ligado,” and those conversations presumably did not take place, said McQuade. 

“We simply make the statement that without that dialogue, it is impractical to believe that the mitigation measures can work,” he said. These mitigation measures also would not be practical if they’re not applied “at operationally relevant timescales and at reasonable cost for DoD systems.”

The NASEM panel, however, did not endorse the approaches used by DoD to test interference and should not be used going forward.

McQuade said the committee believes that “some form of cohesive policy” is needed to address the rights of current spectrum users , what those rights are, how long those rights should prevail, and what the impact of equipment lifetimes are likely to be.

These are debates that should be had “outside of the pressure of an individual spectrum decision,” he said. 

The committee’s work has been officially completed, McQuade said. As the government works to resolve the Ligado issue, “the committee believes that whatever process is followed, that process would be much more much more effective if the FCC and the NTIA both engaged in dialogue.”

Sandra Erwin writes about military space programs, policy, technology and the industry that supports this sector. She has covered the military, the Pentagon, Congress and the defense industry for nearly two decades as editor of NDIA’s National Defense...