NAS Report Warns of Ligado 5G Network Interference to Iridium, Possibly DOD

“When we say there will be harmful interference into Iridium, that’s not a statement on the margin,” Carnegie Mellon University’s Michael McQuade, chairman of the NAS committee that wrote the report, told reporters. “It’s a pretty significant possibility of interference,” he said. McQuade said the committee wouldn’t comment directly on the FCC’s decision authorizing the network or whether the agency had all the data it needed from the DOD.

“We received the report and are reviewing it closely,” an FCC spokesperson said: “We appreciate the efforts of the panel on the tasks presented to them and will analyze the report’s findings carefully.”

“Most commercially produced general navigation, timing, cellular, or certified aviation GPS receivers will not experience significant harmful interference from Ligado emissions as authorized by the FCC,” the report said: “High-precision receivers are the most vulnerable receiver class, with the largest proportion of units tested that will experience significant harmful interference from Ligado operations as authorized by the FCC.” The report warned “Iridium terminals will experience harmful interference on their downlink caused by Ligado user terminals … while those Iridium terminals are within a significant range of a Ligado emitter -- up to 732 meters.”

The study “confirms that Ligado's system will interfere with DOD GPS receivers, which include high-precision GPS receivers,” DOD said in a news release: “The study also confirms that Iridium satellite communications will experience harmful interference caused by Ligado user terminals. Further, the study notes that when DoD’s testing approach, which is based on signal-to-noise ratio, is correctly applied, it is the more comprehensive and informative approach to assessing interference.” The report also said the FCC’s proposed mitigation and replacement measures are “impractical, cost prohibitive, and possibly ineffective,” DOD said.

Ligado said the report makes clear NTIA and DOD should allow the network to launch. “The NAS found what the nation’s experts at the FCC already determined: A small percentage of very old and poorly designed GPS devices may require upgrading,” the company said: “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two years ago to upgrade or replace federal equipment, and we remain ready to help any agency that comes forward with outdated devices.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.” NTIA and DOD should “stop blocking Ligado’s license authority and focus instead on working with Ligado to resolve potential impacts relating to all DOD systems, including but not limited to GPS,” Ligado said.

NAS' findings “are consistent with the opposition from 14 federal agencies, more than 80 stakeholders, and Iridium’s concerns that Ligado’s proposed operations will cause harmful interference,” an Iridium spokesperson emailed. “The NAS study clearly demonstrates what the rest of the industry has known for years: the prior FCC order failed to fully consider the risk of harmful interference posed to mission-critical satellite systems,” the spokesperson said: “Iridium urges the FCC to take swift action to reverse the order before Ligado starts its technical demonstrations this Fall.”

“These findings align with the concerns across the vast federal and commercial user base of GPS, satellite communications, and weather forecasting services,” the Satellite Safety Alliance said in a statement. “Ligado’s harmful interference will disrupt day-to-day operations and cost billions of dollars to the consumers of these mission-critical services,” the group said: “The FCC must stay or reverse the Ligado order to address the imminent -- but preventable -- harm from the company’s proposed terrestrial network that it intends to deploy a test network this fall.”

The report is “pretty smashing great for Iridium,” said a lawyer who has been active in the proceeding: The report “validates everything they’ve been saying for years. This is unequivocal.”

Receiver Standards

NAS underscored the importance of receiver standards, the topic of a recent FCC notice of inquiry (see 2208050044). “It is within the state-of-the-practice of current technology to build a receiver that is robust to Ligado signals for any GPS application, and all GPS receiver manufacturers could field new designs that could coexist with the authorized Ligado signals and achieve good performance even if their existing designs cannot,” the report said.

“We do believe that thinking about receivers, either in best practices and/or receiver standards, is an important element going forward,” McQuade said. The FCC’s receiver NOI was “a good and positive move,” he said. McQuade noted that receiver issues are complicated. “A good receiver design can mean a lot of things -- it doesn’t mean just what the filter path looks like or what the filter cutoff frequencies look like,” he said. Good receiver designs could offer “different or other protection mechanisms,” he said.

“It’s clear when you look at a 60 dB difference in the ability of receivers to be insensitive to the Ligado signals, there’s a lot of different designs out there for the filtering and on front ends,” said Mark Psiaki, Virginia Tech professor of aerospace and ocean engineering, who also served on the committee. “Some of them might have been state of the art a long time ago, and as filters get better, the state-of-the-art is going to move,” he said: “Don’t think about it as a static thing. Something that was reasonable in 1985, or even in 2000, may be obsolete now or very soon.”

“The NAS Report confirms that the FCC should adopt ‘more definitive receiver performance standards,’” emailed consultant Preston Padden: “NAS did not enforce halfway measures -- they called for full-on Receiver Standards. Nothing less will restore integrity to spectrum allocations.”