A report by the National Academies of Sciences, Engineering, and Medicine on Ligado Networks found that Ligado’s network plans will not cause harmful interference to most GPS receivers, but will cause harmful interference to some Iridium Communications mobile satellite services that are used by the U.S. Department of Defense (DoD).
The National Academies released its report on Friday after more than a year of study. The report was mandated by Congress and sponsored by the DoD. The National Academies are private, nonprofit institutions that provide expert advice. This report was prepared by a committee and then analyzed by a panel of expert reviews. Michael McQuade, strategic adviser to the president at Carnegie Mellon University and chair of the committee, presented the findings in a virtual press conference on Friday.
Ligado is working to deploy a terrestrial wireless network in the L-band satellite spectrum, and received FCC approval for the plan in April 2020 — a decision that has been hotly contested. U.S. Department of Defense officials, aerospace and satellite companies and other stakeholders that use GPS signals have sought multiple times to stay the FCC’s decision.
The committee was tasked with looking at how to best evaluate harmful interference to GPS, the potential for harmful interference, and the effectiveness of mitigation measures proposed in the FCC order. Notably, the committee was not tasked with deciding whether the FCC reached the correct outcome.
The committee found that most commercially produced general navigation, timing, cellular, or certified aviation GPS receivers will not experience significant harmful interference due to Ligado. However, high-precision receivers are the most vulnerable receiver class and more likely to experience interference. The committee said that GPS receiver manufacturers could field new designs that could coexist with the authorized Ligado signals.
It also found that the Globalstar system is unlikely to experience harmful interference, but Iridium terminals will experience harmful interference to mobile satellite service. Interference will occur with Iridium terminals on their downlink “caused by Ligado user terminals operating in the UL1 band while those Iridium terminals are within a significant range of a Ligado emitter — up to 732 meters.”
Iridium has long fought Ligado’s network plans, alleging interference.
The DoD asserted to the committee that Ligado’s network “will create unacceptable harmful interference for DoD missions” and mitigation techniques “are insufficient to protect national security missions.” The DoD uses the MSS band for applications, including communications, navigation, tracking of military assets and synchronization of timing. The DoD is also Iridium’s largest single customer. The committee said it was briefed by the DoD, which provided classified supporting data that the committee is unable to discuss publicly.
The committee also found that procedures to mitigate Ligado interference may not be practical due to test certifications and other efforts required.
“While the mitigation measures generally make sense, there may likely be circumstances where they are not going to be practical at operationally relevant timescales and at reasonable cost, regardless of who bears that cost,” McQuade said.
Overall, the committee called for a more collaborative approach to resolving spectrum issues, including joint study and testing by the FCC and National Telecommunications and Information Administration (NTIA). The FCC and NTIA have recently made a point to increase collaboration on spectrum issues, updating an MoU last month.
McQuade talked about the value of spectrum, the committee’s call for a cohesive policy that addresses the rights of current users, how long those rights should prevail, and equipment lifetimes across various business models.
“Those considerations we believe should be debated and a cohesive policy should be arrived at outside of the pressure of an individual spectrum decision,” McQuade said. “In the committee’s view, the better way to do that is to debate and arrive at policy separate from the proceedings around an individual decision.”
The committee said it has presented its findings to the FCC and its work is now complete. It is unclear if the FCC will revisit the Ligado decision, the commission previously denied a request for a stay last year. Last month, a bipartisan group of eight senators sent a letter to the FCC this week, asking the commission to reconsider the decision.
Both Ligado and those opposing Ligado’s put out statements indicating the study supported their point of view.
Ligado’s response said the interference is focused around old GPS devices. “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. 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,” Ligado said. “Now that the review is completed, 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, including but not limited to GPS. We will continue working with all involved stakeholders to determine a mutually beneficial way forward.”
The DoD put out a statement about the report, saying the report’s conclusions on interference and mitigation procedures 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. DoD looks forward to continuing to work with the National Telecommunications and Information Administration, FCC and Ligado on this complex and important issue.”
Iridium also called Friday for the FCC to take action. “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. Iridium urges the FCC to take swift action to reverse the order before Ligado starts its technical demonstrations this fall,” Iridium said in a statement.
NTIA said in a Friday statement that it is reviewing the National Academies report and “we believe this offers the Commission [FCC] an important opportunity to reconsider Ligado’s authorization.”
