INSIGHTi

DOD Concerns About the FCC-Approved
Ligado Network
May 21, 2020
On April 20, 2020, the Federal Communications Commission (FCC) unanimously approved an
application by Ligado Networks LLC (Ligado) to “deploy a low-power [9.8 decibel watts (dBW)]
terrestrial nationwide network in the 1526-1536 MHz, 1627.5-1637.5 MHz, and 1646.5-1656.5 MHz
bands [of the electromagnetic spectrum] that will primarily support Internet of Things (IoT) services.”
These frequency bands are traditionally used for satellite operations. The Department of Defense (DOD)
opposed this decision—along with the Department of Homeland Security, Department of Transportation
(DOT), Department of Interior, Department of Justice, the Federal Aviation Administration (FAA), and
others. That opposition related to concerns that Ligado’s proposed network could interfere with signals
from satellites to Global Positioning System (GPS) receivers. Congress may consider federal agency
concerns, including DOD concerns related to mission-critical systems and the FCC’s response, as it
conducts oversight of the FCC’s ruling. Congress may also consider broader issues related to fifth
generation (5G) mobile technologies, such as the allocation of spectrum among competing users and the
impact of spectrum decisions on national security.
DOD Concerns and Related Studies on GPS Interference
In both its formal response to the FCC’s ruling and its May 6, 2020, testimony before the Senate Armed
Services Committee, DOD cited two primary studies that shaped its belief that the Ligado network
“would cause unacceptable operational impacts and adversely affect the military potential of GPS”: a
2018 DOT study and a 2016 classified study conducted by the U.S. Air Force (USAF). The 2018 DOT
study assessed the extent to which “a typical cellular base station power level of 29 dBW” would interfere
with GPS. (At the time of the study, Ligado proposed a base station power level of 32 dBW.) The study
concluded that a 29 dBW base station would exceed allowable levels of interference, instead
recommending that ground station transmissions not exceed 9.8 dBW to ensure the protection of certified
avionics in “the most restrictive of the certified aviation scenarios examined.” DOD additionally
recommended “that proposals for use of bands adjacent to GPS should not be approved unless they meet
the transmission power levels described in the [DOT test].” Based on these recommendations, Ligado
submitted an amended application to the FCC, reducing its proposed power levels to 9.8 dBW. Per the
FCC ruling, Ligado also agreed to maintain a 23-MHz guard-band of unused spectrum designed to
separate its transmissions from GPS, thus attempting to mitigate potential interference.
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There are limited details available in the public domain that describe the technical parameters of the 2016
classified USAF study; however, the USAF’s formal response to the FCC ruling notes that its study,
which specifically tested potential interference with military GPS receivers, “supported the conclusions
drawn from the DOT testing ... conducted during the same month.” This suggests that the study may not
provide evidence that a Ligado network—using the FCC-approved specifications from the company’s
2018 amended application—would necessarily interfere with GPS. Nonetheless, DOD has continued to
cite these studies in its objections to the ruling.
Dr. Michael Griffin, Under Secretary of Defense for Research and Engineering (USD R&E), has
additionally asserted that any ground transmissions—regardless of power level—“would drown out the
very weak signals that come from [GPS] satellites,” likening the effect of the proposed Ligado network
on GPS to attempting to listen to the rustling of leaves while 100 jet aircraft simultaneously took off.
Statutory Obligations with Regard to Potential GPS Interference
DOD has noted its statutory obligation, pursuant to 10 U.S.C. §2281, to object “to any restriction on the
GPS System proposed by the head of a department or agency of the United States outside DOD that
would adversely affect the military potential of GPS” [emphasis added]. Although the DOT and the
USAF study do not appear to provide assured evidence that a Ligado network would adversely affect
military GPS, neither definitively ruled out the potential for adverse effects. As a result, Secretary of
Defense Mark Esper concluded, “Consistent with my statutory responsibilities, I believe there are too
many unknowns and the risks are far too great to federal operations to allow Ligado’s proposed system to
proceed.”
DOD has additionally noted that Section 1698 of the National Defense Authorization Act for Fiscal Year
2017 (P.L. 114-328) prevents the FCC from approving commercial terrestrial operations in the bands
proposed by Ligado “until 90 days after the Commission resolves concerns of widespread harmful
interference by such operations to covered GPS devices.” DOD asserts that the FCC did not resolve such
concerns prior to approving Ligado’s application.
FCC Response to Concerns About Potential GPS Interference
Although the FCC has not issued a formal response to DOD, Commissioner Brendan Carr noted in a
statement accompanying the FCC’s decision that “after a thorough and multi-year review, the FCC’s
professional staff of engineers and other experts determined that we can advance America’s 5G leadership
while protecting GPS and other adjacent band services.” Commissioners Jessica Rosenworcel and
Geoffrey Starks termed the decision “an extremely close call,” but similarly noted in their joint statement
of concurrence that, despite the concerns of DOD and others about potential GPS interference, “in the
end, we are compelled to support the expert technical analysis done by the [FCC’s] engineering staff.”
Potential Independent Review of Test Results
Due to the ongoing disagreement about the impact of Ligado’s proposed network on GPS, some
analysts have suggested that relevant tests should be independently reviewed by a “neutral
arbiter,” such as the National Academy of Sciences. According to a Pentagon spokesman, DOD
would “support an impartial third party, one with demonstrated expertise in GPS testing,
conducting a thorough examination of all data collected during the preceding decade of testing.”
Such a review, which would delay execution of the FCC decision, would likely require the
approval of the congressional commerce committees.

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Author Information

Kelley M. Sayler
John R. Hoehn
Analyst in Advanced Technology and Global Security
Analyst in Military Capabilities and Programs





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