GPS is facing a major interference threat from a 5G network approved by the Federal Communications Commission, US military officials told Congress in a hearing on Wednesday.
In testimony to the Senate Committee on Armed Services, Department of Defense Chief Information Officer Dana Deasy disputed the FCC’s claims that conditions imposed on the Ligado network will protect GPS from interference.
When the FCC approved Ligado’s plan last month, the agency required a 23MHz guard band to provide a buffer between the Ligado cellular network and GPS. Deasy argued that this guard band won’t prevent interference with GPS signals:
The Order includes the 23MHz “guard band” to protect GPS L1 receivers from Ligado’s terrestrial-based network. GPS receivers are designed to receive signals from space and would be overpowered by this terrestrial network regardless of this protection. Despite this guard band, many varieties of GPS receivers would still suffer interference. To be clear, Ligado’s proposal is to field a terrestrial-based network. GPS has a satellite-based space segment that transmits radio signals to users. This means that GPS L1 receivers are designed to tolerate interference from space systems in adjacent spectrum, but not to tolerate interference from terrestrial systems in the adjacent band.
Results from tests by federal agencies show that “conditions in this FCC order will not prevent impacts to millions of GPS receivers across the United States, with massive complaints expected to come,” Deasy said.
The FCC unanimously approved Ligado’s application, but the decision is facing congressional scrutiny. “I do not think it is a good idea to place at risk the GPS signals that enable our national and economic security for the benefit of one company and its investors,” Sen. James Inhofe (R-Okla.) said at the hearing, according to CNBC. “This is about much more than risking our military readiness and capabilities. Interfering with GPS will hurt the entire American economy.”
FCC calls opposition “baseless fear-mongering”
A spokesperson for FCC Chairman Ajit Pai called the military’s concerns “baseless fear-mongering” in a statement quoted by Multichannel News.
“The FCC made a unanimous, bipartisan decision based on sound engineering principles,” the spokesperson said. The FCC said “the metric used by the Department of Defense to measure harmful interference does not, in fact, measure harmful interference,” and that “testing on which they are relying took place at dramatically higher power levels than the FCC approved.”
The FCC also said that “the Department of Defense (and every executive branch agency that is part of the Interdepartment Radio Advisory Committee) was given our draft decision last autumn, so the assertion that they were blindsided by it this April is preposterous.”
“Ligado said Wednesday in a statement that it has gone to great lengths to prevent interference and will provide ‘a 24/7 monitoring capability, a hotline, a stop buzzer or kill switch’ and will ‘repair or replace at Ligado’s cost any government device shown to be susceptible to harmful interference,'” CNBC reported.
In addition to the guard band, the FCC imposed a power limit of 9.8dBW on Ligado’s downlink operations, with Pai saying “that represents a greater than 99 percent reduction from what Ligado proposed in its 2015 application.”
Deasy, though, said that “even these substantial reductions fail to meet the power levels that can be tolerated in bands adjacent to GPS L1 signals that were studied by the DoT [Department of Transportation].” GPS L1 operates at 1575MHz, while Ligado’s frequencies include the 1526-1536MHz, 1627.5-1637.5MHz, and 1646.5-1656.5MHz bands.
Deasy also criticized the coordination plan, which the FCC said “requires Ligado to protect adjacent band incumbents by reporting its base station locations and technical operating parameters to potentially affected government and industry stakeholders prior to commencing operations, continuously monitoring the transmit power of its base station sites, and complying with procedures and actions for responding to credible reports of interference, including rapid shutdown of operations where warranted.”
According to Deasy’s Senate testimony, this coordination system won’t be enough to protect many military and consumer GPS devices. He said:
Coordination and notification requirements normally work well with spectrum sharing and DoD often champions such measures. However, there are millions of GPS receivers in use by federal agencies, industry, and general consumers that are mobile. Given the massive scale, there is no way to protect those mobile operations. This challenge is compounded by the fact that most GPS users would never know if Ligado disrupted their equipment in the first place or who to call about a problem.
Deasy also said that “the FCC Order expects Ligado to protect US government GPS receivers and to repair or replace affected receivers identified before Ligado terrestrial operations commence.”
“But this overlooks the classified nature of military GPS use and the sheer number of government receivers and military platforms affected,” Deasy continued. “The FCC expectation is unreasonable and could never be employed in real practice. To avert significant mission impacts, the government would need to undertake unprecedented accelerated testing, modification and integration of new GPS receivers on existing platforms. This is cost- and schedule-prohibitive and would significantly degrade national security.”
FCC vote “must be reversed”
Ultimately, Deasy says, the FCC should have rejected the Ligado application because “no practical solution or mitigation is available that would permit Ligado to operate without high likelihood of widespread interference.” Deasy told senators that “the FCC’s Ligado decision is flawed and must be reversed.” Congress has the power to reverse agency decisions, as happened in 2017 when lawmakers killed the FCC’s broadband-privacy rules.
Senators also heard opposition to Ligado’s plan from Michael Griffin, undersecretary of defense for Research and Engineering, and Thad Allen, a retired US Coast Guard admiral. “Signals from GPS satellites, as from all satellites, are extremely weak, and earthbound GPS receivers must be highly sensitive in order to use them,” Griffin said. “For this reason, they are assigned to portions of the radio spectrum—frequency bands—reserved exclusively for their use. Any nearby transmitter operating in or close to the frequency bands that have been set aside for GPS would overwhelm their signals.”
The hearing did not include FCC officials. “There are two sides to this,” and it is difficult to “reverse a decision based upon the presence of one side,” Sen. Tim Kaine (D-Va.) said at the hearing, according to CNBC.