Neither of two commonly used approaches to reduce significant interference with GPS receivers would work in the case of Ligado Networks‘ proposed 5G terrestrial wireless network in the L-band satellite spectrum, according to a new study from the National Acadmies of Science, Engineering and Medicine (NASEM).

The NASEM report may add to the pressure on the Federal Communications Communication (FCC) to postpone the implementation of Ligado’s operations in the 1526-1536 MHz band–a start to happen on or after Sept. 30.

The two often used interference mitigation strategies–a signal-to-noise (SNR) interference protection criterion and another based on a device-by-device measurement of the GPS position error–do not provide “analytical, repeatable, or straightforward criteria to evaluate new entrants,” NASEM said.

“Both approaches have a role to play in evaluating harmful interference to existing receivers,” per the study. “The
signal-to-noise approach is inflexible as a determinant or threshold, providing what in some circumstances may be an overly conservative emission limit because no single value for signal-to-noise degradation determines when the various types of possible harm to receiver performance will become significant. The position measurement approach is dependent on the test sampling approach and is too narrow in its applicability to the many and varied uses of the GPS system.”

In April, 2020, under FCC Order 20-48, Ligado received FCC approval to deploy its network, and soon after the Department of the Air Force said it would try to take mitigation steps, in case the network was deployed (Defense Daily, Apr. 22, 2020). Pentagon, aerospace and satellite company officials have sought to stay the FCC decision, but the FCC denied those requests in January last year

“Ultimately, both proposed [GPS interference migitation] approaches are cumbersome, owing to the intensive, device-by-device testing required,” NASEM said in its new study, mandated by section 1663 of the fiscal 2021 National Defense Authorization Act.

The mitigation approaches “do not provide an engineerable, predictable standard that new entrants can readily use to evaluate impact,” NASEM said. “As such, these approaches impede progress in making more efficient and effective use of the spectrum. A new applicant for emissions in a new adjacent channel will have great difficulty in determining the emitter power levels and stand-off distances that will be guaranteed not to cause Harmful Interference to the installed base of GPS receivers. A GPS receiver designer will be unable to design a receiver that will be guaranteed to tolerate unknown potential future allowed levels of adjacent-band power.”

Last month, a bipartisan group of eight senators, led by Senate Armed Services Committee Chairman Jack Reed (D-R.I.) and Ranking Member Jim Inhofe (R-Okla.) asked the FCC to reconsider its April 2020 decision (Defense Daily, Aug. 19). Inhofe has said that Ligado should pay the costs needed to mitigate all potential GPS interference, not just that associated with government systems.

The new NASEM study “confirms that Ligado’s system will interfere with DoD GPS receivers, which include high-precision GPS receivers,” DoD said on Sept. 9.

In addition, the report says that Iridium [IRDM] satellite communications “will experience harmful interference caused by Ligado user terminals,” the Pentagon said. “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 study also concludes that the Federal Communication Commission’s proposed mitigation and replacement measures are impractical, cost prohibitive, and possibly ineffective. These conclusions are consistent with DoD’s longstanding view that Ligado’s system will interfere with critical GPS receivers and that it is impractical to mitigate the impact of that interference.”