DOT Study Points to Ligado Dangers to GPS Receivers

BALTIMORE -- Transportation Department test results on adjacent-band compatibility to GPS reinforces the belief of a threat that Ligado's plans for a broadband terrestrial low-power service (TPLS) pose to high-precision GPS receivers, Bradford Parkinson, vice-chairman of the Space-Based Positioning, Navigation and Timing (PNT) National Advisory Board, told us Wednesday during the board's meeting. "I want [Ligado] to succeed, but not at the expense of PNT." He said concerns remain that the FCC could move on approval of the company's applications. The FCC didn't comment.

The proposed transmitter power is tolerable to certified aviation receivers but could cause interference with or degradation to most other categories of GPS or Global Navigation Satellite System (GNSS) receivers, said DOT's technical report last month. It focused on transmitters in bands adjacent to the 1559-1610 MHz radionavigation satellite service band used for GPS Link 1 (L1) signals and looked at equipment susceptibility testing involving 80 GPS or GNSS receivers. Ligado wants to introduce TLPS service in the 1526-1536 MHz band.

GPS received signal power is such a miniscule fraction of a watt that "therein lies the clash," Parkinson said. He has been critical of the Ligado plans (see 1801170028). Part of the problem is that GNSS involves more than just L1C/A signals, and the upper end of Ligado's proposed TLPS sits in the midst of a variety of other signals like the Galileo high-precision service, he said. Interference tests have come up with conflicting claims, but DOT's is the most credible adjacent band compatibility testing since it covered all PNT Advisory Board criteria, he said: It used a 1 dB noise floor as a metric, included all classes of receivers and all modes of operation, focused on stressed conditions, addressed impact on emerging GNSS and included GNSS experts and the public.

The results show most GPS receivers except mobile phones are under threat, Parkinson said. Even the 20 watt transmitters Ligado discussed with the board could pose problems for high-performance GPS, he said. That approach would likely require denser spacing of transmitters, but Ligado hasn't provided details and multiple towers would contribute additive noise, he said. He showed modeling that said 36 towers in Washington would completely block reception for some receivers downtown and result in a minimum 1 dB degradation across a much broader area of the metropolitan area.

Separate from general aviation, a certified avionics analysis found Ligado-proposed handset levels shouldn't affect certified avionics receivers, said Karen Van Dyke, DOT PNT Program director. The study results will be incorporated in the 2019 update to the Federal Radionavigation Plan underway now, said Van Dyke. That plan will help guide DOT views on issues like GPS adjacent band compatibility, she said.

Ligado in a statement said that since DOT "finally publicly released the complete data set which has been available to decision-makers for several months, we look forward to continuing to work with the NTIA -- the government agency tasked with oversight of federal spectrum use -- and the FCC regarding our plan to more efficiently utilize our L-band spectrum. While this study used an unreliable and inaccurate metric to assess the impact on GPS devices, the good news is that the complete body of research, including the testing done by the Departments of Defense and Commerce ... assessed what spectrum regulators care about: harmful interference. That testing and our agreements with GPS manufacturers confirm that we can have both a robust, protected GPS and all the benefits of efficient use of this spectrum. Advancing our proposal will not only result in a more resilient GPS system, but it will also unlock critical mid-band spectrum to support innovation in our digital infrastructure, create jobs and help the U.S. catch up in the race to deploy 5G. We believe both agencies have the information needed to consider and advance our proposal.”

The first GPS III platform satellite is set for launch in October, with the second to follow in April and the third in July 2019, said Air Force Lt. Col. Andy Menschner, GPS Directorate materiel lead. He said by the end of 2020, the GPS III technical capabilities should be in place and ready for use.

GPS signal jamming and spoofing is an increasing problem, as is data spoofing, said Stormy Martin, Space-Based PNT National Coordination Office director. Part of the problem is GPS receivers lack cyber resilience, he said, saying the Department of Homeland Security has been pushing best practices to increase GPS equipment resilience. GPS spectrum "is no longer a safe haven," with jamming being seen both by state actors and non-state actors, he said. Martin said the FCC has a no-jamming effort underway, citing a $34.9 million fine levied by the agency in 2016 against a Chinese jammer maker (see 1605250071).