Ligado's TLPS Plans Seen as 'Existential Threat' to GPS

Parkinson said any repurposing of mobile satellite service bandwidth for wideband terrestrial high-power use would be inherently problematic. "They crank up the power, they destroy a whole lot of things," he said.

Ligado responded that its proposal "has been publicly discussed for nearly two years; there are thousands of pages containing all of the relevant information in the record. It is therefore stunning that the PNTAB meeting opened with remarks that indicated that the board does not know and has not made the effort to understand the details of the very proposal they were trying to address. It reflects a refusal to consider basic facts and runs counter to the scientific method.”

High-performance full-band receivers are most vulnerable to adjacent band interference, and testing has shown the Ligado proposal would jam those, said Parkinson, Stanford University professor of aeronautics and astronautics emeritus, Wednesday, according to provided recordings of the PNTAB meeting. He said, given the potential number of broadband transmitters operating at power levels 5 billion times the received power GPS provides, "This is a tough law of physics problem." He also said he wasn't clear on the full details of Ligado's proposal.

PNTAB and Ligado are miles apart on even how to test for the potential of high-power terrestrial transmitters interfering with GPS-adjacent bands, PNTAB said. Parkinson said PNTAB developed criteria for testing that include accepting a 1 dB rise in carrier-to-noise ratio as the standard for tolerance of interference; verification interference for all classes of GPS receivers; testing of all those receivers in all operating modes; and focusing analyses on worst cases, including maximum authorized transmitted interference power: "We are interested in worst cases, not best cases or nominal cases." He also said interference to the emerging Global Navigation Satellite System (GNSS) signals must be considered. Parkinson said none of Ligado's recent testing met any of those criteria: "That gives us great pain." Ligado opposed use of the 1 dB benchmark (see 1605240021).

Numerous potential, high-precision GNSS use cases -- such as precision control of construction vehicles, precision survey in construction of high rises, and delivery and reconnaissance drones -- must be explored in depth before decisions are made on reallocating adjacent-band spectrum, Parkinson said.

Iridium, which raised concerns about interference from Ligado's broadband terrestrial network (see 1612140061), also brought its critiques to the PNTAB meeting. In a briefing submitted to PNTAB, it said the proposed Ligado network -- with planned uplinks -- "will potentially deploy millions of mobile devices" that would use the 1627.5-1637.5 MHz band for uplinks, 1 MHz away from 1617.775-1626.5 MHz, for which Iridium is licensed. Iridium said Ligado's out-of-band emissions "result in unwanted emissions ... and cause interference" to Iridium's services.

Ligado and the FAA have been in talks about power limits to protect certified aviation GPS (see 1702270037), but Karen Van Dyke, Transportation Department director-positioning, navigation, and timing and spectrum management, said the FAA and the FCC aren't having any discussions of their own about such power levels. She also recapped 2016 DOT testing of 80 GPS receivers across six categories, with the agency looking at 100 MHz on either side of the radionavigation satellite service band. She said high-precision and space-based receivers were found to be particularly sensitive to adjacent band power. She also said the next step is DOT completion of its GPS adjacent band compatibility assessment final report, and then public review and comment on that report. She said the report could be done and submitted in August.