Aviation Raises Red Flags Over Ligado Proposed Lower Power Limits
Some aviation interests and allies remain largely unswayed Ligado's proposed power limits on use of the 1526-1536 MHz band (see 1805310069) for a terrestrial broadband service will effectively protect GPS receivers. That 9.8 dBW power level "is a first step" toward certified aviation GPS device protection, but helicopter interference issues remain unaddressed, Garmin said in a docket 11-109 filing posted Tuesday. No, Ligado said, "The substantial analysis and testing in the docket" makes clear its proposed ancillary terrestrial component (ATC) operations wouldn't cause harmful interference.
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Those lower levels, while consistent with FAA recommendations, still represent "significant operational problems for the aviation community," Aviation Spectrum Resources said. It criticized the Ligado proposal of notifying the FCC and FAA in advance of base station work that changes antenna specifications, saying operators wouldn't be notified and they would have to keep searching FAA and FCC public notices. Garmin sought restrictions on tower spacing and some antenna parameters.
Ligado's power limits aimed at precision aviation GPS worries don't resolve Iridium interference concerns regarding Ligado operations in the 1627.5-1637.5 GHz band or weather interest concerns about Ligado operations in the 1675-1680 MHz band, the satellite operator said. Boeing said the FCC needs to ensure small aircraft GPS receivers are protected, that Ligado reaches agreement with the aviation industry about modifications that will have to be done to Inmarsat aviation transceivers and that there can be coexistence between Ligado's operations and Iridium transceivers.
Ligado has supporters. Public Knowledge and X-Lab said that given the years since Ligado began "the torturous process" of trying to address interference protection demands of adjacent services, the FCC "owes [it] either a grant of its request or a denial that it can appeal to court." They said by now the burden is on objecting parties to show the proposed modified services would cause harmful interference. The Information Technology and Innovation Foundation said that given the modifications Ligado made to band plan and power levels, "this chapter in the L-band saga should be closed" with the requested waiver granted.
"The dramatic reduction" in transmission power Ligado is proposing in its license modification request means no GPS devices should see harmful interference, Roberson and Associates said. It said filtered antennas could take care of remaining concerns about high precision GPS devices.
Air medical services provider Metro Aviation said it's "fully confident" Ligado's ATC deployment under the modification terms won't interfere with safe helicopter operation. Ligado said the amendment ensures its proposed ATC will protect even the helicopter use case of certified aviation GPS receivers, which the Transportation Department has said would be the most restrictive of such use cases.
Deere, Garmin and Trimble argued for the value of the 1 dB noise floor as an interference protection yardstick. Trimble said the 1 dB standard has long been used in various spectrum contexts and Ligado "provides no basis for abandoning this longstanding U.S. and international standard of harmful interference." Deere said Ligado-suggested key performance indicators don't fully account for the key attributes of GPS -- accuracy, integrity, continuity and availability -- or the diversity of GPS receivers and uses.