Ligado, FAA Nearing Specific Downlink Power Limits for LTE Network
Ligado put a potential number, 15 dBW, on what kind of equivalent isotropically radiated power (EIRP) power limits it might end up using for its "left-hand" downlink spectrum to protect certified aviation GPS devices from potential interference from its planned terrestrial LTE network, the company said in reply comments posted Tuesday in docket 11-109. That limit is still being hammered out with the Federal Aviation Administration, with the company previously having committed to operate at whatever levels the FAA says are required to safeguard aviation, Ligado counsel Gerry Waldron of Covington and Burling told us Tuesday. Monday was the deadline for oppositions to its modification applications, with replies due June 16.
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Ligado said the FCC's authorization for terrestrial operations in the 1526-1536 MHz band would nominally allow a 32 dBW power limit, but the 15 dBW level proposal is "substantially lower to address the hardest use case of helicopters." It also said FAA rules require fixed wing aircraft to stay at least 500 feet from obstructions such as radio towers, but helicopters generally have no distance requirements. The company also said if it were somehow to resolve the helicopter use case, it "could consult anew with the FAA about the possibility of implementing a higher power limit.”
Aviation concerns have become a hurdle for Ligado's TLPS plans. Aviation, GPS and satellite representatives continue to raise concerns about interference from Ligado's planned terrestrial LTE network (see 1605240021). "We have agreed to comply with and fully support current and future FAA standards," company CTO Tamara Casey, herself a licensed pilot, said in a blog post Monday, noting the company has been talking with the FAA since December about power limits and ensuring compliance. "Aviation operators are key customers for our network, so we consider ourselves to be part of the aviation community," Casey said, saying the company is "highly motivated to develop and solve any and all potential interference problems.”
In its filing, Ligado included a report on its uplink emissions' compatibility with certified aviation GPS receivers. The company said the report "establishes that operations in the uplink bands on the right-hand side of the GPS signal allocation present no problems to certified or noncertified aviation devices at the power and OOBE levels" laid out in in its modification application. "We think the record is very clear and well grounded to conclude the right-hand spectrum doesn't have any issues," Waldron said.
Even "the old and less-stringent limitations" first proposed in 2013 for terrestrial uplinks wouldn't affect existing or emerging Global Navigation Satellite System (GNSS) operations, Ligado said, pointing to a 2015 analysis. That testing looked at the effects on overhead aircraft from Ligado devices on the ground, on taxiing aircraft from passengers using Ligado and other scenarios. "In each use case, even using worst-case assumption ... emissions from Ligado's proposed handheld devices would not affect aviation GPS operations," it said.
The GPS/GNSS camp still has concerns. In a series of filings, a variety of firms said they backed NovAtel's analysis that Ligado hasn't sufficiently looked into coexistence between LTE and GPS/GNSS systems or its argument for keeping a 1 dB rise in carrier-to-noise ratio as the standard for tolerance of interference (see 1605200064) -- a standard Ligado has criticized. "The suggestion that ongoing L1 GPS users might be required to modify their existing systems with band-pass filters or other devices -- possibly putting the onus for retrofits onto the existing user segment -- is problematic," Orion Monitoring said in a filing Tuesday in docket 12-340.
Ligado said Tuesday it's “actively engaged in ongoing conversations with parties including NovAtel to understand and address their concerns, and we are committed to finding solutions just as we’ve found with other manufacturers.”