NOAA Sees Existing Interference as Harbinger of Ligado TLPS Problems
Despite Ligado assurances its terrestrial low-power service plans, with proper protection zones, won't cause interference to National Oceanic Atmospheric Administration downlinks sharing the same spectrum, NOAA said interference problems it has from Ligado operations (see 1608120033) demonstrate otherwise. "Nothing Ligado has done or shown ... has changed our view," said NOAA Chief Information Officer Zach Goldstein told us. On whether sharing is possible at all in the 1675-1680 MHz band, he said, “We don't know. We don't have a technical solution as we stand here today -- that doesn't mean we can't develop one.”
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Wallops Island, Virginia, involves only Ligado test signals with a few test towers, but the company's plan for 1675-1680 MHz "is a growing commercial concern" with different power levels and far more transmitters, Goldstein said. Added David Franc, NOAA deputy director-radio frequency, "Changing five megahertz upward isn't going to change propagation characteristics. It is more transmitters, same propagation conditions."
The infrequent interference to NOAA satellite operations at Wallops Island from Ligado operations in 1670-1675 MHz isn't an indicator those same problems will be inevitable in 1675-1680 MHz, company Chief Legal Officer Valerie Green said Monday. Ligado's current test network at 1670-1675 MHz -- using a small number of tall, high-powered towers -- has some important differences from the company's TLPS plans at 1675-1680 MHz, which would be based on lower power and smaller towers, she said. "Those two things alone … will lead to less likelihood of interference."
The company also urged that any commercial licensee of 1675-1680 MHz be required to use protection zones and coordination to safeguard NOAA operations, Green said. She said Ligado and NOAA do collaborate at Wallops to prevent interference and mitigate it when it occurs. And not all interference there is due to Ligado, she said, pointing to other test networks in that spectrum and instances when NOAA alerted the company about interference and subsequently said it wasn't due to the satellite firm after all.
NOAA drafted a proposal seeking funding through the federal Spectrum Relocation Fund to study sharing 1675-1680 MHz. "We are simply asking for the resources" to an engineering analysis on the problem, Goldstein said: "It may very well be tractable, maybe a different shape to an exclusion zone. All we want to do is get the engineering facts."
The Wallops Island interference problems seem to involve ducting -- an atmospheric inversion with high humidity -- Franc said. The island interference is only occasional, Goldstein said, but "you can't predict when it's going to happen [and] when it does, by the time we discover it we have already had imagery impaired or blacked out by interference. If we are tracking a hurricane when it happens, we have got a problem."
Many weather and emergency management enterprise users of NOAA imagery and data get them straight from satellite feeds, Goldstein said. "You can't set up protection zones for them; they're everywhere." Ligado pushed the idea of a web- or cloud-based content delivery network (see 1607280022), but that alternative CDN "isn't a new idea," Goldstein said. "We do it today," with imagery from NOAA's Geostationary Operational Environmental Satellite (GOES) system available online. NOAA users often invest in antennas "to get the information right off the bird" for numerous reasons, including avoiding any possibility of an Internet delay or outage, Goldstein said. "The Internet has issues, the Internet also isn't accessible everywhere," he said, saying some mechanisms for Internet distribution are subject to the same threats with which emergency management personnel deal.
Goldstein said Ligado's alternate CDN "might be a piece in the puzzle." The agency needs better understanding of the latency and survivability requirements of emergency management and weather enterprises supported by NOAA data, he said, "understanding that we may be able to come up with solutions that deal with this direct satellite feed.”
Some FCC commenters raised concerns in docket 13-213 about NOAA’s GOES data distribution for such programs as the Physical Oceanographic Real Time System and its Natural Water Level Observation Network. Ligado said the data downlinked to NOAA receive stations over the agency’s current data collection system also is distributed via other means such as NOAA’s domestic communication satellite service at 12 GHz and the U.S. Geological Survey’s Emergency Data Distribution Network. Ligado said in comments in August its TLPS plans “may affect one method through which entities receive information for these NOAA programs (i.e., direct reception from NOAA’s GOES satellites) [but] several other mechanisms exist through which the same information can be accessed.” That data also could be distributed through the proposed alternative CDN, Ligado said.
Coordination and protection zones would safeguard third parties that get NOAA data via satellite because those users generally aren't mobile, Green said. Firefighters battling wildfires “are not going around with satellite dishes that get a direct feed," she said, saying concerns about weather and emergency management users of that GOES data and imagery are getting mistakenly lumped in with concerns about GOES downloads spectrum.