Representatives of the US Federal Aviation Administration (FAA), Department of Defense and US aerospace companies aim to determine the potential threat to GPS navigation posed by a new broadband service.
Telecoms provider LightSquared received conditional approval from the Federal Communications Commission (FCC) in January to proceed with the service, set to start this year.
LightSquared required a waiver to operate the system as a ground-based, rather than satellite-only, service.
It will funnel L-band satellite signals into ground stations then rebroadcast the signal to an estimated 92% of the US population via high-power transmitters on 40,000 towers.
However, the possible impact on GPS is alarming the industry.
"Indeed, because of the ambitious [FAA NextGen] programme to shift air traffic management to a satellite-based navigation and communications system, GPS will become even more important over the next decade," wrote the Air Transport Association (ATA) in a letter to the FCC in January.
"For these reasons, we are extremely concerned about spectrum issues and the possibility of inadvertent interference," the letter adds.
Along with the National Telecommunications and Information Administration (NTIA), Motorola, Cessna, Hawker Beechcraft and others, ATA publicly asked the FCC to perform a thorough analysis of the potential for GPS interference before approving the waiver.
The FAA and Department of Defense had informally petitioned the agency to slow down but ultimately failed to sway the FCC when it approved LightSquared's plan less than three months after submittal.
However, it said LightSquared must work with the industry to determine the impact and develop mitigations before commercial operations can begin, and deliver a final report to the FCC by 15 June. LightSquared has pledged up to $20 million for the effort.
Issues do not appear to involve out-of-band spillover interference from antenna sites (operating at 1525-1559MHz) into the GPS band (1559-1610MHz) as much as GPS units becoming overloaded by legal LightSquared communications traffic in the historically-tranquil neighbouring band.
A study prepared last month by Garmin - which has GPS units installed in more than 43,000 aircraft, 70% in the USA - found units such as its GNS430W would detect interference or jamming more than 22km (12nm) from an antenna site and loss of positional fix at 5nm.
Garmin says its results were conservative and these distances could increase.
However Jeff Carlisle, LightSquared's executive vice-president for regulatory affairs, says Garmin did not account for special filtering incorporated into its transmitters to prevent them from "pushing emissions into the GPS band".
Carlisle says the filters, which LightSquared spent $9 million developing, result in out-of-band emission limits that are tighter than FCC requirements.
LightSquared has taken the lead in organising the industry working group.
"The first thing we need to do is make sure we're talking to the GPS industry, to make sure their assumptions are the same as ours," says Carlisle.
He added that LightSquared had made some "preliminary outreach" to the GPS community, including NTIA, and "more significant" efforts were underway.
"The thing to understand is this is a solvable problem and testing should take place to figure this out before we assume everything will be impacted," he says.
"We'll figure out which receivers are susceptible, then we can figure out mitigation. The issue really is receivers that may, regardless of what we're doing, be susceptible to our spectrum," he adds.
Carlisle says some manufacturers' receivers may be susceptible, but others will not.
"When you have that subset, the question is what you do from a technical standpoint," he says, adding that some solutions could be as simple as antenna filters costing less than $0.50 each.
"You need a wide variety of units that you can test in the lab and field environment to determine whether or not they're being interfered with," says Carlisle. "That is why you need to proceed on a co-operative basis."
Source: Flight International
