LightSquared to divulge scope, scale of GPS issue

Wed, 06/15/2011 - 8:05am
Maisie Ramsay, Wireless Week

LightSquared is expected to notify the FCC today that its planned LTE network could crash critical government GPS systems if allowed to go live, and the company is expected to make suggestions on mitigating the interference.

The company was ordered by the FCC in January to test whether its hybrid satellite-terrestrial LTE network would interfere with GPS systems as a condition of its waiver to provide land-based mobile broadband service in its satellite spectrum.

LightSquared has already signed up Best Buy and Leap Wireless International as wholesale customers for its mobile broadband network. Any FCC-mandated delays in the launch of the network would be a major setback for the venture-backed company.

The company's final report, due today, will disclose the results of FCC-mandated tests measuring interference to GPS receivers.

LightSquared's network sits in spectrum directly adjacent to bandwidth used for GPS, making GPS receivers vulnerable to interference from LightSquared's transmitters.

Advance details of the tests have not been provided by LightSquared, but independent tests conducted by government agencies provide mounting evidence that the company's transmitters drown out weaker transmissions from GPS satellites, making it impossible for sensitive GPS receivers to pick up an accurate signal.

The National Space-Based PNT Advisory Board reported last week that LightSquared's network interfered with GPS receivers used by the Federal Aviation Administration (FAA), John Deere, U.S. Coast Guard, NASA and Garmin. Military-grade GPS receivers are significantly more sensitive than those used in consumer devices, making them more vulnerable to interference from LightSquared's LTE network.

The board proposed that LightSquared either deploy its network in bandwidth farther away from GPS spectrum or lower the power of its transmitters, increasing the number of base stations LightSquared would need for its network.

The board said its simulations showed that LightSquared's network, when fully deployed with 40,000 base stations, would "degrade or result in loss of GPS function (ranging, position) at standoff distances ranging from few kilometers and extending to space operations."

"No universal mitigation approach identified," the board said in its report.

LightSquared told the PNT Advisory Board that "mitigation is possible along several different dimensions," including adjusting the frequencies its uses for its network, its transmitting power, geographic coordination and network synchronization.

"LightSquared has not taken any options for mitigation off the table," the company said in its presentation.

GPS systems are also used in industry applications, such as agriculture. Farm equipment maker John Deere has released unofficial tests showing that LightSquared's network could knock out its GPS-based StarFire precision agriculture system. John Deere wants the FCC to guarantee that LightSquared's network poses no threat to GPS systems before allowing the service to become commercially available.

Tests also indicate LightSquared's network hampers GPS in devices with less sensitive receivers, such as smartphones, tablets and in-car navigation systems. General Motors reported Tuesday that its GPS-based OnStar system was knocked out within a quarter-mile radius of LightSquared's base stations.

LightSquared could not be reached for comment on last week's tests. The company previously said it installed special filters on its base stations to limit out-of-band transmissions, but preliminary tests suggest that the filters have not been enough to protect GPS systems.

Some GPS receivers pick up on additional signals from neighboring spectrum bands to get more precise coordinates, making them particularly susceptible to interference from LightSquared's LTE network. John Deere's StarFire system receives "augmentation signals" from satellites in the MSS band, where LightSquared plans to deploy its LTE network.

The FCC said in its January order it will not allow LightSquared's network to go live until the company resolves the GPS interference issue after several federal agencies, including the Defense Department and Homeland Security, expressed concerns that LightSquared's network could knock out their GPS systems.

The lower portion of LightSquared's spectrum stretches from 1525 MHz to 1559 MHz, adjoining spectrum allocated by the FCC for GPS, which runs from 1559 MHz to 1610 MHz.

LightSquared's spectrum was formerly reserved for mobile satellite service with land-based transmitters as an ancillary component. The FCC's controversial waiver allows the company to set up nearly 40,000 base stations sending out a signal in the mobile satellite band.

LightSquared spokesman Jeff Carlisle said in a call with reporters earlier this month that the company's plans to launch its LTE network in early 2012 were still on track, but did not say how the company planned to address the GPS interference issue.

"I don't think there is any plan to change the launch as a result of these test results," Carlisle said.

Some experts have suggested LightSquared outfit GPS receivers with filters that would cancel out interference from its network, but the cost of equipping millions of affected devices with the filters could make such a plan impracticable.

Advocates for the GPS industry are calling for LightSquared to deploy its network in spectrum located further away from GPS bands. Jim Kirkland, general counsel for Trimble Navigation and spokesman for the Coalition to Save Our GPS, says there is no other fix for the problem.

"They can't change the laws of physics here. The answer is they should use other spectrum that doesn't have this problem," Kirkland says.

Dan Hays, director at consulting firm PRTM, says that in the GPS industry's "insatiable thirst for precision," it made poor engineer decisions that made GPS receivers more vulnerable to interference from neighboring bands.

Suggestions that GPS receivers have been poorly designed have been repeatedly refuted by the GPS industry. However, none of the tests that have been conducted into the interference issue over the past few months show LightSquared's transmitters interfere with the GPS signal itself, suggesting the issue lies with receivers, not the transmitters.

The FCC, which does not generally regulate receivers, is unlikely to take the matter of blame into consideration when reviewing LightSquared's report.

"To me, in a lot of ways, this is like an automotive recall issue. Someone put together a design, whether it's well-intentioned or not doesn't really matter," Hays says. "The issue comes down to how you remediate it. That's the question here."


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