GPS Interference

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Interference Confirmed, LightSquared Offers Concession to GPS Industry

Industry Rebuffs LightSquared's Proposed Remedy

December 15, 2011

It's official. The federal government has confirmed previously leaked reports that tests conducted by a joint Technical Working Group show that signals from a proposed nationwide wireless broadband network interfered with the majority of commercial GPS receivers tested.

LightSquared has been quick to respond with an olive branch of sorts to the Federal Communications Commission (FCC) and the GPS industry, but NBAA and other industry stakeholders said the offer does not resolve concerns about interference.

A joint statement issued December 13 by the Department of Transportation and the Department of Defense confirms that “testing did show that LightSquared signals caused harmful interference to the majority of other tested general purpose GPS receivers.” A separate battery of tests, conducted by the Federal Aviation Administration (FAA), has shown the signals also interfered with terrain avoidance and warning systems (TAWS), which rely on GPS signals for reference to an aircraft's proximity to the ground.

In response, LightSquared executive vice-president Jeffrey Carlisle sent a December 12 letter to FCC Secretary Marlene H. Dortch, offering to cede authority over use of the upper part of its allotted spectrum to the National Executive Committee for Space-Based Positioning, Navigation and Timing, a government organization of military and commercial telecommunications officials. The move would effectively shut LightSquared out from utilizing that bandwidth, which is immediately adjacent to the spectrum used by GPS receivers.

In exchange for the concession, the company asked for immediate access to the lower 10MHz of the spectrum initially granted to LightSquared by the FCC in December 2010. LightSquared claims signals utilizing this bandwidth will not interfere with most GPS devices, provided they are fitted with special filters. “LightSquared recognizes as legitimate the concern on the part of some federal agencies that, for budgetary and planning purposes, they cannot know that GPS fixes they deploy today will be sufficient unless they understand how the spectrum may be used in the future,” Carlisle wrote in his letter.

That proposal was met with wariness from the Coalition to Save our GPS. “LightSquared’s proposal to make use of the upper 10 MHz subject to the approval of National PNT Executive Committee, the government body primarily responsible for protecting the integrity and operational effectiveness of the GPS constellation and critical GPS uses, is a constructive step,” the group stated. “Unfortunately, LightSquared ties its agreement to do that to being given the green light to proceed full steam ahead in the lower 10 MHz, which is very premature.”

In a statement, the National Business Aviation Association – a founding member of the Coalition – noted LightSquared's proposal “doesn't alleviate our overall concern that its signals will still interfere with GPS signals, potentially jeopardizing the safety of countless pilots and others who rely on GPS for navigation. NBAA and its Members are not opposed to new satellite technologies, as long as it is conclusively proven that they will not interfere with GPS systems currently in use, and the newest plan offered by LightSquared does not alleviate that concern.”

LightSquared CEO Sanjiv Ahuja announced December 7 that testing on three companies’ existing GPS receivers showed those devices, after being retrofitted with filters, were not disrupted by LightSquared's immensely strong signal bleeding onto bandwidth used by the weaker GPS signals. The next day, Javad GNSS President Dr. Javad Ashjaee provided details about his company's proposed solution to GPS interference problem: retrofitting a series of filters to existing receivers, at a quoted cost between $300 and $15,000.

As NBAA has reported, documents surfaced December 9 that LightSquared's signal interfered with 69 out of 92 GPS receivers tested by the TWG. The full report will now be reviewed over the next several weeks by representatives with the National PNT Executive Committee, with that group's report scheduled for submission to the FCC in the first quarter of 2012.