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LightSquared says GPS makers ignored filtering rules

11.08.2011
GPS vendors have not complied with a 2008 Department of Defense recommendation that called for much better filtering of signals from adjacent spectrum bands, mobile startup LightSquared told the U.S. Federal Communications Commission on Thursday.

LightSquared over interference between the navigation system and its planned cellular LTE (Long-Term Evolution) network. That network would transmit on frequencies close to those used for GPS. The company has long argued that makers of GPS equipment are to blame for the interference because they don't use strong enough filters to keep their receivers from searching for signals in LightSquared's bands. But this is the first time LightSquared has accused the vendors of flouting a specific rule.

The DoD's GPS Standard Positioning Service Performance Standard called for GPS receivers to filter out transmissions on frequencies adjacent to the GPS band, LightSquared told the FCC in a filing related to the agency's ongoing consideration of the company's network proposal. The standard, issued in September 2008, recommends that receivers reject all transmissions on frequencies that are more than 4MHz outside the GPS band, said Jeffrey Carlisle, LightSquared's executive vice president for regulatory affairs and public policy. That 4MHz buffer is essentially a "guard band" to protect operations on either side, he said.

LightSquared plans eventually to use frequencies adjacent to the GPS band for its LTE network, but after mandatory tests earlier this year showed strong interference in that area, the company said it would start out in a slightly lower frequency block.

"Had the GPS industry complied with the DoD's recommended filtering standards for GPS receivers, there would be no issue with LightSquared's operations in the lower portion of its downlink band," Carlisle wrote in the filing. With its latest plan, LightSquared is proposing a guard band of 23MHz, but the GPS industry wants to keep the new carrier out of the lower frequencies also, effectively asking for an even wider 34MHz guard band, he said.

There is no mandatory standard for filtering in GPS receivers, and the FCC does not certify the devices for this, Carlisle acknowledged in an interview. The DoD document is a recommendation of how manufacturers should build their receivers to achieve maximum performance, he said. The DoD is responsible for first developing and building the GPS system.