GPS vendors have not complied with a 2008 US Department of Defence recommendation that called for much better filtering of signals from adjacent spectrum bands, mobile startup LightSquared has told the US Federal Communications Commission.
LightSquared is at loggerheads with makers and users of GPS (Global Positioning System) 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.
However, "if they want to claim that they're entitled to some performance from the GPS constellation, then [the DoD standard is] the standard they need to meet. ... If they go outside those standards, they're taking a risk," Carlisle said.
LightSquared has said for several months that the GPS industry is to blame for the interference, but it did not make any claims about the DoD recommendation earlier, partly because it needed to verify that the highly technical document was still current and was considered authoritative, Carlisle said. "You don't want to say something like this prematurely," he said.
In addition to the DoD recommendation, the International Telecommunication Union, a United Nations agency, has also warned since 2000 that stronger filtering might be necessary to protect GPS from nearby transmissions, LightSquared said.
The Coalition to Save Our GPS, which has led the fight against LightSquared's proposed network, defended the filters used in GPS devices and called the latest filing a sign of "desperation" by LightSquared.
"GPS receivers incorporate filters that reject transmissions in adjacent bands that are hundreds of millions of times more powerful than those of GPS. What LightSquared is proposing, however, is to transmit signals that are at least one billion times more powerful," the group said in a statement.
"There has never been, nor will there ever be, a filter that can block out signals in an immediately adjacent frequency band that are so much more powerful, nor has LightSquared put forward any credible, independent expert opinion or other evidence that this is possible."
The FCC is still accepting comments on LightSquared's proposal, which includes operating both a satellite and an LTE network and selling capacity wholesale to other carriers. The FCC said earlier this week that it would not allow the LTE service to launch unless the interference issue was resolved. LightSquared has said it is confident the plan will be approved next month. Opponents say the carrier should be required to operate the network on other frequencies.