New government test results show that a proposed high-speed wireless broadband network could jam GPS systems used for everything from aviation to public safety and military operations. The results released Friday by a federal working group come amid mounting concern that LightSquared\'s planned network could cripple GPS systems embedded throughout the nation\'s infrastructure, some of which are critical to national defense. \"LightSquared\'s network could cause devastating interference to all different kinds of GPS receivers,\" said Jim Kirkland, vice president and general counsel of Trimble Navigation Ltd., which makes GPS systems. In January the Federal Communications Commission gave LightSquared approval to build a nationwide fourth-generation wireless network that would compete with super-fast systems being rolled out by AT&T and Verizon. The new network will wholesale access to other companies that will rebrand the service under their own names. It would cover at least 92 percent of the U.S. by 2015. The FCC sees the LightSquared network as one part of a broad government push to bring high-speed Internet connections to all Americans. The company\'s plans have set off alarm bells among GPS equipment makers and the many government agencies and companies that rely on GPS systems, because LightSquared\'s network would use airwaves right next to those already set aside for GPS. The concern is that sensitive satellite receivers — designed to pick up relatively weak signals coming from space — could be overwhelmed when LightSquared starts sending high-powered signals from as many as 40,000 transmitters on the ground. Test results were compiled by a working group of the National Executive Committee for Space-Based Positioning, Navigation, and Timing — a government organization that advises and coordinates among federal agencies that rely on GPS technology. It found that wireless signals from LightSquared\'s planned network interfered with GPS receivers used by the Coast Guard and NASA. They caused Federal Aviation Administration GPS receivers to stop functioning altogether. The tests — most of which were conducted by various federal agencies at Holloman Air Force Base and White Sands Missile Range in New Mexico in April — also caused GPS receivers used by New Mexico state police and by fire and ambulance crews to lose reception. In addition, GPS receivers built into farm equipment made by John Deere lost signals, as did most General Motors\' OnStar navigation systems. Last week RTCA, a nonprofit group that advises the FAA, released the results of its own interference tests and found that LightSquared\'s use of airwaves closest to the GPS spectrum would cause a \"complete loss of GPS receiver function\" over large metropolitan areas. Faced with these concerns, the FCC has made clear that LightSquared cannot launch its network until the interference issues are resolved. It is requiring the company to participate in a technical working group with GPS manufacturers and users to study the matter. That group conducted GPS interference tests using LightSquared equipment in Las Vegas last month. The FCC is due to receive the results next week. The agency will then seek public comments before on whether to let LightSquared turn on its network. In a statement Friday, the FCC said it \"will not allow LightSquared\'s commercial service to proceed if that would cause widespread harmful interference with GPS.\" Despite the test results that have come out so far, the agency added that the interference questions are far from settled. \"Some of the tests to date may have relied on different assumptions, metrics, and mitigation assumptions, and so may not accurately reflect the potential for interference as a result of how the network may be operated,\" the FCC said. LightSquared executive vice president Jeffrey Carlisle said he remains confident that the company\'s new network and GPS systems can co-exist. After all, he noted, findings of interference do not come as a surprise. What matters, he said, is what can be done about the interference. Among the solutions outlined by the government working group: modifying LightSquared\'s antenna patterns and reducing the power levels of its base stations; limiting the slices of airwaves that LightSquared can use or moving the company to a different part of the spectrum; and installing better filters on GPS receivers to screen out LightSquared\'s signals.