By Graham Warwick
Initial testing of GPS receivers confirms aircraft navigation systems will experience significant jamming from thousands of broadband-wireless transmitters planned to be deployed across the U.S.

The tests were conducted to determine the susceptibility of GPS receivers to interference from the high-power terrestrial transmitters to be deployed by LightSquared, a company developing a 4G wireless network. All GPS receivers tested by the National Position, Navigation and Timing Engineering Forum (NPEF) were effected by the high-power transmissions, the FAA’s Deane Bunce, NPEF co-chair, told a meeting in Washington June 9.

The testing indicates LightSquared’s plans to deploy a terrestrial network of 40,000 base stations will result in the degradation or loss of GPS function at distances from the transmitters ranging from a few kilometers to 300 km for space-based receivers, he said.

Simulations conducted by aviation standards developer RTCA for the FAA concluded “GPS is likely to be unavailable over the whole U.S. East Coast,” based on LightSquared’s deployment plans, Robert Frazier, of the FAA’s spectrum planning and international office, told the meeting.

Potential mitigations identified by the NPEF include relocating LightSquared’s transmissions to a different frequency band and retrofitting GPS receivers with filters. But redesiging receivers and re-equipping aircraft “will take a minimum of 7-8 years, and possibly up 15 years, and would not necessarily buy back the performance lost because of the filter,” Bunce said.

“There is now overwhelming evidence of interference, and no evidence of a mitigation solution in the results,” Jim Kirkland, general counsel for GPS manufacturer Trimble Navigation, told the meeting. “It’s time to stop this and look for alternative spectrum.”

Jeff Carlisle, executive vice president of regulatory affairs and public policy for LightSquared, says, “We believe we can deploy in a way that we can coexist with GPS. We always knew there would be interference. The hard question is how to mitigate it.” Potential mitigations being proposed by LightSquared include using only the lower of its two frequency bands, the furthest away from GPS, to launch its service; reducing transmitter power; adjusting base station locations; and modifying the timne scale (geological time scale) for full use of its frequency allocation.

A working group led by LightSquared and the U.S. GPS industry Council, representing manufacturers, has conducted receiver testing mandated by the Federal Communications Commission. The results, including mitigation recommendations, will be submitted June 15.


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