GPS satellites transmit their navigation signals in the range 1559 to 1610 megahertz. Telecoms firm LightSquared of Reston, Virginia, has long communicated with its satellites using low-power signals in the adjacent frequency band, from 1525 to 1559 MHz, part of the "L band". Despite the closeness of the frequencies, satnav receivers have so far operated without any interference problems.
But in January, the US Federal Communications Commission (FCC) gave preliminary approval to a plan by LightSquared to build 40,000 new 4G base stations on the ground. These stations would broadcast much stronger signals in the 1525 to 1559 MHz range, to link to cellphones.
Based on lab simulations of the new transmissions, Scott Burgett and Bronson Hokuf, engineers with satnav manufacturer Garmin International in Olathe, Kansas, say this will seriously damage GPS reception. In a report to the FCC last month, they say that overlaps between the two systems are inevitable, and that this "will result in widespread, severe GPS jamming [and] will deny GPS service over vast areas of the United States".
Jeff Carlisle of LightSquared says it is the GPS receivers, not his company's base stations, that are at fault. "The issue is that some GPS receivers may be able to see into the L band where we operate," he told New Scientist.
The stakes are high. By 2015, LightSquared expects to spend $6 to $8 billion to complete the network, which promises to bring download speeds of 5 to 10 megabits per second to cellphone users. Meanwhile, over a billion GPS receivers are in use worldwide.
LightSquared has until 25 February to submit a plan to the FCC for working with the GPS industry and federal agencies to analyse interference issues; a final report detailing a solution is due by 15 June. LightSquared wants all future tests to be performed with real transmitters rather than simulators.
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