A government agency that conducted tests with proposed wholesale network operator LightSquared's base station said Thursday that it found interference in the higher portion of LightSquared's spectrum bands and minimal GPS interference in the lower portions of of the spectrum. The interference issues exist because LightSquared's spectrum sits adjacent to the frequencies used by GPS receivers, which might have to peek into LightSquared's spectrum. The concern is that LightSquared's high-powered base stations will cause interference.
The National Space-Based PNT Advisory Board, a government entity that advises and coordinates federal departments and agencies on GPS matters, conducted the tests in May. The PNT tests showed that some GPS receivers lost signal strength while others were completely disabled by LightSquared's signal.
Separate tests commissioned by the Federal Aviation Administration, and conducted by RTCA, an organization that writes GPS avionics specifications for the FAA, found that "GPS operations below elevations of 2,000 feet would be unavailable over a large radius of metro" areas for aircraft. The RTCA tests foudn that there was more interference with the the upper portion of the 1525-1559 MHz L-band than the lower portion.
According to Deane Bunce, the co-chair of the National PNT Engineering Forum, there are several mitigation steps that could be taken to stop the interference, including moving LightSquared's service to a different frequency band, limiting the service to the lower chunks of L-band spectrum it owns and installing filters on GPS receivers to reject or limit LightSquared transmissions. However, Bunce said that the filters could taken between seven to eight years to put in place at a significant cost, and that even with the filters, receivers might still face interference.
Jeff Carlisle, LightSquared's executive vice president of regulatory affairs, said that the company is committed to finding a solution. "I think we do need to talk about the possibility of accommodations on our side in order to figure out if there is a way forward," he said. Carlisle added that LightSquared believers there are a variety of solutions to the problem, including adjusting the base station transmitting power and reviewing the frequencies LightSquared is using to launch its network as opposed spectrum it might use to expand its network.
Carlisle said that in 2005, the FCC removed the limit on the number of base stations that could be deployed using MSS spectrum, and made the power limit the one that LightSquared is using. Several members of the Advisory Board said that the wider GPS community and GPS users in general had not been properly informed of the changes by the FCC, which is why the arguments over potential GPS interference only began to percolate late last year. Carlisle conceded that the kind of GPS interference testing being conducted now could have been done six years ago.
Jim Kirkland, vice president and general counsel at Trimble Navigation which has opposed LightSquared's operations as currently designed, said the tests showed that LightSquared's network clearly cause interference with GPS.
"There is not a solution here," he said. "In our view, it's time to stop squandering resources on this and look for alternative spectrum for this operation. What LightSquared is trying to do is a great thing. It's very important that we have more competition in broadband and more spectrum in broadband wireless. However, there is one place in the satellite band where this does not work," and that is the spectrum next to GPS, he said.
- see this WSJ article (sub. req.)
- see this release
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