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GPS Industry Worries About Lightsquared Interference

2/21/2011 12:01:00 AM Eastern
NEW YORK — The GPS industry and Lightsquared, which plans a satellite- and terrestrial-based 4G voice and data network, will conduct field tests to determine whether Lightsquared’s terrestrial cell sites will, as Garmin believes, interfere with the operation of GPS-equipped consumer products and commercial devices.

Based on lab tests using one of its portable navigation devices (PNDs) and one of its aircraft GPS units, Garmin found that its PND would completely lose GPS satellite signals within 0.66 miles of a cell site when the PND is under an open sky. In urban areas where some GPS satellites’ signals are more likely to be blocked, the PND would lose its position within 1.79 miles of Lightsquared cell sites, which would operate in the 1525MHz to 1559MHz band next the 1559MHz to 1610MHz GPS band.

Lightsquared plans to build 40,000 cell sites, most of which will be in urban areas where most consumer GPS devices are used, a Garmin spokesman warned. The terrestrial network will cover 92 percent of the U.S. population in 2015 if all goes according to plan.

The effects of base-station jamming on Garmin’s avionic device were greater, with the device losing its location within 5.6 miles of a cell site while in the open sky.

Although Garmin’s PND test involved only one PND model, Garmin said the device was representative of most PNDs and that the PND test likely replicates the interference that other types of consumer GPS devices would encounter. Those devices would include GPS-equipped cellphones and in-dash navigation systems.

For its part, Lightsquared said it is working cooperatively with the GPS Industry Council to develop test methodology that will use real base stations and multiple GPS devices in real-world situations. But Lightsquared’s Jeff Carlisle, regulatory affairs executive VP, noted that the base stations were designed with filters endorsed by the GPS Industry Council about eight years ago to prevent out-of-band emissions from entering the GPS band. The filters were developed by Lightsquared’s predecessor company, Mobile Satellite Ventures, which proposed a hybrid satellite-terrestrial network to the Federal Communications Commission (FCC), and the filters abide by “stricter out-of-band emission restrictions that those required by the FCC,” Carlisle noted.

Carlisle also contended that Garmin’s lab tests simulated a filter that “allowed significantly more interference to get through” than the filters that will be used in the base stations.

As a result, Carlisle said he suspects “a significant number of GPS receivers are adequately filtered” and won’t suffer interference issues. However, “some subset of receivers are looking into the L band,” he said. Field testing will determine how many GPS receivers are susceptible and will help Lightsquared “know exactly how our transmitters are operating,” he said.

Once the tests determine the types, models and number of devices affected, the next step would be to determine how the affected devices are used and whether users would come close enough to a base station to be affected, Carlisle said. The tests might be able to isolate the susceptible products to a specific GPS chipset, he noted.

The FCC has asked for a final report by June 15.

For his part, the Garmin spokesman said his company’s lab tests used conservative parameters and noted that the tests simulated full-strength reception of the maximum number of 12 GPS satellites that a navigation device could see at one time.

Although future versions of susceptible navigation devices could be built with additional filtering, Garmin said a filter would not be a “cheap easy fix” and that it’s unclear how existing devices in use could be filtered. The FCC, he noted, wasn’t clear on who would be responsible to fix any problems. Although Garmin’s test involved only one PND model, Garmin said the device was representative of most PNDs and that the test likely replicates the interference that other consumer GPS devices would encounter. Those devices would include GPS-equipped cellphones and in-dash navigation systems.
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