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Experts: GPS at risk with LightSquared 4G

A government/industry panel will warn the Federal Aviation Administration (FAA) in a 3 June report that the expected interference from a new 4G ancillary terrestrial broadband network will cause "complete loss" of GPS receiver functionality.

RTCA special committee 159 (SC-159), which includes representatives from 4G provider LightSquared, Airbus, Boeing, navigation system providers CMC, Honeywell and Rockwell Collins, as well as the FAA and Transport Canada, composed the report following several months of analytical and actual test measures. The work is part of a broader six-month technical investigation called for by the US Federal Communications Commission (FCC) in its conditional approval of the 4G network in January.

LightSquared's approval will allow the company to re-transmit L-band satellite signals (1525 - 1559MHz) from a network of 40,000 ground transmitters to provide wholesale mobile and broadband services across the continental US. Interference can occur in GPS receivers that operate in the 1559-1610MHz band due to several factors, including the high relative power of the LightSquared ground transmitters compared to the GPS signal, and FAA-approved design parameters of the GPS receivers themselves.

LightSquared is planning a three-phase rollout of its service, initially using a 5MHz slot (1550-1555MHz, called Phase 0), followed by the addition of a lower frequency 5MHz slot (1526-1531MHz, Phase 1) and lastly, the implementation of two larger bands (1526-1536MHz and 1545-1555MHz, called Phase 2).

Along with analytical calculations, the RTCA group tested four brand name airborne GPS receivers in the vicinity of LightSquared ground transmitters.

Results were both encouraging and discouraging.

All four GPS units were able to handle significantly more interference than the minimum levels set out by the FAA, and could function normally with only the lower portion of Phase 1 deployment (1526-1531MHz) operating.

Performance worsens as the frequencies go higher: Officials say GPS receivers would have a "small margin" for tracking with LightSquared's 4G in the 1526-1536MHz range only (the lower portion of the Phase 2 signal) but that locking in the GPS signals initially could be a problem. With the frequencies above 1536MHz active however, the group will report that they expect "complete loss" of GPS functions. "From an aviation perspective, operations at Phase 0, 1 and 2 spectral deployments, the upper channel should not be permitted," the report will read.

"Further study is recommended to make sure we have found the worst case operational scenario; to confirm we are properly modelling receiver susceptibility, and to make sure path loss is done appropriately," the group concludes. While the report is written as a consensus position, views of members who did not concur with certain points are included.

The FCC has set 15 June as the final date for LightSquared and industry to issue a final report on the potential impacts for operating the 4G network, including technical and operational steps to avoid the interference and possible mitigation strategies.

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