June 1, 2011
By Dan Namowitz
A special advisory committee will issue a report that validates the aviation community’s concerns about threats to GPS by a proposed broadband communications network.
The committee, part of the not-for-profit organization RTCA that serves as a federal advisory panel on navigation and air-traffic management policy, issued a summary of its report May 26 stating its conclusion that elements of the cellular network proposal by LightSquared are “incompatible” with aviation because of potential GPS signal interference. It added that modifications could be made “to allow the LightSquared system to co-exist with aviation use of GPS.”
“This confirms our message that the concerns expressed by the aviation community and other industries dependent on GPS are warranted, and are now helping to shape the review process,” said Melissa Rudinger, AOPA senior vice president of government affairs.
The report summary called for more study to determine frequency limits versus power for the network’s base stations, taking into consideration the effects of altitude on signal interference and the “lowest path loss for the low altitude enroute scenario.” The report, which discussed operation at the low and high channel, said the impact of LightSquared’s deployment of upper channel spectrum is expected to be “complete loss of GPS receiver function.”
RTCA Special Committee 159 at the FAA’s request studied the LightSquared plan, which holds a conditional waiver from the Federal Communications Commission’s International Bureau but remains the object of strong opposition from the aviation community, other GPS users, and government stakeholders allied in a Coalition to Save Our GPS. AOPA is an RTCA member and served on the review committee.
Members of Congress also recently weighed in on the side of the GPS community. A May 19 letter signed by 33 senators including 13 members of the Senate General Aviation Caucus called on FCC Chairman Julius Genachowski to rescind LightSquared’s conditional approval and bring the full FCC into the review process.
On May 12 AOPA reported the beginning of testing the effect of LightSquared’s proposed network of 40,000 ground stations on GPS signals. The testing was required under the FCC waiver’s conditions. AOPA urged pilots to report any signal interference encountered in the area.
The RTCA committee’s report focused on compatibility between operating a terrestrial wireless broadband network in the bands 1525 to 1559/1626.5 to 1660.5 MHz and airborne GPS receivers. The executive summary, made available ahead of the June 3 official release of the full document, said that “the report addresses the issues analytically based on existing domestic and international standards and includes results of tests of four certified aircraft GPS receivers.”
“The study concludes that the current LightSquared terrestrial authorization would be incompatible with the current aviation use of GPS, however modifications could be made to allow the LightSquared system to co-exist with aviation use of GPS,” it said.
The summary added that “conclusions and recommendations are strictly based on an assumed set of operational parameters for the LightSquared system and identified source mitigations.”
The report published two detailed recommendations for the FCC’s further review.
1. From an aviation perspective, LightSquared upper channel operation should not be allowed. 2. Further study is recommended to more carefully determine a refined terrestrial base station power versus frequency limit considering: a. determination of the lowest path loss for the low altitude enroute scenario, b. confirmation of acceptable receiver susceptibility for GPS initial acquisition and signal tracking in the presence of the 10 MHz bandwidth terrestrial network interference, c. computation of the cumulative probability distribution function for the aggregate path loss.
Dan Namowitz is an aviation writer and flight instructor. He has been a pilot since 1985 and an instructor since 1990.
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