The debate over LightSquared's terrestrial use of frequencies originally allocated for satellite communications is still a hot topic. The latest development comes in a letter from the National Space-Based Positioning, Navigation and Timing (PNT) Executive Committee (EXCOM) to the National Telecommunications and Information Agency (NTIA). PNT asserts that LightSquared's system would cause significant interference to GPS. LightSquared responded by saying that EXECOM's studies were biased and invalid.
"It is the unanimous conclusion of the test findings by the National Space-Based PNT EXCOM Agencies that both LightSquared's original and modified plans for its proposed mobile network would cause harmful interference to many GPS receivers," EXECOM wrote. "Additionally, an analysis by the Federal Aviation Administration (FAA) has concluded that the LightSquared proposals are not compatible with several GPS-dependent aircraft safety-of-flight systems."
The communication went on to state that "based upon this testing and analysis, there appears to be no practical solutions or mitigations that would permit the LightSquared broadband service, as proposed, to operate in the next few months or years without significantly interfering with GPS. As a result, no additional testing is warranted at this time."
LightSquared issued a press release that included comments from former FCC chief engineer Edmond Thomas, who questioned EXECOM's test results and conclusion.
"The testing protocol deliberately focused on obsolete and niche market devices that were least able to withstand potential interference," the company claimed. "When LightSquared finally obtained a list of the devices tested, after all testing in this first phase of tests had been completed, it was able to determine that the testing included many discontinued or niche market devices with poor filters or no filters. "
LightSquared stated that units tested "represent less than one percent of the contemporary universe of GPS devices." The company further stated that "the only mass market device alleged to 'fail' during this round of testing performed flawlessly during the Technical Working Group testing, which used best practice protocols agreed to by all parties…"
The company said that this incident caused raised doubts about the "integrity" of the testing procedure used by PNT EXCOM.
"The testing standard does not reflect reality," LightSquared said. "To guarantee favorable results, the PNT EXCOM selected an extremely conservative definition of failure--one dB of interference. Independent experts agree that a one dB threshold can only be detected in laboratory settings and has no impact on GPS positional accuracy or user experience. In fact, GPS devices are designed with the ability to withstand eight dB or more of loss of sensitivity due to man-caused and natural interference."
LightSquared contended that placing the interference threshold at one dB rigged the outcome, ensuring that most receivers wouldn't pass. The company additionally noted that the one dB threshold came from an ITU standard, but cautioned that the standard "explicitly states that it does not apply to general purpose GPS receivers."
The debate over LightSquared interference to GPS is complicated by LightSquared's use of frequencies originally allocated exclusively for satellite use, and by GPS manufacturers making and selling receivers based on the incorrect expectation the FCC would never allow that spectrum to be used for terrestrial base stations.
Tests have shown that that the reported GPS interference problems are not due to LightSquared's devices radiating energy outside their allocated bands into the GPS spectrum, but by GPS receivers--which are designed to work with weak satellite signals--being unable to handle strong signals in adjacent spectrum.
(Traditionally the FCC has not set receiver performance standards. Many analog TV channels were blocked from use early on due to the "taboos" necessary to protect UHF TV receivers from interference on image frequencies that weren't adequately filtered in the TV set's front end. FM broadcasters have also been troubled with complaints of interference to older aircraft receivers operating on spectrum adjacent to the FM band. When the FCC allocated spectrum for the DTV transition, it based the allocations on receiver performance criteria, but did not require receiver manufacturers to meet that criteria except in the case of NTIA-subsidized DTV set-top converter boxes.)
I expect that we will see additional interference issues arise as spectrum demands collide with market pressures to produce the more compact and lower cost consumer devices. While few interference issues are likely to attract the same attention as this one, such interference will be annoying to users of affected consumer items
