Tests Show LightSquared Interference with 75 Percent of GPS Receivers
December 14, 2011U.S. government tests conducted in November showed that 75 percent of GPS receivers examined were interfered with at a distance of 100 meters from a LightSquared base station. The report states that "No additional testing is required to confirm harmful interference exists," and "Immediate use of satellite service spectrum for terrestrial service not viable because of system engineering and integration challenges."
The tests showing interference by the LightSquared Lower 10 terrestrial signal with an overwhelming majority of general-purpose GPS receivers. Data from LightSquared handsets was collected, and analysis is underway, but no results were presented on December 14.
The presentation of findings was made to the National Executive Committee for Space-Based Positioning, Navigation, and Timing (PNT ExCom) by Lt Col Robert Erickson, U.S. Air Force, and Dean Bunce, Federal Aviation Administration, the two co-chairs of the National Space-Based PNT Systems Engineering Forum (NPEF).
The findings came from November tests conducted at the behest of the National Telecommunications and Information Administration according to very specific parameters, described in the slide below, for general location and navigation receivers. Wideband and military receivers were also tested, but neither the specifications nor the results were shared with general audience; a classified session was convened for that purpose.
The tests were conducted at White Sands Missile Range in New Mexico in early November, with participants as listed in the following slide.
The testing was done in the lower LightSquared band (1526–1536 MHz), the so-called Low 10. Testing during the past summer showed widespread interference from LightSquared’s terrestrial transmitters at the higher band (1526-1559 MHz), known as the Upper 10, adjacent to GPS L1. LightSquared had offered after those tests to (perhaps) abandon or at least (partially) delay terrestrial broadcast in the Upper 10, while pressing forward with plans for the Lower 10, which the company maintained should not present any problems for GPS receivers.
Of the 92 receivers for which full data sets were compiled, 75 percent of them failed a 1db test, showing harmful interference, at a distance of 100 meters from a LightSquared base station. These 69 receivers failed at a broadcast level of around -15dBm from the LightSquared transmitter, as shown in the plot below.
On December 7, in the latest of multiple filings with the FCC, LightSquared further revised its publicly promulgated plans to say that it would "limit its power on the ground when transmitting in the lower 10 MHz from 1526-1536 MHz to no more than -30 dBm until January 1, 2015, -27dBm until January 1, 2017, and -24dBm thereafter." According to the plot above, at -30dBm, approximately 17 percent of GPS receivers would be affected; at -27dBm, approximately 25 percent of GPS receivers would be disrupted; at -24dBm of LightSquared power, approximately 36 percent of GPS receivers would be interfered with. Proceeding with this scenario would require the assumption that the FCC, or indeed anyone, believes anything that LightSquared says at any given instant, for any given duration.
The report concludes:
1. LightSquared signals caused harmul interference to a majority of GPS receivers tested.
No additional testing is required to confirm harmful interference exists.
2. Immediate use of satellite service spectrum for terrestrial service is not viable because of system engineering and integration challenges.
The [LightSquared proposed] Lower 10 mitigations are unverified, neither do they rectify previously found problems in the Upper 10.
If the end state [of the conditional waiver] should be deemed compatible, a long-term transition/implementation plan is necessary to protect GPS service and users. The presenters cited the U.S. Air Force process to discontinue semi-codeless GPS, with a transition extending through 2020.
3. Millions of fielded GPS receivers are not compatible with proposed LightSquared signals.
Statement by Anthony Russo, Director, National Coordination Office
On Behalf of the National Executive Committee for Space-Based Positioning, Navigation, and Timing, December 14, 2011
WASHINGTON -- "Today [December 14], a government technical group reviewed the findings from last month's testing of LightSquared's proposal to provide new broadband service. The final test report will be sent to the National Telecommunications and Information Administration (NTIA), which advises the President on telecommunications policy, and represents federal agencies to the Federal Communications Commission (FCC).
"Preliminary analysis of the test findings found no significant interference with cellular phones. However, the testing did show that LightSquared signals caused harmful interference to the majority of other tested general purpose GPS receivers. Separate analysis by the Federal Aviation Administration also found interference with a flight safety system designed to warn pilots of approaching terrain.
"The findings were presented to the technical steering group which represents the nine federal agencies that make up the National Executive Committee for Space-Based Positioning, Navigation, and Timing. Over the next several weeks, the final analysis of the findings will be completed and a final report will be transmitted from NTIA to the FCC."
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