NASTCN Report Should End LTE Proceeding, Ligado Says
The FCC now should have all the information it needs to decide on Ligado's LTE plans with release of National Advanced Spectrum and Communications Test Network (NASCTN) findings on LTE signals effect on GPS devices, Ligado officials said Thursday. The 428-page test report released Wednesday backs up the company's contention that its broadband terrestrial low-power service in spectrum adjacent to GPS won't interfere with the real-world functioning of various device categories, the company said. Ligado argued its proposed power and out-of-band emission levels protect GPS (see 1602250032). GPS Innovation Alliance didn't comment.
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The NASCTN testing is "objective and impartial," said Ligado Chief Legal Office Valerie Green, saying NASTCN had briefed the FCC and NTIA on the findings. Ligado outside counsel Gerard Waldron of Covington & Burling expressed some optimism that the report, coupled with indications new FCC Chairman Ajit Pai is serious about not delaying proceedings, could mean action on the Ligado application. The FCC didn't comment.
The report deliberately didn't use pass/fall criteria, NASTCN said. Said Green: "That's not NASTCN's responsibility. They would have exceeded their authority." Much of the study went over the methodology of the testing itself, which was proposed by Ligado in April, the agency said. The report said the testing looked at a variety of metrics: carrier-to-noise density, 3D position error, timing error, time to first fix, time to first reacquisition and number of GPS satellites in view. It focused on general location and navigation (GLN), high performance positioning (HPP), real-time kinematics, and GPS-disciplined oscillator receivers, the agency said. The 1,476 hours of testing was done over three months at a National Technical Systems facility and the National Institute of Standards and Technology broadband interoperability testbed, NASTCN said. NIST, NTIA and NASTCN are part of the Commerce Department.
The report repeatedly shows LTE signals like those Ligado is proposing wouldn't affect real-world usage of most GPS devices, and that the 1 dB noise floor interference metric used by the Transportation Department (see 1610180048) should be abandoned since it found no correlation between changes in that and actual device performance, Green said. The noise floor metric "doesn't even make sense," Green said, saying when exposed to an LTE signal steadily increasing in power, "Why is it jumping all over the place?”
The study at times shows changes in the noise floor of more than 1 dB when there's no LTE signal at all, or during times when GPS devices otherwise are performing accurately, Green said. Ligado repeatedly argued against the 1 dB standard as an interference threshold (see 1510140052). Testing of GLN devices shows position error fluctuations "of maybe a couple feet, maybe less than that" when exposed to an LTE signal that exceeds the power levels proposed by the company, and those fluctuations remain well within GPS manufacturer specifications, Green said.
NASTCN testing of HPP devices such as those used in surveying and agriculture showed interference from LTE, as did previous DOT testing, but the latest report also shows different antennas reduce that noise floor fluctuation substantially, Ligado said. Numerous GPS companies signed off on Ligado's TLPS plans since it revised them (see 1612060023). The company still faces interference-related pushback from Iridium (see 1609020029) and parts of the weather community (see 1612060023).