Inside GNSS Media & Research

MAR-APR 2018

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20 Inside GNSS M A R C H / A P R I L 2 0 1 8 www.insidegnss.com the Space-Based PNT Advisory Board (PNTAB). ) T h a t 's i mp or t a nt b e c au s e t h e PNTAB measures interference using the internationally accepted 1 dB deg- radation Interference Protection Cri- terion (IPC) — that is a one-decibel (1 dB) degradation in C/N0, the carrier-to- noise power density ratio. Ligado has been seeking to redefine the yardstick to a more favorable method: a change in positioning and timing accuracy. Of the five tests (listed below) the first three were designed and conducted by the government. Tests four and five were sponsored and framed by Ligado. 1) Federal Communication Commis- sion (FCC)-mandated Technica l Working Group (TWG) 2) National Space-Based PNT Systems Engineering Forum (NPEF) 3) Depa r t ment of Tr a nspor t at ion (DoT) Adjacent Band Compatibility (ABC) 4) Roberson and Associates (RAA) 5) National Advanced Spectrum and Com mu nicat ions Test Net work (NASCTN) e NPEF also was to determine if there were any unanswered questions or untested conditions that would hinder the GPS community from determin- ing the "maximum aggregate power level of out-of-band transmissions to ensure that the existing and evolving uses of space-based PNT services are not affected." Tests 1 and 2, which were done in 2011, showed there was debilitating interference to GPS receivers from the telecom network as it was designed at the time by Ligado's predecessor firm, LightSquared. e results of these tests contributed to an FCC decision to set the LightSquared plan aside. The final report on the third test, t he Adjac ent B a nd C ompat ibi l it y Assessment (ABC study), has yet to be released. ere were preliminary results presented in 2017 and the NPEF appears to have had access to either these or the final report. "W hile the exact va lues var y by receiver categor y and LTE network architecture and the resulting aggre- gate power," wrote the Gap Analysis authors, "the test results indicate that the maximum tolerable EIRP of inter- ference sources in the frequency bands adjacent to GPS are in the milliwatt or microwatt range." e NPEF found there is sufficient data to assess the risk of using frequen- cies near the GPS band for a ground- based communications network. While there were also some research gaps, that is questions that would be valuable to have answered, the study team also concluded that the data from the three government tests — when used in com- bination — was sufficient and appropri- ate to determine the maximum tolerable aggregate power level of transmissions in the band adjacent to GPS L1. As for the Ligado-sponsored tests, the NPEF acknowledged the effort that went into them but determined that their scope and framework were insuf- ficient when evaluated against the mini- mum criteria. Also noted in the back of the report was the fact that the studies had limited to no input from the public or from GPS experts. Garmin Weighs In Ligado has been going to great lengths to put a positive spin on its tests and its settlements with different GPS receiver manufacturers — suggesting, for exam- ple, that the deals with the five receiver f irms indicate that the interference problems have really been addressed. Garmin, it seems, disagrees. In a letter filed March 19, the GPS receiver manufacturer stated that, while it " has long supported the domestic development of new broadband ser- vice;" it believes that "broadband devel- opment generally should not come at the expense or harm to the nation's well functioning, innovative, and economi- cally important global positioning ser- vice (GPS)." e company said that in its legal settlement with Ligado it agreed not to object to proposals regarding its non- certified aviation and general location / navigation lines of business — pro- vided that certain technical param- eters were met. The f irm, however, had reserved the right to comment on issues related to certified aviation and its deal with Ligado did not constitute an endorsement by Garmin of Ligado's proposal. Garmin also said that the two com- panies had not reached an agreement about the 1 dB Interference criterion as an appropriate metric to evaluate inter- ference. Moreover, in previous filings, Garmin has made clear that it supports the 1 dB standard and disagrees with Ligado's approach. The NPEF underscored that the 1 dB standard is the right approach. Going forward they strongly recom- mended that decisions impacting the GPS radio frequency environment be informed by data from tests aligned with the PNTAB's set of minimum cri- teria, including the 1 dB standard, and that full consideration be given to the potential operational, scientific, and economic impacts. Taking Out the Spin e NPEF assessment goes a long way toward sorting out the various tests and judging their usefulness. e NASCTN test for example — the second one to be sponsored by Ligado — was actually a reasonable effort to design a test meth- odology, and was never intended to be an interference test. Ligado, however, has repeated ly claimed that the NASCTN data was supportive of their assertion that GPS receivers could exist along side Ligado's revised network. WASHINGTON VIEW The NPEF found there is sufficient data to assess the risk of using frequencies near the GPS band for a ground- based communications network.

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