THOMAS: FCC NOTICE WOULD OPEN DEBATE ON GPS PROTECTION LIMITS
Planned FCC notice on what changes, if any, are needed in certain protection levels for GPS is meant to open debate on issue that has surfaced since ultra-wideband proceeding began, Office of Engineering & Technology (OET) Chief Edmond Thomas said. In recent mobile satellite service (MSS) order, FCC chose less conservative limits to protect GPS from ancillary terrestrial operations than power levels NTIA backed. But 2 agencies agreed OET would seek comment on potential changes, if any, in protection levels for GPS in future. Agencies agreed “it was a reasonable course that we will start a public debate about whether it is necessary to set a generic limit for devices that have spurious emissions into the GPS band,” Thomas said. “If the answer to that is yes, what should that limit be?”
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Public notice will be out shortly, Thomas told us: “We are moving forward on it. It’s useful and important.” Leading up to MSS order, one issue debated related to GPS band was out-of-band emission limits into that spectrum from ancillary terrestrial component (ATC) base stations and terminals. Order upheld FCC’s existing GPS protection limits of -70 dBW/MHz for ATC operations. That level is less restrictive than agreement on out-of-band emission limits reached last summer by U.S. GPS Industry Council and Mobile Satellite Ventures, with Defense Dept.’s approval. Those limits were -100 dBW/MHz for ATC base stations and initially -90 dBW/MHz for terminals operating in ATC mode. NTIA had sought -95 dBW/MHz power limit for ATC mobile units to maintain performance level for GPS receivers needed to meet certain Enhanced 911 accuracy requirements.
Thomas said -70 dBW/MHz level in MSS order to protect GPS reception was “placeholder” that could change later, depending on feedback FCC received. “It could go up or down or stay the same,” he said. “I think we have the luxury of time,” he said, in part because number of ATC handsets will take time to reach market. MSS order also limits number of those handsets in first place, he said.
Before MSS order, some concerns had been raised in UWB proceeding about potential interference risk to GPS systems from UWB devices. NTIA in UWB proceeding had questioned vulnerability of GPS to interference from spurious emissions, Thomas said. Similar concern was raised in MSS order that ATC in lower band would create higher-than-acceptable interference risk for GPS, he said. Thomas said he expected possibility of generic limit to protect GPS, as well as potential drawbacks of applying such catch-all number across different technologies, would be raised in forum for debate stirred by public notice. In MSS proceeding, NTIA also had stressed to FCC that technical factors differed when considering interference to GPS receivers in aviation versus terrestrial scenarios. NTIA said previous limits of -70 dBW/MHz were based on protection levels for GPS reception in aviation scenarios. But Thomas said -70 dBW/MHz figure was arrived based on “engineering judgment of what we thought was reasonable.” It happened to come out to be same figure, but wasn’t aviation-based number, he said.
While scope of notice will be limited to GPS, FCC’s Spectrum Policy Task Force has lined up idea of “interference temperature” that addresses some similar issues, but more broadly. Interference temperature would put ceiling on noise environment in which receivers would be required to operate. Then to extent “temperature” in certain band wasn’t reached, user that emitted below that threshold would gain benefit of additional operational flexibility. Of GPS notice, Thomas said: “It’s kind of the tip of the interference temperature iceberg. At the end of the day, it’s a good thing to do to start the debate. It’s unclear to me where it will wind up.”