Metro Aviation, Garmin at Odds Over Ligado Helicopter Worries; GPS 'Threat' Challenged

Questions about interference to certified aviation GPS receivers continue to hover over Ligado's terrestrial low-power service plans. In a docket 11-109 filing posted Tuesday, Mike Stanberry, president of air medical services company Metro Aviation, said it's confident Ligado operations wouldn't interfere with safe helicopter operations and disagrees with the assertion Ligado's proposal wouldn't protect safe helicopter navigation. Metro said Ligado's commitment to keep its transmit power below FAA minimum operational performance standards, except in areas 250 feet horizontally distant or 30 feet above any Ligado antenna, should mean it won't affect safe helicopter operation. Aviation interests raised GPS receiver concerns in June (see 1706210030). Garmin Tuesday cited an RTCA review of Ligado's proposal that it said showed unresolved issues that potentially could affect aviation safety. Garmin said the issues include whether Ligado will provide a public database of base station locations, concerns of helicopter operators about relying on GPS-based navigation and GPS-enabled capabilities for obstacle avoidance when near Ligado standoff cylinders, signal acquisition scenarios that haven't been analyzed, and lack of agreement about base station antenna height and inter-site distance Ligado. The GPS device maker said there should be no Ligado authorization until the FAA acknowledges operation of the Ligado network is compatible with certified aviation devices. Meanwhile, in a LinkedIn column Wednesday, Roberson and Associates CEO Dennis Roberson criticized the Space-Based Positioning, Navigation and Timing Advisory Board (PNTAB) for insisting on a 1 dB noise floor interference metric as the best way to assess the effects on GPS (see 1706290043). He said 1 dB metric advocates have been asked repeatedly for references to standards or documentation for use of a noise floor in identifying out-of-band emissions problems, but none have been identified. He also said PNTAB calling Ligado's plans within the 1526-1536 MHz band an "existential threat to GPS" lacks scientific grounding, since there's "a very expansive 27 MHz 'guard band' " between the lower edge of GPS signals and the proposed Ligado terrestrial network.