Garmin's Talk of 1 dB Interference Threshold Isn't Solid, LightSquared Says

Not even Garmin really believes a 1 dB rise in the carrier-to-noise ratio represents a good measurement of harmful interference, LightSquared said in a filing posted Tuesday in docket 12-340. In a filing last month, the GPS company -- in talking about LightSquared's planned ground-and-satellite-based LTE broadband network and possible interference with GPS signals in adjacent spectrum bands -- had argued even a 1 dB rise in noise floor can affect GPS (see 1509240033). However, LightSquared said, "the 1 dB proposal is a proxy Garmin wants to use for no other reason than Garmin's belief that it is just too hard to show harmful interference any other way." Noise floor actually pales compared with satellite position and atmospheric conditions when talking about causes of GPS errors, and Garmin has not shown any evidence a 1 dB increase in noise floor actually has any effect on GPS device performance, LightSquared said. The ITU has recommended 1 dB increases "for a very limited purpose" -- co-channel interference applicable only to GPS devices using assisted GPS, LightSquared said. In a separate filing Tuesday in 12-340, the Alliance for Telecommunications Industry Solutions said it is concerned about GPS interference woes coming from use of adjacent spectrum bands. The group said its Copper/Optical Access Synchronization and Transport Committee would be taking part in the Department of Transportation's GPS Adjacent-Band Compatibility Assessment, and that the results of that DOT study, plus any related technical work, "should be considered before any decision is made to change the use of bands adjacent to GPS signals, to avoid any impact to voice and data services on existing and future networks." The group also said it is "crucial to consider how signals in adjacent bands may impact this sector and recommends that test plans for this complex testing be reviewed by neutral parties." LightSquared has commissioned a test aimed at determining the scope and degree of any L-band LTE network interference to GPS (see 1508250070).