GPS interference concerns with Ligado remain unaddressed, experts say

Ligado’s proposed broadband service continues to pose a significant risk of harmful interference to GPS, several key signatories wrote in a July 18 letter to U.S. Federal Communications Commission (FCC) Chairman Ajit Pai.

The interference would occur despite Ligado’s May 31 amendments to its license modification applications, the letter states. “The proposed Ligado operations, even after the recently proposed amendments, will harmfully impact a wide range of GPS receivers and thus should not be permitted.”

Key signatories include representatives of Iridium Communications, the International Air Transport Association, Lockheed Martin, Rockwell Collins and Satelles.

The letter urges the FCC to deny Ligado’s pending license modification application unless Ligado can show it has addressed the substantive GPS interference concerns. Also encompassed in the letter are concerns over interference with satellite communications (SATCOM) and weather data.

“The record, augmented by recent government reports, makes clear that the interference will be particularly impactful to the countless government and commercial entities that rely on GPS and SATCOM services for aviation safety and other critical services,” the signatories wrote.

“A number of data points underscore the flaws in the way in which Ligado has attempted to measure interference to GPS from its proposed terrestrial operations.”

Ligado recently acknowledged that its license modification request as originally filed is insufficient to protect certified aviation GPS receivers, but offered no changes to its proposal to address interference to uncertified GPS systems, SATCOM services (also important for aviation safety), or concerns of the weather data community.

“Evaluating Ligado’s recent amendment to ensure protection of uncertified GPS receivers and protection of GPS receivers from aggregate interference will be necessary,” the letter states.

1-dB Criterion. One issue is the 1-dB degradation interference protection criterion, which Ligado has suggested is “neither accurate nor reliable.” That criterion was used in the U.S. Department of Transportation’s Adjacent Band Compatibility assessment issued in April, which examined the maximum transmitted power levels of adjacent band systems that can be tolerated by GPS receivers. The study endorsed and strictly applied the 1-dB criterion, and termed it the “accepted, worldwide standard for PNT and many other radio communication applications.”

“Each of these government efforts supports one conclusion — the proposed Ligado operations, even after the recently proposed amendments, will harmfully impact a wide range of GPS receivers and thus should not be permitted…

“We urge the Commission to deny Ligado’s pending license modification application as proposed unless Ligado can show it has addressed the substantive GPS, aviation, SATCOM, and weather data interference concerns still outstanding in the record,” the letter concludes.

Read the full letter here.