Working group seeks to address perceived problems with GPS signal interference

An FCC-mandated testing process is evaluating 150 different GPS devices, and when that testing is complete, mitigation solutions will be explored by all the parties


A few weeks ago, I wrote a story about potential interference with Global Positioning Satellite (GPS) signals due to the deployment of a new cellular telephone and data network by a communications company called LightSquared. When I was preparing that article I attempted to contact LightSquared to get their side of the issue, but got no response to my emails and phone calls. After the story ran, LightSquared did reach out to me, and on April 28, 2011, I spoke via phone with Jeff Carlisle, their executive vice president for regulatory affairs and public policy.

LightSquared is historically a satellite telephone services company, founded in 1989. They launched their first communications satellite in 1996, and have a wide range of clients using their services, including many public safety agencies. Carlisle pointed out that LightSquared is the only provider of a push-to-talk satellite-based network that can be provided on an ad hoc basis to first responders.

The frequency band that LightSquared licenses from the FCC is underused by their satellite subscribers. In 2001, LightSquared petitioned the FCC to repurpose this frequency band for a combination of satellite and ground-based wireless communications. When users were within range of a ground station, their communications would run through that system, as with conventional cellular telephone service. When users were out of range of a ground station, their handset or data modem would communicate with an overhead satellite. The switch from ground to satellite communications would be seamless and transparent to the user.

Because LightSquared’s portion of the communications spectrum is adjacent to the channels used by the GPS network, the GPS community expressed concern that LightSquared’s ground-based signal would bleed into the GPS bands and overwhelm the relatively weak transmissions from the GPS constellation, roughly 13,000 miles away from the typical GPS receiver.

LightSquared responded to this issue by spending $9 million to develop filters to ensure their signals would not interfere with GPS transmissions. Carlisle says these filters are 1000 times more effective than the FCC required in a 2003 ruling. In 2004, the GPS industry indicated to the FCC their support for LightSquared’s technical parameters.

GPS frequencies lie within bands designated L1 to L5, with the L1 band being closest to the LightSquared spectrum. In 2010, the GPS industry told the FCC that some GPS receivers “look” outside of the GPS bands, and strong transmissions in the adjacent frequency bands — namely, the ones used by LightSquared — could render those receivers useless. Many receivers do not have appropriate filtering because of older technology or poor design, and can pull in signals outside the designated GPS bands, so that a strong adjacent transmission could overwhelm the GPS signals.

Carlisle co-chairs a working group to resolve this problem with Charlie Trimble who founded Trimble Navigation, a major GPS manufacturer. There are seven subgroups within the working group that include representatives from Garmin, APCO, Boeing, Rockwell, Lockheed-Martin, public safety organizations, the Dept. of Defense, NASA, and the FAA. Testing of solutions is underway, with a final report to the FCC due on June 15, 2011.

The FCC-mandated testing process is evaluating 150 different GPS devices. After that testing is complete, mitigation solutions will be explored by all the parties. Carlisle assured me that both the FCC and LightSquared are cognizant of the critical role that GPS has not only for land, sea, and air navigation, but for maintaining other aspects of our infrastructrure.

We’ll all still be able to find Grandma’s house at Thanksgiving.

About the author

Tim Dees is a retired police officer and criminal justice professor. He has been writing on criminal justice technology issues for virtually every U.S. police publication and commercial website since 1988. Tim holds a bachelor’s degree in biological science from San José State University, and a master’s degree in criminal justice from The University of Alabama. He serves on the executive board of the Public Safety Writers Association.

He can be reached at tim.dees@policeone.com.

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