The FCC's Decision on LightSquared: High-Precision Users Would Be Affected Most
February 16, 2011 By: Eric GakstatterSurvey Scene, Early February 2011
If you’ve been tuned into gpsworld.com, subscribe to our newsletters, or belong to various online GPS/GNSS newsgroups, there’s no doubt you’ve heard the name "LightSquared" being mentioned. There's a lot of concern about LightSquared's potential jamming of GPS.
This is a drama that is playing out only in the U.S. at this moment, but since GPS/GNSS is becoming more ubiquitous in our lives and will continue to proliferate, it’s an interesting case study on frequency spectrum management that everyone in the world can, and should, learn from. And, by the way, the drama is not over. In fact, I’m not sure it has even peaked yet.
Background
LightSquared is a new business venture (~$7B investment) that is focused on providing 4G wireless broadband communications (mobile Internet). According to its website:
“LightSquared will deploy an open wireless broadband network using a technology called Long Term Evolution (LTE), the most widely adopted 4G standard in the world. Its LTE network will be combined with one of the largest commercial satellites ever launched, to provide coverage of the entire United States. This integrated LTE-satellite network is a world first.”
That’s the background in a nutshell. If you want to read more about LightSquared, visit the company's website.
How is LightSquared Related to GPS?
The issue for GPS users is that LightSquared is planning to use frequencies that are immediately adjacent to the frequencies used by GPS. Since LightSquared’s technology is based on deploying high-powered transmitters in selected metro areas, the concern is that it will interfere with the function of GPS receivers. In the past two weeks, Garmin published test results where they simulated LightSquared’s broadcast strength.
The results were alarming. Following is an excerpt from a GPS World article published on February 1.
Testing Confirms Potential Problem
Representatives of the U.S. GPS Industry Council and two prominent GPS manufacturers, Garmin and Trimble, presented a report, “Experimental Evidence of Wide Area GPS Jamming That Will Result from LightSquared’s Proposal to Convert Portions of L Band 1 to High Power Terrestrial Broadband,” to five members of the FCC’s Office of Engineering and Technology, including its chief, two members of the FCC International Bureau, one from the Public Safety and Homeland Security Bureau, and two from the Wireless Telecommunications Bureau.
The document conveys results of testing on a common portable consumer automotive navigation device and on a common general aviation receiver. The consumer GPS device began to be jammed at a power level representing a distance of 3.6 miles (5.8 kilometers) from the simulated LightSquared transmitter. The consumer device lost a fix at 0.66 miles (1.1 kilometers) from the transmitter.
The industry report concludes: “As shown by the Garmin testing described in this document, the proposed LightSquared plan to add 40,000 high-powered transmitters in the band adjacent to GPS will result in widespread, severe GPS jamming. This will deny GPS service over vast areas of the United States.”
Lightsquared has stated that it will work with the GPS industry to determine which GPS equipment needs "filtering so that they don't look into our band." The FCC wants to start the testing process on February 25 and have it completed by June 15. "It's a fast process," noted Jeff Carlisle, Lightsquared executive vice president for regulatory affairs and public policy.
What About High-Precision Users?
Garmin only tested two receivers from one manufacturer, themselves. There are several ways to skin a cat when designing a GPS receiver, so a lot of testing is still required to understand the impact of LightSquared's transmitters on GPS receivers.
Unfortunately, it's likely that LightSquared would affect high-precision GPS receivers even more than the consumer GPS receivers tested by Garmin. High-precision GPS receivers are those GPS L1 receivers that try to squeeze the most out of the GPS L1 signal to well under a meter. They are typically used for mapping, GIS, and navigation over a broad range of applications such as forestry, agriculture, utilities, environmental, infrastructure, marine, aviation, federal/state/local government, and many others.
"Our RF (radio frequency) front-end is wide relative to some of the lower end (consumer) GPS receivers since a wide front-end gives better code tracking performance," said Michael Whitehead, vice president of technology at Hemisphere GPS. "But, it also opens you up more to jamming susceptibility."
"The LightSquared tests need to use samples from many different manufacturers, including receivers using GLONASS," said Whitehead. "They also need to test applications using carrier phase such as RTK since the jamming may cause carrier slips that go unnoticed otherwise. Better yet, look for anomalies in the code/carrier observations."
The Federal Aviation Administration's Wide Area Augmentation System (WAAS), which thousands in the professional GPS user community use on a daily basis for high-accuracy GPS L1 positioning, would be equally affected.
"SBAS and GPS are at the same frequency and similar power levels, so the effect from jamming should be similar," said Whitehead. "There is one difference, though, and that is how they modulate the navigation message. SBAS uses 2 ms per symbol, whereas GPS uses a full 20 ms. So there could be some difference in ability to decode the navigation data. Furthermore, the frequency of occurrence of carrier track-loop slips caused by the jamming may be higher in SBAS (GPS can employ a slightly more robust carrier tracking Costas loop, but this will depend on manufacturer implementation)."
"He might be right, but we are doing the testing to know for sure," said Leo Eldridge, GNSS program manager for the FAA.
On December 21, 2010, the FAA sent a letter to the National Telecommunications and Information Adminstration expressing its concern. Among other things, the FAA recommends that FCC explore the "spurious emission limits for the band 1559-1610 MHz (that is allocated exclusively to the radionavigation-satellite service and used by many GPS applications) derived under the new terrestrial-only assumptions."
On December 28, 2010, NASA, the Department of Transportation, the Department of Commerce, the Department of the Interior, the Department of Homeland Security, the U.S. military departments (Army, Navy, Air Force), and the U.S. Coast Guard jointly issued a memo to the National Telcommunications and Information Adminstration's Interdepartment Radio Advisory Committee (IRAC) Chairman expressing "a number of serious concerns with the LightSquared proposal as submitted."
High-Performance GPS Receivers Are More Susceptibile to Jamming
The problem with high-performance receivers is that they are designed with a wide bandwidth front-end for superior code tracking and multipath mitigation techniques. Therefore, their antennas are designed to accomodate even a wider bandwidth in order to cover the bandwidth range design of the RF front-end and other variables.
"The RF front-ends for our receivers is greater than 18 MHz to improve multipath mitigation," said Walter Feller, director of engineering at Hemisphere GPS. "Any high-end receiver for multipath mitigation needs at least 6-10 MHz of bandwidth. E-911 commercial receivers (low-end) only require 2 MHz of bandwidth."
For consumer receivers, the front-end is designed for a narrow bandwidth (2 MHz). This translates into poor code tracking (poor accuracy) and poor multipath mitigation, but it does make the receiver more immune to interference. The opposite is true for high performing receivers where the front-end is designed for wide bandwidth (up to 20 MHz or more) which improves code tracking (accuracy) and improves multipath mitigation. With such a wide bandwidth, there becomes overlap between high-performance GPS receivers and LightSquared's frequency spectrum. The result may be that the GPS receiver will be jammed.
If Garmin's experiments show interference using a narrow bandwidth consumer GPS receiver like they did (nuvi 265W), imagine what might happen when a high-performance, wide bandwidth GPS receiver is used?
OmniSTAR Users
OmniSTAR receivers in North America use frequencies that may be affected by LightSquared's transmitters. Furthermore, some GPS receivers designed to use OmniSTAR require a super-wide bandwidth antenna (60 MHz), which would be even more susceptible to interference from LightSquared's transmitters than the high-performance GPS receivers mentioned above.
However, note that most OmniSTAR users operate in rural areas rather than the urban metro areas where LightSquared seems to be focusing their efforts. Furthermore, OmniSTAR offers an NTRIP service that enables users to access OmniSTAR corrections via internet connection (mobile phone, WiFi).
More Testing Is Needed
One thing is for sure. More testing with models from many different manufacturers of GPS receivers, GPS/GLONASS receivers, and GPS/SBAS receivers needs to be done before any sort of conclusions can be drawn. Given that GPS/GNSS is a multi-billion dollar industry; all eyes are on the FCC to be prudent and approach this with the caution that it deserves. I think it is imperative that we let our representatives in Congress know about our concerns so that attention will be focused, and the right decisions made. Please click on the following link for guidance in contacting the appropriate people in Congress:
GPS Community Urged to Contact Congress Regarding FCC Proposal
Thanks, and see you next time.
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About the Author: Eric Gakstatter
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