LightSquared Wireless LTE Broadband Kills GPS, Government Tests Find

By Wayne Rash  |  Posted 2011-06-13

LightSquared Wireless LTE Broadband Kills GPS, Government Tests Find

Two government bodies have now reported their findings confirming that LightSquared's satellite-based LTE broadband data system interferes with the global GPS system run by the U.S. government.

The short answer is that the interference ranges from significant to completely blanking out GPS signals required for navigation and many other uses. When this issue first emerged, LightSquared said that there would be no interference.

I looked at the specs, and I looked at the frequencies, and I relied on my tests of GPS receivers performed over the years for eWEEK, The Washington Post, Plane & Pilot magazine and others. During those tests it was clear that strong signals from nearby transmitters would desensitize GPS receivers. The amount of desensitization depended on how close in frequency the offending transmitter was, how strong the signal was and how well-designed the GPS receiver was.

In May the National Space-Based PNT (Positioning Navigation and Timing) National Executive Committee and the Federal Aviation Administration in concert with contractor RTCA tested the LightSquared system's interference with GPS. Both agencies revealed in a panel discussion in June that the interference was significant.

In some tests, all receivers including those used by the U.S. Coast Guard completely lost the ability to navigate. GPS systems used by NASA for scientific use were seriously impacted. The FAA had similar results in its tests, concluding that aviation GPS receivers would be rendered useless at altitudes below 2,000 feet in urban area. You'll note that aircraft frequently fly below 2,000 feet in urban areas because that's where the airports they most frequently use for landings are, and GPS plays a significant role in approach navigation.

What this means is that in spite of LightSquared's claims to the contrary, its LTE system, which operates on frequencies adjacent to the GPS frequency bands, would effectively kill most GPS uses. There has been a great deal written about this issue, with many, especially those supporting LightSquared, suggesting that this is all the fault of poor GPS receiver design. Apparently the GPS manufacturers should have anticipated that a high-powered broadband Internet service would decide to locate next door.

But the fact is that millions of GPS receivers were built long before LightSquared was a gleam in anyone's eye, and long before the Federal Communications Commission gave approval for operations. The result now is that if the FCC continues to authorize it, GPS will effectively be compromised.

Whats the Solution?


Fortunately, FCC Chairman Julius Genanchowski has now said that the FCC will not authorize commercial operations for LightSquared until the GPS interference issue is resolved. The problem is that finding a resolution won't be easy or cheap. The solution recommended by the government agencies studying the problem is for LightSquared to move its operations to frequencies that aren't adjacent to GPS. Considering the amount of money that LightSquared has sunk into its service, this isn't a popular alternative.

The other option most frequently mentioned is to equip all GPS receivers with filters to block LightSquared or to design new GPS receivers and replace existing receivers with new ones. Considering that perhaps there are hundreds of millions of GPS receivers in the world that would be impacted, it's hard to take this suggestion seriously, especially since it's LightSquared that's creating the interference.

There are other options: LightSquared can put filters on its transmitters. It can only use the bottom half of its spectrum, which interferes less. It can use lower power. It can use some combination of all three methods. Or it can be sent to a different frequency band.

As you can see, there are no really practical answers here. The tests that demonstrated LightSquared's interference with GPS could have been conducted years ago, long before the company had sunk an estimated $14 billion into the project. The FCC could have been more responsible by requiring the tests to be conducted before it let LightSquared begin building out its system. GPS makers could have used their magical powers to see the future and designed around LightSquared.

So now clearly we're in a conundrum caused by the government's haste to see its dream of broadband everywhere realized and caused by a disregard of obvious consequences by a company in a hurry. There are no good answers, but perhaps there's a workable solution.

First, tell LightSquared that it can only use the bottom half of its spectrum for transmitting its information and even there it can only do so in a way that doesn't interfere with GPS in any way, even a little. This may mean reducing power, filtering, antenna design and reducing the transmitter density. Next, tell GPS makers that from now on they have to design their receivers with better rejection of adjacent frequencies. After a decade or so, it should be safe for LightSquared to ease into full operation, subject, of course, to testing along every step of the way.

The sad part is that a widespread means of delivering broadband communications is badly needed, and had the FCC and LightSquared acted responsibly, we'd be seeing it happen. But in a classic example of haste making waste, now we won't see such a deployment for a long time, if ever.

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