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Electronics

Threats to GPS from Land-Based Signal Boosters

Spectrum Space Invaders

LightSquared would rely on ground-based transmitters, as well as satellites, as shown in this rendering.We’ve all had voyages that weren’t quite what we signed up for when we first got into boating—trips in which the wind and sea on the nose have made passagemaking quite a bit slower—and less fun—than it’s meant to be, and when rain has reduced the passing scenery to intermittent glimpses of a gray smudge.

But suppose, just when the first sight of harbor lights promises the reward of shore power, showers, and a decent meal—your plotter loses its fix. Wouldn’t that just put the cherry on it?

That could become absolutely standard if one particular company has its way.

Offshore, GPS would still work just fine. But a few miles off the coast, you’d start to lose a lock on satellites. And for the last couple of miles into a harbor approach, canny captains would bank on being without GPS altogether, as powerful ground-based transmitters drown out the faint signals from GPS satellites.

The company behind this nationwide jamming project—now known as LightSquared—is a perfectly respectable business that has been operating satellite communications for more than 15 years. It was one of the founders of XM satellite radio, and it’s rumored that Barack Obama invested a five-figure lump of his personal capital in it.

And there’s nothing wrong with its ambition of making mobile Internet available to every American, at prices that almost every American could afford—about $7 per gigabyte.

The problem is, that would generate more traffic than could possibly be handled by a couple of satellites, so LightSquared intends to “fill in” its satellite service with a network of 40,000 ground-based transmitters, each transmitting at up to 16 kilowatts, in a part of the radio frequency spectrum that is supposed to be reserved for satellite services.

LightSquared is not intending to transmit on the same exact frequency as GPS: It operates on 1545.2-1555.2MHz and 1526-1536MHz, while the primary frequency of GPS is 1575.42MHz.

But GPS signals come from solar-powered 50-Watt transmitters 12,000 miles out in space.

Having so many powerful transmitters operating in what was intended to be a quiet part of the radio spectrum has been compared to a rock band playing in a library, but even that is a serious understatement. A rough calculation suggests that a LightSquared transmitter 20 miles away could produce a signal more than a hundred million times stronger than is received from a GPS satellite.

In spite of this, the FCC gave LightSquared conditional approval to go ahead with the project, subject to further testing and with safeguards built in to protect GPS.

Even conditional approval sparked massive concern in the GPS industry, but it wasn’t just commercial interests that were worried by the prospect: General William L. Shelton, of the U.S. Air Force Space Command, warned the House Committee on Armed Services that “virtually every GPS receiver out there would be affected.

An interagency working group was set up to investigate the issue. Three months later, when it published its report, its co-chairman Charles Trimble, who is chair of the U.S. GPS Industry Council, described its job as “an extraordinary challenge.” They had been, he said, “trying to determine if the laws of physics would allow the high-power LightSquared signals to coexist with the low-power satellite signals of GPS.”

Technicians prepare a satellite for launch.“In the end, the laws of physics won out,” Trimble added. “There is no single, simple solution that can eliminate interference for all classes of GPS receivers in the near term.” 

LightSquared’s response was to issue its own report, claiming that only “legacy” receivers would be affected.

“Legacy GPS receivers,” the report read, “do not adequately reject transmissions from base stations operating in the adjacent frequency band because the GPS receivers have been deliberately or, sometimes, inadvertently, designed or manufactured with the assumption that there would be no adjacent-band transmissions.”

The company offered to reduce its transmitter power and to limit its transmissions to only one frequency block, and asked for a second round of testing. 

But LightSquared was disappointed: The second round of tests showed that although the revised plans would have less impact on GPS, they also showed that many receivers would still suffer. In one damning sentence, the National Executive Committee for Space-based Positioning, Navigation, and Timing said, “there appear to be no practical solutions or mitigations that would permit the LightSquared Broadband service to operate in the next few months or years without significantly interfering with GPS.”

Two weeks later, the president signed the National Defense Appropriation Act, including a section that prohibits the FCC from approving the LightSquared plan unless it can demonstrate that there will be no interference with military use of GPS.

Far from putting an end to the war of words, the Defense Appropriation Act seems to have turned up the heat, prompting LightSquared to claim that the test process “was rigged by manufacturers of GPS receivers and government end users to produce bogus results.”

Jim Kirkland, VP of Trimble and a founder of the Coalition to Save Our GPS hit back: “At each and every turn in this process, whenever LightSquared does not like a test result or ruling, it either seeks to change the parameters or cries foul—and frequently both.” 

“The technical evidence,” Kirkland continued, “speaks for itself and no individual, company, or government body can legitimately be blamed for the clear defects of LightSquared’s ill-conceived proposal or the failure of that proposal to pass an extensive, fact-based review process.”

But LightSquared is not about to kiss its $14 billion project goodbye without a fight. Building on its claim that “the interference experienced by the commercial GPS receivers is the result of an industry decision to design and sell poorly filtered devices,” the company has now asked the FCC to set performance standards for GPS receivers that would outlaw devices that look outside their assigned frequency band. 

Most boaters, I’m sure, will be rooting for GPS, but there’s a nagging little voice at the back of my mind that says LightSquared may have a valid point. After all, if the GPS receiver in your smartphone can filter out transmissions from GSM, WiFi, and Bluetooth transmitters built into the same slim casing, surely it’s not impossible to filter out interference from LightSquared towers?

It seems the FCC is thinking along the same lines. Its latest move—as I write this—is to rescind its conditional approval of LightSquared. But in the same public document, it’s firing a warning shot across the bows of the GPS and satcom industries: “Congress, the FCC, other federal agencies, and private-sector stakeholders must work together to reduce regulatory barriers and free up spectrum for mobile broadband,” said FCC spokesperson Tammy Sun, adding that “part of this effort should address receiver performance.”

For the moment, it looks as though GPS has won the battle. The interesting bit now is what is going to happen in the next few years.

This article originally appeared in the May 2012 issue of Power & Motoryacht magazine.

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