LightSquared's Proposed Fix for GPS Interference Requires New Testing

News Analysis: LightSquared is offering to reduce the power output of its transmitters and shift to the lower range of its permitted spectrum to resolve problems that were confirmed in the GPS Technical Working Group report released on June 30.

The long-awaited 750-page report of the GPS Interference Technical Working Group released June 30 confirmed what multiple government agencies and private industry interests have been saying since the group began its work: that LightSquared's plan for a wireless broadband network would prevent GPS devices from working properly anywhere in the United States.

But even before the official release of the Technical Working Group Report, LightSquared went public with a proposed workaround to the GPS interference problem.

LightSquared is proposing that it use the lower 10MHz of the L-band spectrum the company was given for its 4G LTE service and that it reduce the power to 10 percent of what the Federal Communications Commission originally authorized. According to LightSquared Executive Vice President Martin Harriman, this would mean that the LightSquared transmitters would only produce about 1,500 watts of radio power, versus the 15,000 watts the FCC originally authorized.

It's worth noting that the test transmitters that the TWG used in its study were running at about 1,500 watts of power, but they were using the upper 10MHz of the frequency allocation as well as the lower 10MHz section. There has been little formal testing of GPS interference using just that lower frequency slice. Because of this, many of the Working Group tests would have to be rerun to see if the interference problem has been solved.

The problem is one of basic physics and radio design. The GPS frequencies span 20MHz of the L-band immediately adjacent to the upper 10MHz segment granted to LightSquared. A strong signal in an adjacent channel can cause the receiver's RF amplifiers to become saturated, even when the strong signal isn't on the same frequency.

This can cause receiver desensitization, and it can create spurious signals inside the receiver. The result is that radio reception in the affected receiver may be impossible, degraded, distorted or some combination of the three.

Compounding the problem is that you can only do so much about it. GPS receivers have to be very sensitive to pick up the faint signals from the GPS satellites. Because they are so sensitive, they are also prone to interference. You can design in what's called a band-pass filter to block some of this interference, but a band-pass filter introduces its own problems, including reducing the signal strength of the receiver being protected. When that happens, the whole issue of high sensitivity comes to the fore again, and you're basically stuck with diminishing returns.