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Satellite and Terrestrial Signal Differences

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Why the two signals need separation

At the heart of the threat to the nation's Global Positioning System (GPS) is the proposal to place two signals—one faint and one blaring—in operation immediately adjacent to each other.

For the same reason radio station signals are not placed right next to each other in the same city— because of the strong interference that would inevitably result—the Federal Communications Commission (FCC) has traditionally kept a wide separation between the spectrum used for satellite signals and spectrum used for ground-based signals.

But the FCC's recent grant of a conditional waiver to a company called LightSquared focuses attention on the huge disparity between the strength of faint satellite-generated signals emanating from 12,000 miles away and the blaring signals that LightSquared's planned 40,000 ground stations would generate here on Earth.

And the term "blaring" actually considerably understates the situation. As astonishing as it might sound, the strength of the signal from a LightSquared ground station would be one billion or more times greater, as received on Earth, than the signal from a GPS satellite.

Why such a difference in signal strength?

GPS provides its highly accurate navigational information via 24 satellites placed in orbit by the U.S. Department of Defense. The satellites circle the earth in a precise orbit, sending signal information to earth that GPS receivers take and, using triangulation, provide data that helps guide such widely varying activities as navigating cars; constructing buildings and roads; timing bank transactions; locating pizza parlors or nearby gas stations, synchronizing wireless, computing and utility networks, and many others.

The only means of powering GPS signals from satellites is via solar panels, and GPS signals are sent out from satellites using 50 or fewer watts, about the same wattage it takes to power a light bulb. That GPS signal then travels 12,000 miles before being received. Contrast that to LightSquared's planned ground transmissions powered at up to 1,500 watts—30 times greater than GPS's—transmitting as few as 100 feet away.

One way to think of this is in terms of the amount of light you'd expect to see from a bulb located 12,000 miles away, powered with 50 watts versus the amount of light you'd see from a bulb located 100 feet away, powered with 30 times greater wattage. One you wouldn't see at all; the glare of the other would be blinding.

When one signal is one billion times stronger than the other there is a significant danger that the 40,000 ground stations - with their overpowering signals—will cause significant interference with GPS signals. The possibility of 40,000—dead spots—each miles in diameter - is real.

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